Did tyrannosaurs smile like crocodiles? A discussion of cranial epidermal correlates in tyrannosaurid dinosaurs

Brain 1: "Right, you need an image for your tyrannosaurid facial tissue post."
Brain 2: "OK, here're some Tyrannosaurus rex in a really dark and back-lit scene. Their faces are in shadow, and you can't really see the features."
Brain 1: "This is perfect. After all, only losers want to see the faces of animals in posts about facial tissues."
Brain 2: "Exactly. Hey, since when did I have two brains?"
Brain 3: "Beats me."

Discussing the craniofacial tissues of tyrannosaurid dinosaurs is the palaeointernet equivalent of lighting a match in a straw-filled barn - the slightest spark of opinion spawns a 100-strong comment field about extra-oral tissues, tooth exposure, rictal tissues, facial skin depth and a number of other topics. But despite this keen popular interest, there's been relatively little academic study into tyrannosaurid facial tissues, perhaps because their soft-tissues mostly remain unrepresented in the fossil record. Happily, close examination of tyrant skulls reveals a number of textures and rugosity profiles which were almost certainly created by bone-skin interaction, so we can form some idea of their life appearance even without soft-tissue specimens. The first detailed attempt at interpreting tyrant cranial rugosities was published last year by tyrannosaur expert Thomas Carr and colleagues (Carr et al. 2017 - you might also know Thomas by his super-comprehensive blog Tyrannosauroidea Central). This widely publicised paper proposed a number of hypotheses about the face of Daspletosaurus horneri: that the sides of the jaw were adorned with crocodile-like 'facial scales'; that various scales, dermal armour and cornified sheaths adorned the nasal and orbital region; and that it lacked lips (not explicitly stated in the paper, but restored as such in an illustration and touted in the paper's PR). The idea that tyrannosaurids may have had crocodylian-like facial tissues has since generated a lot of discussion online, some in favour, some against, and as someone increasingly looking at epidermal correlates for palaeoartistic purposes, I thought this topic was worthy of a blog article: are tyrannosaurid jaws really croc-like enough to assume comparable skin types?

(An important caveat before we start this discussion is that the following is based on tyrannosaurids generally, not D. horneri specifically, because the horneri study does not include photographs of its alleged epidermal correlates. The D. horneri paper describes them very well (see Carr et al. 2017, supplementary data), but it's difficult to evaluate them without images of the bone surfaces themselves. Dave "Tyrannosaur Chronicles" Hone needs a shout out here for sharing his expertise and extensive image library of tyrant fossils as I prepared this post - though I have some experience with tyrannosaur bones and their interpretation, this article has been considerably improved by his involvement.)

Tyrannosaurids and crocodylians: face off

An obvious place to begin this discussion is crocodylian facial structure. Crocodylian skulls are so familiar that it's easy to forget how distinctive they are among modern animals, and I don't think it's widely known that their skin plays a significant role in shaping their skull tissues. Crocodylian jaw bones have incredibly high numbers of foramina, with averages of 100 in each major jaw bone (premaxilla, maxilla and dentary) and over 1000 in each bone in some specimens (Morhardt 2009). These openings are the loci around which gnarly ridges and tubercles grow by a process of dermal ossification: tissues from the skin are turned to bone and build up the sculpting on the skull surface (Grigg and Kirshner 2015; de Buffrénil et al. 2015). Simultaneously, the bone immediately surrounding the foramina is resorbed, enhancing the rugosity pattern further and creating that highly distinctive, deeply pitted and grooved crocodylian skull texture (de Buffrénil et al. 2015). This restructuring can be extensive and, over ontogeny, crocodylian snout surface area can increase by as much as 20% (de Buffrénil et al. 2015). That's a major reworking of the superficial bone of the skull, and their skin has a major role in its development.

Skull of a mature American crocodile, Crocodylus acutus, demonstrating that classic crocodylian skull texture. Cropped from public domain Wikimedia image by Daderot.

Among living tetrapods, only some turtles and a couple of geckos show a comparable degree of sharply-defined cranial sculpting (Evans 2008; de Buffrénil et al. 2015) but, among extinct taxa, stem-tetrapods, temnospondyls, parareptiles and many crocodylomorphs present analogous cranial conditions (Witzmann et al. 2010; de Buffrénil et al. 2015). Studies show that temnospondyl skulls developed their sculpting via a similar mechanism of ossifying dermal tissues (Witzmann et al. 2010), perhaps indicating croc-like skin properties in these animals, too. Until recently it was thought that crocodylian facial skin was scaly, but new research shows that it is actually a sheet of toughened skin which cracks through growth, creating a scaly appearance, but not true epidermal scales akin to those seen in lizards (Milinkovitch et al. 2013). Regardless of whatever other conclusions are drawn here, this has to be a minor amendment to Carr et al.'s (2017) interpretation: if tyrannosaurids (or any other extinct animal) have croc-like textures on their jaw bones, we should be visualising tight, tough skin, not epidermal scales.

Juvenile alligator, Alligator mississippiensis, showing virtually crack-free facial skin - it's only adults that develop the extensively cracked, superficially 'scaly' faces. Photo by Joxerra Aihartza, from Wikimedia, FAL 1.3.

Whether tyrannosaurid jaws are truly crocodylian-like is open to question, however. Carr et al, (2017) are clear that they consider tyrants and crocodylians jaws as identical in superficial appearance ("The texture in crocodylians is identical to that of tyrannosaurids, except that the entire face of crocodylians is coarse in texture" - p. 21; Supplementary information to Carr et al. 2017) but I disagree: there are a number of ways in which they differ and, given the link between crocodylian skull development and dermal tissues, these differences may be critical to our considerations of facial anatomy. Many of these contrasts pertain to jaw foramina, which we know are important in defining crocodylian cranial sculpting (de Buffrénil et al. 2015) and may have a deeper relationship with jaw tissue properties (Morhardt et al. 2009; Hieronymus et al. 2009).

Firstly, although tyrannosaurids have elevated numbers of jaw foramina compared to other dinosaurs, their numbers are, on average, significantly lower than those of crocodylians (Morhardt 2009). No tyrannosaurid jaw bone reported by Morhardt (2009) exceeds 81 foramina, which is high for a dinosaur, but still short of the crocodylian average, and well below the 1000+ figure reported for some croc jaws. Interestingly, data in Morhardt (2009) suggests that foramina numbers weakly correlate to jaw size: the longer a jaw is, the more foramina it generally has. This trend is particularly well shown in her tyrannosaurid sample but seems true of other fossil and extant animal groups as well, and might also be reflected in ontogeny (smaller Tyrannosaurus have fewer foramina, on average, than large ones). The cause behind this trend seems to be elusive at present - might it reflect a change in tissue type with age (Morhardt 2009)? does it reflect demands of supplying an absolutely larger jaw with nervous and vascular tissues? - but whatever the reason, it implies that we should consider foramina frequency proportionate to jaw size when analysing rugosity profiles. Under this metric, foramina values in crocodylian jaws are even more impressive as, compared to some extinct animals, their skulls are of middling size. By contrast, the slightly above-average foramina counts of even the largest tyrannosaurines seem less significant because, even with extreme jaw size, they don't attain a value comparable to a much smaller alligator. If we remove size from our consideration by comparing similarly-sized tyrant and croc jaws, we find they are worlds apart in terms of jaw perforation. Indeed, the foramina values of smaller tyrants are nothing special - they are comparable to most other similarly-sized tetrapods (Morhardt 2009). Presumably, this explains why - as many internet conversations have pointed out - tyrannosaurid jaws simply don't have that same obvious, pitted surface as those of crocodylians.

Further differences might be noted in relative foramina sizes. Those foramina occurring high on tyrant snouts - such as at the top of the maxilla - are much smaller than the broader, obviously deep labial foramina paralleling the jaws (Brochu 2003; Carr et al. 2017). In crocodylians however, jaw foramina seem to have a lower size range. Foramina shape and size is an important consideration for facial tissues (Hieronymus et al. 2009) and this might imply different tyrant facial tissues over the side of the snout vs. those at the jawline, whereas the more uniform foramina sizes of crocodylians are entirely consistent with their homogeneous jaw skin.

Schematic drawing of Tyrannosaurus skull FMNH PR2081 (the specimen better known as 'Sue') showing the distribution and (somewhat conservatively) size differences in jaw foramina. This huge skull is said to be one of the most rugose Tyrannosaurus skulls known (Brochu 2003), but it fails to meet the high foramina numbers, sculpting extent and uniform foramina size of mature crocs. Image from Brochu (2003).

A related issue concerns a possible link between extra-oral tissues and foramina counts. Morhardt (2009) noted that, as a general rule, extant animals with average foramina counts below 50 in each jaw bone have tooth-covering extra-oral tissues; that those above 50 but below 100 have immobile facial tissues; and only those with 100 or more are reliably excluded from having lips or other means of tooth coverage. Average tyrant jaw foramina counts are well below that upper threshold for exposed teeth so, by this metric, they should have lips, and would not look like bipedal crocodiles. This might match what we're seeing with tyrant foramina size: perhaps those large labial foramina are something to do with nourishing and innervating extra-oral tissues, while those on the side of the snout need only access the overlying skin. There are some complications to Morhardt's data (if anyone is looking for a PhD project, a more extensive follow up would be terrific) but, at face value, her research does not support crocodile-like facial tissues for tyrannosaurids.

Finally, we can observe that the ontogeny of tyrannosaurid skull textures is not at all crocodylian-like. Tyrants do have some sculpting on their jaw bones and, as with most reptiles, these become better defined with maturity (e.g. Evans 2008; de Buffrénil et al. 2015). However, even the most rugose tyrannosaurid skulls do not match the complex and sharply pitted rugosity patterns of mature crocodylians (e.g. Osborn 1912; Carr et al. 1999; 2017; Brochu 2003; Hone et al. 2011). Given that ossifying facial skin is a direct factor in jaw bone sculpting in crocodylians, the lack of comparable development in tyrannosaurids is a blow to the idea that their faces bore the same dermal regime. Histological examination of tyrannosaurid jaw bones for might have further insight here, as the resorption/remodelling pattern might reveal details about bone/dermal interactions (Witzmann et al. 2010; de Buffrénil et al. 2015) but, for now, this inconsistency seems to be a big hole in the idea that tyrannosaurids had crocodylian-grade facial tissues.

Does the tyrannosaurid EPB help here?

Collectively, these points seem to suggest that tyrant jaws are not as croc-like as argued, and that it's not a given that the two groups had similar facial tissues. A counterargument to this is that crocodylians are the best tyrant analogues in their extant phylogenetic bracket (EPB), and thus give us our best, most phylogenetically informed insight into tyrannosaurid faces. Indeed, the croc-snouted tyrant hypothesis was informed primarily by comparisons with taxa from the tyrannosaurid EPB - specifically the skulls of birds and alligators (Carr et al. 2017) and, sure, crocs and tyrannosaurid jaws may not be exactly alike, but they're undeniably more similar to each other than either is to a bird. Might we concede that the comparisons aren't perfect, but that this is simply the best we can do without violating the tyrannosaurid EPB?

Our issue is that, while the EPB is a terrific method for predicting ancient anatomies, it really struggles with the complexity of archosaur facial tissue evolution, perhaps to the extent of being redundant. One major issue is that we can be near certain early archosaurs had neither croc- or bird-like facial tissues because no species representing the earliest phases of archosaur evolution have comparable skin-influenced jaw textures (see Nesbitt et al. 2013, and papers therein). Rather, we only see these features developing in relatively crownward archosaur groups, implying independent development of their respective facial anatomies well after the croc-bird split. This being the case, the common archosaur ancestor must have had a different set of facial tissues, and the facial anatomy of extant archosaurs may tell us little about the faces of Mesozoic dinosaurs.

Like crocodylians, birds have jaws with surface textures shaped by their overlying skin: networks of branching neurovascular canals and oblique foramina underlie cornified sheaths (their beaks). The prominence of such jaw rugosities in living archosaurs allows us to predict the facial condition of fossil archosaurs and stress test the tyrannosaur EPB, and it doesn't seem to hold up well. This skull is a marabou stork (Leptoptilos crumenifer), photo by me.

A second major issue is evidence that living archosaur faces don't reflect tissues known from their fossil cousins. In addition to tight facial dermis and cornified sheaths, a plethora of fossil evidence show that fossil archosaurs had faces with epidermal scales, projecting skin tissues (e.g. pterosaur crests) armoured dermis, and cornified pads (Frey et al. 2003; Hieronymus 2009; Hieronymus et al. 2009; Carr et al. 2017). These go well beyond the anatomical range implied by the EPB and show that fossil archosaur faces sometimes had more in common with non-archosaurs than their closest extant relatives. We must remember that the EPB is a predictive method which should be applied where no other data is forthcoming: in this case, we have enough fossil data to show that our EPB predictions are problematic, and that we can't rely on it for insight into tyrannosaurid faces. I'm hardly the first to suggest EPB approaches don't help discussion of non-avian dinosaur faces (e.g. Vickaryous et al. 2001; Knoll 2008), but these points are worth repeating in this context: I don't think the EPB is a compelling supporting argument for a croc-faced tyrannosaurid.

So, if not croc-like, what might be happening here?

If croc-skinned tyrant snouts are problematic, what are our other options? Our discussion above really only pertains to the maxillary region of tyrannosaurid snouts and, for the rest of the skull, I think Carr et al. (2017) nailed it: what I've seen of tyrannosaurid skulls suggests the orbital region and skull roofs were covered in cornified sheaths, armoured dermis and large scales. There seems to be quite a bit of variation in these tissues, with some taxa having more defined scale correlates over the nasals than others, as well as differences in elaborations of the hornlets above the eye. In all likelihood, different tyrant species would be highly recognisable in life by the development of scales, armour and horn across the top of their faces. These armoured tissues are entirely consistent with what we understand of tyrannosaurid behaviour: if you were being routinely bitten about the face by another tyrannosaur, you'd want some protection too (see opening image).

Dorsal view of the snout of a red river hog (Potamochoerus porcus). These pitted, grooved bone textures are fairly widespread across tetrapod skulls and don't seem to correlate to any one skin type, but might indicate the presence of tough, well-cornified skin (these hogs wrestle with their faces, so need protected snouts). Note the projecting rugosities on the side of the snout and on the ascending maxillary projection - these anchor vast skin projections in life. Red river hog skulls are awesome. Photo by me.

But what of that maxillary portion of the snout - the lateral region suggested as being crocodile-like? The surfaces of tyrannosaurid maxillae are pretty complex with a hierarchy of rugosity profiles (Carr et al. 2017). Very obvious features include many pits and short, branching neurovascular grooves: these might not necessarily indicate of particular tissue type in themselves, but are often associated with a well-cornified, tough epidermis (above). The high number of foramina in tyrant maxillae implies immobile facial tissues (Morhardt 2009), which I guess we probably expected in a reptile anyway.

Holotype maxilla of Zhuchengtyrannus magnus: check out that network of elliptical depressions bordered by raised regions. Note how they terminate about a few centimetres above the line of labial foramina - we'll come back to this in a moment. From Hone et al. (2011).

Underlying these pits and grooves are a series of large, elliptical shallow depressions surrounded by low ridges (above). These vertically-aligned structures are found in many tryannosaurids and are especially obvious in large tyrannosaurines like Tyrannosaurus, Tarbosaurus and Zhuchengtyrannus. You can see them easily in museum mounts, even from across the room. Some taxa have single rows of these structures below the antorbital fenestra (Tyrannosaurus), but others have tessellating networks of depressions and ridges that extend to the top of the maxilla (Tarbosaurus), terminating beneath the scaly region overlying the nasal bone. They're unusual structures which are almost certainly epidermal in origin: they're in a place where epidermal correlates often form; are more pronounced in mature individuals; are regularly and consistently arranged across the surface of the skull; and are not associated with any pneumatic or neurovascular openings. They broadly recall the 'hummocky' rugosity profile seen under epidermal scales (Hieronymus et al. 2009) and, if so, the convex, ridged areas probably underlay vertically aligned scales, or rows of scales. Some tyrant skulls, such as the especially rugose Tyrannosaurus skull AMNH 5027, have especially sharp and rugose ridges which, to me, recall the facial ridges of certain iguanine lizards: specifically, anoles, chameleons and basilisks. These are often quite rugose and sculpted, but smoother, more tyrannosaurid-like conditions exist in a number of species (I'm thinking of things like helmeted basilisks and smooth chameleons). Prominent, ornamental rows of relatively large and often colourful scales overly these structures in these iguanines and I wonder if the same was true for tyrants. Alternative hypotheses, such as scales sitting in the depressions between the ridges, aren't consistent with the relationship between scales and bone in living species, and there's no indication that other tissue types (e.g. cornified sheaths, armoured dermis) were present in these areas, so I think the ornamental ridge hypothesis is sensible (or, at least, not outrageously daft given the available data). I must admit to liking this hypothetical juxtaposition of fancy ornamental scales around the mouth and tough, reinforced tissues over the snout: perhaps tyrannosaurs weren't just big biting machines, but liked to look nice, too.

AMNH 5027 is just riddled with interesting surface textures that probably relate to epidermal features. To my reckoning, in addition to those depression/ridge pairings on the maxilla, the dorsal region of the lacrimal bears coarse projecting rugosity (armoured dermis); the top of the premaxilla and postorbital has a series of coarse hummocks (probable scales); and the ascending processes of the postorbital, lacrimal and maxilla are covered in a dense, anastomising network of neurovascular foramina (cornified sheath). What a neat looking animal Tyrannosaurus must've been - my take on this data is seen in the paintings accompanying this post. Image from Osborn (1912), in public domain.

Significantly, I can't find any tyrannosaurid skulls where these possible scale correlates extend right to the base of the maxilla (see photos, above). Rather, they terminate a few centimetres above the line of labial foramina, and this might have bearing on ongoing discussions about dinosaur lips. Scleroglossan lizards (the group that includes geckos, skinks, varanoids and amphisbaenians) frequently have osteoderms on their faces which cover their snouts (including the maxillae) except for a region around the labial foramina, which is smooth. This seems to relate to the presence of lip tissues displacing the scales from the skull and prohibiting formation of a epidermal correlate adjacent to the toothrow. Their maxillary juxtaposition of epidermal correlates is the same configuration that we see in tyrannosaurids as well as a number of other non-avian dinosaurs with maxillary epidermal correlates (e,g, pachycephalosaurids, ankylosaurids, some ceratopsids) and this has to be regarded consistent with hypotheses of extra-oral 'lips' in tyrannosaurids and other dinosaurs. If we add this to the evidence from foramina counts (Morhardt 2009, also see above) as well as other arguments for extra-oral tissues the case for crocodylian-like exposed teeth is looking increasingly doubtful. I must admit to thinking that proponents of exposed dinosaur teeth really need to start making better cases for this idea: most ways we can slice this particular debate suggests that extra-oral tissues are looking likely (and no, the common argument that their teeth were too big to be sheathed isn't valid: it's simply a speculation based on incredulity, not actual data from dinosaur skulls).

So...

To sum up this long, detail-heavy post:

1. Crocodylian skull textures are basically built by their skin, and we should expect any prehistoric animal with croc-like facial tissues to have a croc-like cranial rugosity profile. What we see in tyrannosaurs is a little croc-like, but only superficially. Differences between croc and tyrant skull tissues may be more significant than their similarities and seem to contradict the notion of croc-like facial tissues in tyrannosaurids.
2. Attempts to ground discussions of dinosaur facial tissue in the EPB are problematic: a great deal of what we know about archosaur facial tissue refutes what the EPB predicts. Basic comparative anatomy, framed by a wide phylogenetic bracket, might be the way forward for understanding dinosaur faces.
3. Tyrant faces - as largely predicted by Carr et al. (2017) - seem to have been adorned with scales, cornified sheaths and armoured dermis, but their jaw regions may have been covered in vertical (perhaps ornamental?) bands of epidermal scales, not croc-like skin. Distribution of epidermal correlates around the jaws of tyrannosaurids (and other dinosaurs) is suspiciously reminiscent of many lizard skulls, and may favour a lipped condition.

Tyrannosaurus rex portrait, based on my take of epidermal correlates of the AMNH 5027 skull. No, you tell it that its ornamental ridges look a bit silly.

Perhaps unsurprisingly, I couldn't research and write all this without wanting to draw my take on tyrannosaur facial anatomy. I'll leave you with my take on the face of AMNH 5027 (above): I'm sure it'll need modifications as more details on tyrannosaurid faces come to light, but I won't pretend it wasn't neat to draw a Tyrannosaurus based on relatively objective reading of available data. Palaeoart is at it's most exciting when we join dots between data rather than, as is so often the case, largely imagine huge swathes of our subject species. The duelling Tyrannosaurus that welcomed you to the post are based on the same model.

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References

• Brochu, C. A. (2003). Osteology of Tyrannosaurus rex: insights from a nearly complete skeleton and high-resolution computed tomographic analysis of the skull. Journal of Vertebrate Paleontology, 22, 1-138.
• Carr, T. D. (1999). Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria). Journal of vertebrate Paleontology, 19(3), 497-520.
• Carr, T. D., Varricchio, D. J., Sedlmayr, J. C., Roberts, E. M., & Moore, J. R. (2017). A new tyrannosaur with evidence for anagenesis and crocodile-like facial sensory system. Scientific reports, 7, 44942.
• De Buffrénil, V., Clarac, F., Fau, M., Martin, S., Martin, B., Pellé, E., & Laurin, M. (2015). Differentiation and growth of bone ornamentation in vertebrates: a comparative histological study among the Crocodylomorpha. Journal of morphology, 276(4), 425-445.
• Evans, S. E. (2008). The skull of lizards and tuatara. Biology of the Reptilia, 20, 1-347.
• Grigg, G. (2015). Biology and evolution of crocodylians. Csiro Publishing.
• Hieronymus, T. L. (2009). Osteological correlates of cephalic skin structures in amniota: Documenting the evolution of display and feeding structures with fossil data. Ohio University.
• Hieronymus, T. L., Witmer, L. M., Tanke, D. H., & Currie, P. J. (2009). The facial integument of centrosaurine ceratopsids: morphological and histological correlates of novel skin structures. The Anatomical Record, 292(9), 1370-1396.
• Hone, D. W., Wang, K., Sullivan, C., Zhao, X., Chen, S., Li, D., ... & Xu, X. (2011). A new, large tyrannosaurine theropod from the Upper Cretaceous of China. Cretaceous Research, 32(4), 495-503.
• Frey, E., Tischlinger, H., Buchy, M. C., & Martill, D. M. (2003). New specimens of Pterosauria (Reptilia) with soft parts with implications for pterosaurian anatomy and locomotion. Geological Society, London, Special Publications, 217(1), 233-266.
• Knoll, F. (2008). Buccal soft anatomy in Lesothosaurus (Dinosauria: Ornithischia). Neues Jahrbuch für Geologie und Paläontologie-Abhandlungen, 248(3), 355-364.
• Milinkovitch, M. C., Manukyan, L., Debry, A., Di-Poï, N., Martin, S., Singh, D., ... & Zwicker, M. (2013). Crocodile head scales are not developmental units but emerge from physical cracking. Science, 339(6115), 78-81.
• Morhardt, A. C. (2009). Dinosaur smiles: Do the texture and morphology of the premaxilla, maxilla, and dentary bones of sauropsids provide osteological correlates for inferring extra-oral structures reliably in dinosaurs?. Western Illinois University.
• Nesbitt, S. J., Desojo, J. B., & Irmis, R. B. (2013). Anatomy, phylogeny and palaeobiology of early archosaurs and their kin. Geological Society, London, Special Publications, 379(1).
• Osborn, H. F. (1912). Crania of Tyrannosaurus and Allosaurus; Integument of the iguanodont dinosaur Trachodon. Memoirs of the AMNH; new ser., v. 1, pt. 1-2.
• Vickaryous, M. K., A. P. Russell, and P. J. Currie. (2001). Cranial ornamentation of ankylosaurs (Ornithischia: Thyreophora): reappraisal of developmental hypotheses. In K. Carpenter (ed). The Armored Dinosaurs. 318–340. Indiana University Press.
• Witzmann, F., Scholz, H., Mueller, J., & Kardjilov, N. (2010). Sculpture and vascularization of dermal bones, and the implications for the physiology of basal tetrapods. Zoological Journal of the Linnean Society, 160(2), 302-340.

Revenge of the scaly Tyrannosaurus

Reworked version of my 2012 Tyrannosaurus painting, now in it's third guise. There's something about this painting which recalls reconstructions from 1906 rather than those of 2016.

The skeletal anatomy of Tyrannosaurus rex is probably better known and studied than the skeletons of many living animals, but its soft-tissues - and thus much about its life appearance - are poorly represented by fossil remains. Thus, virtually all of our ideas about muscle bulk, soft-tissue body shape and integument have to be reconstructed by phylogenetic proxy and functional prediction. As with all dinosaurs, we've historically felt pretty confident that Tyrannosaurus was entirely scaly, but relatively recent discoveries of filamented tyrannosauroids in China (Xu et al. 2004, 2012), as well as a growing mountain of fuzzy coelurosaur fossils, point to a different conclusion: that Tyrannosaurus was adorned in simple filaments - hair-like equivalents of feathers. Skin impressions for more derived tyrant species - the tyrannosaurids - have proven rare in fossil record (Hone 2016) and, though rumours have circulated about some, they have largely escaped formal description and publication. In the absence of better evidence, the most parsimonious modern takes on everyone's favourite tyrant have involved a fuzzy covering.

In the recent months two papers have challenged this idea. The first, by Thomas Carr and colleagues (2017), purports to find osteological correlates of scales on the facial anatomy of the tyrannosaurid Daspleteosaurus, which they argue (along with other lines of evidence), to suggest crocodylian-like facial tissues and sensitivity. The second, by Phil Bell et al. (2017), describes scaly skin impressions from multiple postcranial regions of a Tyrannosaurus skeleton, and argues that the distribution of these impressions implies a uniform (or near uniform) covering of scales across the body, without much in the way of fuzz.

Because this is Tyrannosaurus, media sites and bloggers have spilled great amounts of ink over these stories. The scientific press has often been far from objective or unbiased. Popular articles have suggested Jurassic World fans might have 'won' the debate over scientists, that science fans are 'due' a return to scaly tyrants after 'losing' Pluto, and that the findings mean 'all is well in the dinosaur world'. The implication is a ridiculous one, like evidence of scalier tyrants is a moral victory rather than a test of a scientific hypothesis. But while the popular press has been celebrating the new papers, members of the palaeoblogosphere have been less enamoured with the findings. Trey the Explainer suggests that Bell et al.'s work doesn't really change what we already knew about tyrant integument, and thus does not invalidate many existing reconstructions. Andrea Cau posits that interpretations of scaly tyrants reflect our prejudices more than science, and that taphonomic factors may explain the absence of filaments. Brian Switek has concerns that the skin patches are too small and spread too widely to give a complete picture of the integument, and echoes concerns about taphonomic interference. The collective response seems to be a defensive one, protecting concepts of filamented tyrannosaurids from a resurgence of a more traditional, scaly model. Would any other dinosaur get this treatment? Perhaps not: as Brian explains in his recent post, this reaction is the T. rex celebrity effect at full bore.

Supermegafluffy Tyrannosaurus, from 2015. They were simpler times.

I've painted many fluffy Tyrannosaurus in the last few years (above) and quite like the idea of everyone's favourite 6 tonne dinosaur bonecrusher being a giant plush toy. However, we also have to concede that our ideas of Tyrannosaurus skin have been largely informed by prediction, not direct data, and that popular, long-held notions are as ripe for scientific revision as any other (lest we forget other famous examples of this - Brontosaurus and Ornithoscelida). Moreover, although some critics are suggesting the papers don't tell us anything new - rumours of scale impressions have been circulating for years - these recent studies give us the first rigorously documented, peer-reviewed glimpse into Tyrannosaurus skin anatomy. This is new, allowing us to form our own opinions on Tyrannosaurus appearance based on actual data, not hearsay. So, rather than putting our gloves up to defend our prior model, I wonder if we should be exploring how this new data might transform our perception of Tyrannosaurus life appearance. That these new studies present conflicting data to our expectations is not grounds to be upset, annoyed or defensive. To the contrary, they allow us to use real data - not predictions - to refine our ideas of tyrannosaurid appearance and evolution. For those of us interested in dinosaurs as real entities, and not movie monsters, that's a good thing.

What, exactly, has been argued about scaly tyrants?

A lot of the popular write ups of these recent papers include errors and misrepresentation, so let's recap what is actually being argued about Tyrannosaurus skin. A common social media reaction to Bell et al.'s work is that they've presented 'a patch' of skin, and are extrapolating from that. We need to debunk that right away: they've not described a single patch, but multiple small patches from the neck (alas, exactly where on the neck isn't reported), the top of the pelvis, and the base of the tail (below). All the samples stem from the 'Wyrex' specimen (HMNS 2006.1743.01). The most extensively represented area is the tail base, which has the largest single piece of fossil skin - 30 cm². The other skin samples are not as large, some being just a few centimetres across. Each patch shows the same skin type: uniform, tiny 'basement scales', each less than 1 mm across (Take note, artists: you would not see Tyrannosaurus scales until you were being eaten by their owner). Similar scale patches, also described by Bell et al. (2017), have been found on the torso and tail regions of other tyrannosaurid species, implying similarly scaled regions in these taxa.

Tyrannosaurus skin patches from the neck, pelvic area and tail of the 'Wyrex' specimen as illustrated by Bell et al. (2017). The scale bars for the scale imagery are 5 mm (b - e) and 10 mm (f-h). These things are tiny, and we can assume the skin of the animal would look smooth or leathery in life.

Some folks are suggesting that the size of these skin patches allows us to dismiss their scaly signal, or that even that they're anomalous, reflecting unusual taphonomic conditions that cloud their significance. I'm unsure about these ideas. Most skin impressions are small patches (even scaly skin gets a rough ride during fossilisation) and the fact they're small doesn't diminish the fact that each records a cluster of scales. We have to assume these are not unusual or 'special' areas on the body but generally indicative of surrounding skin fabrics. The fact that each patch is consistent with regard to scale size and texture hints at them being part of a continuous, unbroken integument, and not isolated scaly pockets in a sea of fluff.

But what about arguments that the scale patches are tissues stripped of filaments before preservation, like so many 'monster' carcasses? Filament/scale combos do have precedent in dinosaurs, being present on the tail of Juravenator and those scales of Kulindadromeus with fibre-like tassels (Chiappe and Göhlich 2010; Godefroit et al. 2014). We know from modern animals that fibrous epidermal structures are especially vulnerable to decay and physical weathering, but is there evidence that this has taken place on the Wyrex Tyannosaurus skin patches? At present, it's hard to say because we have no idea what tyrannosaur skin looks like as it decays. It might be significant, however, that the scale patches look very similar across the Wyrex specimen, and that they resemble other tyrannosaurid skin impressions closely. We might expect some variation if taphonomy was really distorting these specimens in a major way, and we're not seeing that. Moreover, the Wyrex skin impressions, though small, are pretty high-resolution. The scales, and their intervening areas, have sub-millimetre proportions and sharply defined edges. There's no tatty scale margins, no obvious spaces for filament attachment, or linear structures crossing the scales to imply a rogue filament impression. We'll remain uncertain if these are anomalous, taphonomically-altered samples until we find other examples of tyrannosaurid skin, but there's no reason to be unduly suspicious of the the samples we have.

Of course, the adage that 'absence of evidence is not evidence of absence' is always important when dealing with the fossil record, and it applies here as a sensible caveat. However, we shouldn't wield this phrase as a definitive counter-argument to reasonable interpretations of available evidence. Palaeontologists have to work with data, not suspicions or gut feelings, and the data we have does not include, or hint at, the presence of filaments. I'm not arguing that taphonomy isn't worthy of consideration here (indeed, the omission of details about 'Wyrex' taphonomic history is an issue with the Bell et al. 2017 paper) but we must beware the logical fallacies of appealing to probability (i.e. taphonomy could explain the lack of filaments, so it does explain the lack of filaments) or special pleading (excluding Tyrannosaurus from the same logic we would apply to other fossil animals when presented with this data).

Tyrannosaurus skull AMNH 5027 - note the 'hummocky' textures on the side of the snout, above and below the orbit, and atop the rostrum, likely indications of scaly skin. Image in public domain, sourced from Wikipedia.

Carr et al. (2017) present a different form of evidence for scales: osteological correlates. I consider some aspects of their study problematic in that it only looks to crocodylians and birds for comparative tissues, despite the clear value other tetrapods have in deducing facial tissue types (Knoll 2008; Morhardt 2009; Hieronymus et al. 2010); it lacks illustrations of the bone textures correlated to scaly integuments; and the conclusion of tyrants bearing crocodile-like face scales is flawed: crocodylians do not have face scales, but a tight, highly cracked sheet of facial skin - Milinkovitch et al. (2013). Nonetheless, I think Carr et al. (2017) are right in concluding the bony textures of tyrannosaur skulls seem indicative of scaly skin. These findings echo previous interpretations of bosses and rugosities in tyrant skulls (e.g. Brusatte et al. 2012; Sullivan and Xu 2016) and aren't controversial. Scales closely associated with bone either leave a 'hummocky' surface texture, which is seen on tyrant snouts (specifically their maxillae and nasals) or small bosses and hornlets, which are found in all tyrannosaurid skulls above their orbits (lacrimal and postorbital bones) and on their 'cheeks' (jugal bones). Hornlets and bosses represent the locations of specific scales in living reptiles (Hieronymus et al. 2009) and can thus give especially good indications of life appearance (check out chameleon skulls for especially good correlation between skull and scale features). The presence of hummocky bone textures and hornlets is a strong correlate for scales, as they rule out coverings of naked or feathered skin. Such skin types do not alter the underlying bone surface (Hieronymus et al. 2009).

These osteological correlates combine with the skin impressions to collectively show Tyrannosaurus as scaly across much of its face, somewhere on its neck, over the pelvic region and along the tail base (below). So far as we can tell, this picture seems consistent with osteological correlates and skin sampling from wider Tyrannosauridae. That's pretty extensive coverage, ruling out the presence of fibres in places that we know other dinosaurs - including other tyrannosauroids - were fuzzy, and implies that tyrannosaurids were mostly scaly. I'm particularly startled at the scales over the hip region as they curb even the long 'fibre capes' we see in some modern tyrant reconstructions, like the famous Saurian Tyrannosaurus. The fact that the scales occur in places known to be ancestrally filamented for tyrants is also intriguing: Bell et al. (2017) speculate that they may be modified feathers - that is, the same as bird scales - rather than a reversion to lizard or croc scales. Hold that thought, we'll come back to it soon.

Everyone's doing maps of Tyrannosaurus with integument details nowadays, and I want in. Note that this is Tyrannosaurus specific, and does not feature scale data from other tryannosaurids.

What's in the gaps?

The million dollar question is what was present between these scaly regions: more scales, or fibres? This is a major point for many respondents to the Carr et al. and Bell et al. papers, as it decides whether we keep our interpretation of Tyrannosaurus as an - at least partly - fuzzy animal. With our scale distribution map as a starting point, several options are available. The first is that fuzz was present in regions not yet represented by skin remains or osteological correlates. This would mostly imply the top of the torso (Bell et al. 2017), but may also be parts of the back of the head, some aspects of the neck (depending on where the neck skin impression came from) and maybe the end of the tail. Over on Twitter, Patrick Murphy has presented a reconstruction which shows what this might look like. I must admit to finding it quite amusing, sort of like T. rex has put on a shawl to visit the opera.

But how dense could these fuzzy patches have been? Bell et al. (2017) suggest that dense fibrous coverings are doubtful, noting that large living mammals avoid patches of thick insulating fibres to aid heat loss. This has not gone down well with some critics, who cite studies of feathers preventing over-heating instead of facilitating it. An oft-cited study in this regard is Dawson and Maloney (2004), who found emu feathers block virtually all solar radiation from the skin, preventing them from overheating in solar exposure that causes similarly-sized hairy mammals to seek shelter.

Feathers: great at blocking solar radiation, also great at trapping body heat. Note how cooking hot these ostriches are on their necks, heads and legs, while the feathers are mostly ambient temperature. This isn't because the body isn't warm, but because the feathers block the heat signature entirely, trapping all that heat around the body. As surface area:volume ratios drop as animals get larger, it stands to reason that the benefits of blocking solar radiation give way to a need shed heat. Image from Wikipedia user Arno / Coen, CC BY-SA 3.0.

Feathers, however, are not magic structures that defy fundamental physical laws of insulation, nor do they liberate animals from the challenges of heat loss at reducing surface area:volume ratios. Beyond a certain size, shedding excess body heat is difficult for any terrestrial animal, and it gets tougher as they get larger. King and Farner (1961, p. 249) described feathers as having "an extremely high insulating value to the feathered surfaces" and a rich literature of studies on modern birds shows that feathers are as effective at trapping body heat as they are blocking solar rays (e.g. King and Farner 1961; Kahl 1963; Philips and Sandborn 1994; Dove et al. 2007). We can almost see them as a little too effective, leading many birds to develop heat-dumping adaptations to circumvent their own insulation, such as highly vascularised, non-feathery body parts as well as a repertoire of postures and behaviours (maximising exposure of unfeathered body parts; flapping wings; urinating on their legs) that aid cooling (e.g. Kahl 1963; Arad et al. 1989; Philips and Sandborn 1994). So yes, feathers are terrific at protecting birds from environmental heat, but that limits their ability to release metabolic heat from their own bodies.

If living birds find feathers a little warm, despite their relatively high surface area to volume ratios, we have to assume a theropod weighing anywhere between 6-14 tonnes is going to find big areas of dense filaments a challenge to thermoregulation too. It is not unreasonable to assume blankets of fibres could be a problem for big tyrants. The counterargument here is that Yutyrannus huali, a largish tyrannosauroid, does have dense fibres everywhere. But Yutyrannnus seems more lithe than Tyrannosaurus - perhaps just 10-25% of its mass, depending on the estimates (Bell et al. 2017) - and lived in a more vegetated, and thus shadier, habitat (Bell et al. 2017). A neat comparison Bell et al. (2017) make along this line uses living rhinos, where hairier species live in shadier settings than the virtually naked ones. In light of this, the reduction of filamented regions, and perhaps lessening their density, is a reasonable inference for animals of the size and habitat of Tyrannosaurus, and would reflect thermoregulatory responses to scaling and shade availability seen in living animals.

Large tyrannosauroids, like Yutyrannus huali, show that dinosaurs weighing perhaps 1.5 tonnes could be covered in feathers. But does this reflect the fact that this animal lived in shadier, vegetated habitats than the tyrannosaurids? This idea isn't silly: adaptation to specific circumstances has a major role to play in shaping animal skin anatomy, and could well explain why some tyrants are fuzzy, and others seem less so. (If you want to see the rest of this picture, check out this Patreon post)

Could Tyrannosaurus have had extremely fine, widely-distributed filaments - perhaps similar to something like elephant hair? This isn't entirely falsified by the new data, although the skin impressions we have show no evidence of such a covering despite preserving tiny integument details. Granted, animal filaments can be extremely fine, and they might be beyond the preservation potential and mechanics of even these high-res impressions. However, if we're arguing for filaments of this size and patchiness then - certainly for artistic purposes - we should concede that the animal would be essentially scaly, in the same way that most rhinos, elephants and hippos are essentially naked (below). From a thermoregulatory perspective, short, sparse filaments could make sense as these have the surprising ability to draw heat from the body in modern elephants, helping them stay cool (Myhrvold et al. 2012). Given the potential for overheating under dense filament coats in giant animals (Bell et al. 2017), I see this as more plausible than a 'cloak' of fibres between our scaly waypoints.

Scaly, minimally-filamented Tyrannosaurus. There's some tufts on the neck, but that's it. Is this model more consistent with the thermoregulatory requirements of a 6-14 tonne animal?

A last interpretation of this new data is that Tyrannosaurus was actually just scaly, with no fibres whatsoever. This is the most contested suggestion made by Bell et al. (2017), but it's not unreasonable with our current knowledge. Existing skin data, representing seven parts of the body if you pool all the distinct skull correlates and postcranial points (add several more if you want to extrapolate scale patches from other tyrants), shows enough scales and consistency in the scalation pattern that uniform scale coverage is not a ridiculous or indefensible concept. I appreciate that some folks will point to regional fuzziness of animals like Kulindadromeus in response, and its sharply defined areas of different integument types, and that's valid point. But we can also point to plenty of dinosaurs with extensive or entirely scaly hides and - if there's any value to linking body size and thermoregulatory regimes - they're a better match to Tyrannosaurus body mass than any known fuzzy species. For the time being, wholly scale models fit our existing data just as reasonably as partly fuzzy ones so, archaic and counter-intuitive as it seems - a scaly Tyrannosaurus is not an unreasonable interpretation for the life appearance of this animal, given our current data.

Beyond Tyrannosaurus: 'unlocking' dinosaur skin constraints

My take-home from these new papers is that our models of Tyrannosaurus skin have not crystallised, but we're a little more constrained in how we can imagine this animal, and have to concede a scalier appearance than many of us thought likely. But the implications of the Bell et al. study go beyond Tyrannosaurus in implying new ways to think about dinosaur skin evolution. With incontrovertibly fuzzy animals lining much of the the tyrannosauroid tree and its root, our scalier Tyrannosaurus gives us one of the best examples of a dinosaur replacing fuzz with scales. This is a far-reaching conclusion for those of us interested in dinosaur life appearance, complicating the already confusing evolutionary pattern of scale and fuzz distribution within the group. Ideas that some dinosaurs could be 'secondarily scaled' are supported by this discovery, and we have to wonder if classically fuzzy lineages - including many other theropod lines - are as tightly locked into fuzz, fibres and feathers as we once thought. Could large dromaeosaurs be a little lighter on fuzz than we imagine? Did Therizinosaurus look less like a giant pigeon and more like a walking Christmas dinner? We don't know, but now have reason to wonder.

Fluffy Tyrannosaurus juveniles, one of the possibilities created by the idea that tyrannosaurs might have avian-like 'dynamic' skin. The recovery of scales in non-scaly clades is not as simple as it might first appear!

Furthermore, the notion that Tyrannosaurus scales could be modified feathers (Bell et al. 2017) opens possibilities about mixes of filaments and scales. It's important to realise that not all scales are alike: 'reptile'' scales' are developmentally and genetically distinct from those we see in birds, which are actually secondarily modified feathers (Chang et al. 2000; Dhouailly 2009). Reptilian skin cannot be forced to grow feathers or filaments (Chang et al. 2000) and is developmentally static: once scales are formed, they're with them for life. Bird skin, however, is far more dynamic, and allows for all manner of ontogenetic and even seasonal variation in scale:feather ratios, changes to feather types, and modification of scale size (Lennerstedt 1975; Stettenheim 2000). If, as suspected, our tyrannosaurid skin samples represent fibrous integument masquerading as a scaly one, is this a sign of a bird-like 'unlocked' skin configuration where epidermal dynamism was possible? If so, Tyannosaurus could have changed appearance considerably with age (fluffy when small, scaly when big - above) or season (reflecting changes in climate or behaviour)? It must be stressed that we don't have any direct insight into these sorts of changes at the moment, and the hypothesis of tyrannosaurid scales being modified feathers needs testing. But the irony - we might have data indicating Tyrannosaurus could change its appearance readily, vindicating debaters on both sides of the scaly and fuzzy debate - is not lost on me. Maybe, just this once, everyone wins?

Summing up time

Let's tie this all together. A lot of ambiguity remains about the skin of Tyrannosaurus and its relatives, and it's not wise to hold any opinion about their life appearance too strongly at present. However, unduly downplaying the creep of scaly evidence into the tyrannosaurid fossil record isn't useful or logical. The skull skin correlates and fossil skin patches show that scales were present in numerous, widely-distributed parts of the body, and - until we see evidence to the contrary - this is good reason to assume scalier Tyrannosaurus than we might be used to. And yes, this does mean that some of our favourite, fluffier interpretations are now directly contradicted by fossil data, and consigned to our ever growing book of historic, discredited reconstructions. But this is always a possibility in palaeontology: our views of these animals are only ever hypotheses based on a sparse, biased fossil record, and every new discovery risks overturning someone's favourite concept. The fact we're able to move on from these reconstructions is positive, as it means we're a little less uncertain about the past, and a little closer to the truth.

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References

• Bell, P. R., Campione, N. E., Persons, W. S., Currie, P. J., Larson, P. L., Tanke, D. H., & Bakker, R. T. (2017). Tyrannosauroid integument reveals conflicting patterns of gigantism and feather evolution. Biology Letters, 13(6), 20170092.
• Brusatte, S. L., Carr, T. D., & Norell, M. A. (2012). The osteology of Alioramus, a gracile and long-snouted tyrannosaurid (Dinosauria: Theropoda) from the Late Cretaceous of Mongolia.
• Carr, T. D., Varricchio, D. J., Sedlmayr, J. C., Roberts, E. M., & Moore, J. R. (2017). A new tyrannosaur with evidence for anagenesis and crocodile-like facial sensory system. Scientific Reports, 7.
• Chang, C., Wu, P., Baker, R. E., Maini, P. K., Alibardi, L., & Chuong, C. M. (2009). Reptile scale paradigm: Evo-Devo, pattern formation and regeneration. The International journal of developmental biology, 53(5-6), 813.
• Chiappe, L. M., & Göhlich, U. B. (2010). Anatomy of Juravenator starki (Theropoda: Coelurosauria) from the Late Jurassic of Germany. Neues Jahrbuch für Geologie und Paläontologie-Abhandlungen, 258(3), 257-296.
• Dawson, T. J., & Maloney, S. K. (2004). Fur versus feathers: the different roles of red kangaroo fur and emu feathers in thermoregulation in the Australian arid zone. Australian Mammalogy, 26(2), 145-151.
• Dhouailly, D. (2009). A new scenario for the evolutionary origin of hair, feather, and avian scales. Journal of anatomy, 214(4), 587-606.
• Dove, C. J., Rijke, A. M., Wang, X., & Andrews, L. S. (2007). Infrared analysis of contour feathers: the conservation of body heat radiation in birds. Journal of Thermal Biology, 32(1), 42-46.
• Godefroit, P., Sinitsa, S. M., Dhouailly, D., Bolotsky, Y. L., Sizov, A. V., McNamara, M. E., ... & Spagna, P. (2014). A Jurassic ornithischian dinosaur from Siberia with both feathers and scales. Science, 345(6195), 451-455.
• Hieronymus, T. L., Witmer, L. M., Tanke, D. H., & Currie, P. J. (2009). The facial integument of centrosaurine ceratopsids: morphological and histological correlates of novel skin structures. The Anatomical Record, 292(9), 1370-1396.
• Hone, D. (2016). The Tyrannosaur Chronicles: The Biology of the Tyrant Dinosaurs. Bloomsbury Publishing.
• Kahl Jr, M. P. (1963). Thermoregulation in the wood stork, with special reference to the role of the legs. Physiological Zoology, 36(2), 141-151.
• King, J. R., & Farner, D. S. (1961). Energy metabolism, thermoregulation and body temperature. Biology and comparative physiology of birds, 2, 215-288.
• Knoll, F. (2008). Buccal soft anatomy in Lesothosaurus (Dinosauria: Ornithischia). Neues Jahrbuch für Geologie und Paläontologie-Abhandlungen, 248(3), 355-364.
• Lennerstedt, I. (1975). Seasonal variation in foot papillae of wood pigeon, pheasant and house sparrow. Comparative Biochemistry and Physiology Part A: Physiology, 51(3), 511-520.
• Milinkovitch, M. C., Manukyan, L., Debry, A., Di-Poï, N., Martin, S., Singh, D., ... & Zwicker, M. (2013). Crocodile head scales are not developmental units but emerge from physical cracking. Science, 339(6115), 78-81.
• Morhardt, A. C. (2009). Dinosaur smiles: Do the texture and morphology of the premaxilla, maxilla, and dentary bones of sauropsids provide osteological correlates for inferring extra-oral structures reliably in dinosaurs? (Doctoral dissertation, Western Illinois University).
• Myhrvold, C. L., Stone, H. A., & Bou-Zeid, E. (2012). What is the use of elephant hair?. PloS one, 7(10), e47018.
• Phillips, P. K., & Sanborn, A. F. (1994). An infrared, thermographic study of surface temperature in three ratites: ostrich, emu and double-wattled cassowary. Journal of Thermal Biology, 19(6), 423-430.
• Stettenheim, P. R. (2000). The Integumentary Morphology of Modern Birds—An Overview 1. American Zoologist, 40(4), 461-477.
• Sullivan, C., & Xu, X. (2017). Morphological diversity and evolution of the jugal in dinosaurs. The Anatomical Record, 300(1), 30-48.
• Xu, X., Norell, M. A., Kuang, X., Wang, X., Zhao, Q., & Jia, C. (2004). Basal tyrannosauroids from China and evidence for protofeathers in tyrannosauroids. Nature, 431(7009), 680-684.
• Xu, X., Wang, K., Zhang, K., Ma, Q., Xing, L., Sullivan, C., ... & Wang, S. (2012). A gigantic feathered dinosaur from the Lower Cretaceous of China. Nature, 484(7392), 92-95.

A year of Tyrannosaurus rex artworks

A minor milestone was reached this week at my print store - there's now 50 different bits of art in there. Given that I only started selling prints less than a year ago, I'm happy to see some substantial growth in my catalogue already (albeit with some cheating - many are 'reworked' older pieces, rather than entirely new bits). Lots more will be available in the near future - I'm holding several bits back for various reasons, including a project I'll elaborate more on soon. Working on these in relative secret is why things have been a bit quiet around her for the last month.

Teasers of unreleased artwork: Troodon, Repenomamus, diminutive azhdarchid and Diplodocus. We'll revisit the reason for holding these back in due time.

Scanning through my shop revealed an unexpected bias in my output this year. I make an effort to portray varying subjects and taxa, and find most interest in reconstructing lesser depicted species, scenarios and behaviour. I don't think I do too badly with this - at least within the context of Mesozoic reptiles - so was surprised to find 5 images dedicated to the same species, and one which has been painted, sculpted, animated and rendered to death: Tyrannosaurus rex. Two of these were commissions, but that still leaves three on my own head. I'm forced to concede that I must be a closet Tyrannosaurus fan - I had no idea.

I thought it would be fun to show the last year's worth of king tyrant art: some of them may still be fresh in your memory, but two are new (well, reworked). I realise that I've almost got a growth series across these images, and I've ordered them according to this. As usual, you can grab high quality art prints of these from my store.

Tyrant dinosaurs vs. bees. Bees are winning. Click here for prints.

First up is my tyrants and bees, the image I created to raise money for various bee charity causes in February of this year. Auctioning a framed version and sales of prints raised £249 for the Bumblebee Conservation Trust and a £30 contribution for a new beehive at the Cumberland House Natural History Museum, who also received the image for use on a display board. As you may remember, it shows two infant tyrants checking out a honey bee nest, molecular data indicating that honey bee ancestors were alive in the Late Cretaceous. My favourite bit of the image remains the smaller animal on the right, losing the battle with tiny arthropods. I like the fact its arms aren't really long enough to cover its eyes.

Resting rexes, and bonus moths. Click here for prints.

Next is Chidumebi Browne's reclined teenage Tyrannosaurus commission, from November 2014. These animals are heavily based on BMRP 2002.4.1, the probable half-size Tyrannosaurus with proportions and facial structure quite different to large adults. Of course, some would argue that this makes this image feature Nanotyrannus, but I don't want to get into that here. Those wanting to open that can of worms may want to read Thomas Carr's blog post (and comments) on this topic, as well as Mark Wildman's take on the same debate. 

Dating tip: romantic sunsets don't count for much when you're crushing your partner's skull. If you fancy a physical copy of this scene of violent tyrannosaur copulation, you might be a bit odd. Nevertheless, prints are here.

Something new now - a reworked take on my mating, neck-biting tyrants. Those with long memories will recall the first guise of this image appeared in 2013 with my comments on All Your Yesterdays, the crowd-sourced follow up to All Yesterdays. As explained in that post, a number of tyrants show evidence of having been bitten around the jaws and head, with the area around the braincase of some specimens being badly damaged. I'd been looking at Savannah monitors shortly before rendering the original of this, and found their toughened neck skin - which apparently exists because of rough copulatory behaviour - of interest. I tend to have half my mind on prehistoric animals when looking at modern ones, and it wasn't long before I was wondering if some Tyrannosaurus injuries were the result of similarly violent nuptial encounters. This reworked version includes some very minor anatomical tweaks, slight colouration changes, and a vastly more detailed background.

Triceratops and Tyrannosaurus: finally bro-dogs. Get printed up here.

Another commission from Chidumebi Browne resulted one of the strangest pictures I know of featuring Tyrannosaurus - but hopefully one which is interesting and thought provoking. Alongside this big female (note the similar colour to the red teenage animal in Chidumebi's first commission - this is the grown up version of a female in that 'universe') is a baby Triceratops, the idea being that it's been interspecifically adopted by the tyrant. I provided a long commentary on this image and the likelihood of the scenario back in March, concluding that this image might not be as crazy as it first seems. Quite a few modern animals - including dinosaurs - are known to kidnap or inherit the offspring of other species, although there's not always clear explanations for why it happens. I tried to imply a bit of a story in Chidumebi's concept, those marauding adults in the distance taking clear, hungry interest in the Triceratops infant. I get the feeling this scene wouldn't stay peaceful for long.

A Late Cretaceous evening, ruled by an especially robust tyrant. You can own a copy of him if you click here.  

Finally, one more new image: a major overhaul of one of the first images posted at this blog (end 2012). Changes include anatomical tweaks, a revised pose (now trotting, not standing), new colouration (the cranial pattern is a nod to the judge helmets in Dredd, because scientists predict Tyrannosaurus are some of the few things in life more badass than that movie) and a heck of a lot more background detail. The depicted animal is a 'robust' Tyrannosaurus morph - note it seems the 'robust' and 'gracile' forms are extremes of anatomical variation rather than distinct categories. My goal here was to make the animal look big and heavy - appreciating that tyrants are relatively long-legged and gracile for their size, they're still absolutely huge. I thought of bears a lot when painting this chap - I wanted him to have that same imposing aspect without going all 'awesomebro' on it. Tyrannosaurs - especially big ones - should look like animals you'd instinctively keep a good distance from.

OK, that's all for now. Soon, hopefully, some details on that project alluded to above. 

Of tiny tyrants and Triassic big-heads: Tyrannosaurus rex and Garjainia madiba

This week sees two new pictures of mine being 'released' in one way or another. Much as I'd like to go into lots of detail about each, that realistically isn't going to happen anytime soon. I'm going to attempt a sort of 'picture[s] of the day'-style writing. I'm sure I can do it... right?

Chidumebi Browne's resting Tyrannosaurus teens

Two young adult old male (left) and female Tyrannosaurus on a break from pillaging and destroying the Cretaceous, distracted by a group of ruffian moths. Concept and animal colouration by Chidumebi Browne. Prints are available.

First up is one of my '£100 palaeoart offers', painted for Chidumebi Browne. Featuring Tyrannosaurus, which needs no introduction as an dinosaur most famous for antisocial tendencies, Chidumebi wanted a more relaxed approach to tyrant dinosaur art. The concept called for Tyrannosaurus at the smaller end of their size scale, settling on individuals approximating the size of the 'Jane' specimen - about half the length of a fully-grown animal. There were also requests for contrasting blue and red colours on a male and female. I was happy to oblige, seeing as some degree of dimorphism is defensible for dinosaurs even at on half their full-grown size. Like mammals and non-avian reptiles, Mesozoic dinosaurs hit sexual maturity well before attaining fully ossified, completely grown skeletons and, for Tyrannosaurus, specimens in their early teens were probably reproductively active. In that sense, some features related to sexual behaviour might be expected in 'teenage' animals. Such individuals - better considered very young adults rather than large children - look rather different to their super-size contemporaries with their longer legs and more gracile build. Some of that is obscured here by the extensive feathering covering both animals (if you look very closely, you can just make out the arms of the sitting male), but their long legs at least show through.

The concept called for a a series of moths catching the attention of the male tyrant: initially one was ordered but, even at half-size, Tyrannosaurus is pretty big, so a few more were added to make them more conspicuous. My initial thought was to use butterflies rather than moths for the role of the lepidopterans, but I was surprised to learn that butterflies don't appear in the fossil record until well after the K/Pg event. Moths have a fair, if not especially extensive Mesozoic record, so they seemed a safer bet. They certainly add an air of tranquility to the scene not featured in a lot of theropod art: well done to Chidumebi for an excellent idea.

There'll be more output from the '£100 palaeoart offers' soon, although note that the offer is now full - over-full, in fact. There's some great ideas which I'm hoping to do justice to, so thanks to all who got their orders in - the offer sold out very quickly. If you didn't manage to get something to me on time, prints are still available - wittonprints@gmail.com is the address to contact for them.

Gower et al.'s Garjainia madiba: yes, the head is that big 

Gargainia madiba sp. nov., South Africa's newest erythrosuchid. From Gower et al. 2014.

Art number 2 is a life restoration of a new species of Early Triassic stem-archosaur, the erythrosuchid Garjainia madiba, described by David Gower and colleagues in this week's PLoS ONE. Unearthed in South Africa and named for Nelson Mandela ("Mr Mandela was known affectionately as 'Madiba'" - Gower et al. 2014), G. madiba has been making surprising ripples on Twitter and Facebook because of its rather enormous head. I say surprising because, for an erythroshucid, G. madiba is fairly typically proportioned - so far as anyone can tell, anyway. We don't have anything like a complete skeleton for G. madiba, although many aspects of its anatomy are represented in fragmentary specimens. It is currently distinguished from its relatives by fine anatomical details, perhaps the most notable being its large postorbital and jugal bosses of unknown function (best seen in the reconstructed anterior aspect, above). The discovery of more substantial G. madiba fossils may reveal more obvious distinction from other erythrosuchids, but, for the time being, the best we can do reconstruction-wise is show G. prima with a madiba upgrade package. Still, given how similar the two Garjainia species seem to be, this does not seem unreasonable.

Restoring Garjainia was a lot of fun because it forced a 'back to basics' approach to the artwork where David Gower, Richard Butler and I spent a lot of time discussing proportions, muscle distribution and posture. Many fossil animals - dinosaurs, pterosaurs, etc. - have been restored so often that the basic foundations of their anatomy are very well known, but this is not so for Garjainia and other erythrosuchids. A personal revelation to come from this process was evidence for enlarged areas of axial musculature on erythrosuchid skeletons, indicated by the rather tall neural spines of their necks and backs. This might give some insight into how their large heads were supported: a particularly well-developed, strong set of axial muscles. The posterior faces of their skulls are also wide and robust, providing space sufficient to anchor powerful neck muscles. But erythrosuchid anatomy was likely not held together only by brute strength: there's also some clever biological engineering at work. Like many archosauriforms with huge-looking heads, their skulls are more gracile and lightweight than they first appear, actually being fairly narrow for much of their length and riddled with fenestrae. We tried to show the former in our anterior aspect reconstruction: note how slender the snout of the animal is compared to the cheek region. The result is a head which is undeniably large, but probably much more manageable than it first seems.

For a lot more on Garjainia and other erythrosuchids, including the life restoration in situ, full descriptions of G. madiba anatomy and revisions to the diagnosis of the group, Gower et al. (2014) can be read here (hurrah for open access!). Thanks to David and Richard for bringing me on board, and congrats to them on the paper.

Coming soon: small, brown Mesozoic mammialiaforms! Yes, they are exciting. Really.

Reference

• Gower, D.J., Hancox, P.J., Botha-Brink, J., Sennikov, A.G., & Butler, R.J. (2014) A New Species of Garjainia Ochev, 1958 (Diapsida: Archosauriformes: Erythrosuchidae) from the Early Triassic of South Africa. PLoS ONE 9(11): e111154. doi:10.1371/journal.pone.0111154