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Tuesday, 30 April 2019

The science of the Crystal Palace Dinosaurs, part 1: marine reptiles, Dicynondon and "labyrinthodons"

The southerly approach to one of the most spectacular collections of historic palaeoartworks on the planet: the Crystal Palace prehistoric animals. Over 30 sculptures depict Victorian takes on prehistoric faunas in a remarkable and unique feat of engineering, artistry and scientific outreach. Much about how the models were assembled and the specific science informing their anatomy is lost to history, leaving us to piece it together from written accounts, surviving draft artwork, and the models themselves. This photo is from 2013, some of the models have been restored since then.
The Crystal Palace Prehistoric Park is one of the most spectacular and historically significant pieces of palaeoart in the world. Unveiled in 1854, it features more than 30 models of over 20 extinct species captured in concrete, brick and steel. Each model was crafted by a team of sculptors lead by the zoological artist Benjamin Waterhouse Hawkins under the guidance of Britain's preeminent Victorian naturalist, Sir Richard Owen. The Crystal Palace depictions are often overlooked or dismissed in coverage of early palaeontological history, which is entirely unfair. The scale, ambition and success of the project made it a milestone in not only palaeontological outreach but scientific communication in general, and they had a clear impact on future depictions of fossil animals, both scientifically and educationally (Rudwick 1992; McCarthy and Gilbert 1994; Secord 2004). Most of the models still survive today thanks to ongoing work by conservators and the Friends of Crystal Palace Dinosaurs charity, and a trip to Crystal Palace Park is thoroughly recommended if you're a fan of palaeoart or the history of science. The models are National Heritage Grade 1 Listed Monuments but, on account of their age, exposure to weather and vandalism, they are in continuous need of repair. Phases of renovation have been carried out since at least the 1950s (McCarthy and Gilbert 1994) and are ongoing today. Such work is expensive (conservation began in 2015 is expected to cost £1.2 million when completed) so please consider supporting the Friends of Crystal Palace Dinosaurs if you can. The FOCPD also appreciate volunteers to maintain the landscape around the models, which is a great way to visit them up close - keep an eye on their website for opportunities.

Part of the enduring appeal of the Crystal Palace sculptures is the mysteries of their construction. While the generalities of the project are well documented (e.g. Rudwick 1992; Doyle and Robinson 1993; McCarthy and Gilbert 1994; Secord 2004, also see this FOCPD summary), few documents are known specifying how the sculptures were built, and the scientific rationale behind them. Some details of their construction can be deduced by examination of the models themselves, and both Hawkins and Owen put their general scientific approach on record (Owen (1854) in particular reads like a modern summary of palaeoart practises), but it remains difficult to ascertain exactly how Hawkins decided on the form of each species. Adding depth to the mystery are suggestions that Owen's contributions may have not have been as substantial as generally assumed, and that his expertise was only sought as the designs approached their final phase and full-size construction was set to begin - too late, perhaps, for major revisions to Hawkins' drafts (Secord 2004). Moreover, while Hawkins' attention to Owen's work is clear, he also evidently relied on other sources of information and his own intuition on many occassions. This may explain why Owen sometimes distanced himself from aspects of the models in a 1854 guidebook to the models, as well as in newspaper interviews (Secord 2004). Nevertheless, documents authored by both Owen and Hawkins suggest mutual respect and admiration for one another (Owen 1854; Hawkins 1854), although it's interesting that each eventually claimed to be the greater intellectual influence on the project (Secord 2004).

Scene from Benjamen Waterhouse Hawkins' temporary workshop: a large wooden structure in the grounds of Crystal Palace Park. Images like this - which feature (clockwise from top left) Palaeotherium, Iguanodon, Hylaeosaurus, Dicynodon and "Labyrinthodon" - give valuable insights into the creation of the Crystal Palace Prehistoric Park, but shed little light on the science influencing their restoration from fossil bones. Note the corvid and rodents in the foreground: written accounts suggest Hawkins' workshop was not always a luxurious place to be. Illustration by Philip Henry Delamotte, 1853, image in public domain.
Recently, I've been working with the Friends of Crystal Palace Dinosaurs to restore the Crystal Palace extinct animals as we know them today for their website. As part of that process, I've been providing notes on how accurate the models are to current science as well as against fossil data available to Hawkins in the early 1850s. This has proven fascinating, confirming Hawkins' talents and insight while also raising several questions about his process and palaeoart philosophy. We may never know Hawkins' thought process in detail, but we might be able to 'reverse engineer' his models back to specimens known pre-1854 and, through clues worked into his models, establish what science, artworks and extant species influenced his designs. Over the next three posts, I thought it would be of interest to share some of these thoughts, as well as my modern takes on the Crystal Palace species. Edited versions of these notes also appear at the Friends of Crystal Palace Website, and more will follow in the near future as we wrap up this project up. I'm going to tackle the sculptures more or less as they appear in the park as you walk from the geologically oldest models (the Dicynodon) to the youngest (Megaloceros).

Dicynodon

The Crystal Palace Dicynodon, as seen in 2013. The larger model in the top photo, and the focus of the lower photo, is D. lacerticeps, the smaller sculpture is D. strigiceps. Note the turtle-like bodies and long tails, but also the presence of obvious clawed feet instead of flippers.
There are two Dicynodon sculptures in Crystal Palace, one large, one small. Particulars of their bodies and size indicate that they are meant to be different taxa. Owen (1854) indicates that the larger statue - "with the bulk of a walrus" - is D. lacerticeps, but he did not specify the identity of the smaller animal. He provides a clue, however, in stating it is a species with somewhat owl-like facial features. This must indicate that the smaller model is D. strigiceps, a species Owen named in 1845 that literally translates to 'owl-faced Dicynodon'. D. lacerticeps is the type species of Dicynodon and remains valid today, but strigiceps was regarded as a nomen dubium by Kammerer et al. (2011). The identification of D. lacerticeps as the large animal is peculiar, as it is not a large species - its skull was just over 15 cm long. We know that Hawkins attempted to capture the size of his animals accurately (Hawkins 1854), so perhaps other Dicynodon material factored into this decision.

The Crystal Palace Dicynodon are famously turtle-like in form, a circumstance reflecting Dicynodon being almost entirely represented by cranial material in the 1850s. These skulls demonstrated the basic shape of the skull and their strange turtle-meets-walrus nature but, as noted by Owen (1854), the rest of the sculptures are purely conjectural. Owen regarded dicynodonts as amphibious (Owen 1845, 1854) and Hawkins seems to have ran with this concept, presumably also inspired by the turtle-like features of the skull. Details of the sculptures' feet show that Hawkins was probably modelling these creatures on more terrestrially-adept turtles, and I wonder if the three ridged, sculpted keels, developed claws and long, scute-lined tails specifically indicate influence from snapping turtles. As we'll see, Hawkins often took inspiration, and entire anatomies, from living species in his work.

Today, we imagine dicynodonts very differently to our Victorian colleagues. This image shows Aulacephalodon bainii (the larger species, an animal known to Owen, and possibly referenced in the size of the D. lacerticeps sculpture) and the smaller dicynodont is Cistecephalus microrhinus (a species only distantly related to D. lacerticeps and A. bainii). This image will be featured in an upcoming book, also themed around historic palaeoart.
Dicynodont anatomy is now very well known and contrasts markedly with Hawkins’ sculptures. While their heads are reasonable proxies for dicynodont crania and consistent with contemporary reconstructions (e.g. Owen 1845), they seem a little ‘snouty’ compared to the short, shear-faced muzzles we now known from well-preserved dicynodont skulls. More obvious differnces are that dicynodonts have robust limbs adapted for terrestrial life, and many species were burrowers: they accordingly had rotund, longish torsos, not wide, flat ones. Though no dicynodonts had shells or armour, we are still uncertain what sort of skin they had. Given their relationship to mammals some artists restore dicynodonts with fur, but we have yet to find any evidence of this integument type so deep within our evolutionary history. The recovery of hair from a Permian coprolite (Bajdek et al. 2016) suggests some synapsids from this time may have been furry, but the most parsimonious candidates are our closest Permian ancestors, the cynodonts, not the more distantly related dicynodonts. We still think, as demonstrated in Hawkins’ model, that dicynodont snouts were largely covered with a cornified beak sheath however, with the tusks projecting either side (Kammerer et al. 2011).

“Labyrinthodon”

A trio of "Labyrinthodon", photographed in 2013. There are two species here, the larger being "L. salamandroides", the smaller "L. pachygnathus". Note the palatal teeth in the right animal and the similarity between L. salamandroides and an Owen sketch of Labyrinthodon as the Chirotherium trackmaker.
The three Crystal Palace “Labyrinthodon” reconstructions are attempts to rationalise several pieces of unrelated fossil data, so it is unsurprising that the results are far from the reality of the species they are meant to represent. But while some of the most dated models scientifically, they raise some interesting questions about how Hawkins approached his reconstructions.

Depicted as giant frog-like creatures, Hawkins’ sculptures show close attention to illustrations of “Labyrinthodon” as interpreted by Owen (e.g. Owen 1841a, 1842; also see Benton and Gower 1997) and capture some details of the skull and tooth material then referred to this genus. Particularly notable are the palatal teeth - this excellent attention to anatomical detail, especially given that visitors have to be right next to the sculptures (or looking with binoculars) to see them. Their mix of smooth and warty skin is surely based on living amphibians, and serves to distinguish the models of “L. pachygnathus” (smaller, warty-skinned) from “L. salamandroides” (the larger, smooth-skinned model) (McCarthy and Gilbert 1994). Owen famously linked "Labyrinthodon" with trackways now referred to pseudosuchians, but in doing so rationalised and illustrated the trackmaker as making prints with opposite limb sets, so the left prints were made with the right feet, and vice versa. This detail is absent from Hawkins’ models, despite his general attention to Owenian ideas. Perhaps even he struggled to make this bizarre hypothesis a reality.

Modern takes on “Labyrinthodon” are very different to the creatures displayed at Crystal Palace. What Owen and Hawkins considered “Labyrinthodon” is now rightfully called Mastodonsaurus, the former name being Owen’s attempt to replace Mastodonsaurus with a title he thought better suited the animal (Owen 1841a). Of the depicted species, “L. salamandroides” has been subsumed into M. jageri, and the fossils referred to “L. pachygnathus” are a mix of mastodonsauroids and archosaurian remains (Benton and Gower 1997; Damiani 2001). The latter point vindicates Hawkins' now archaic-looking approach to restoring Mastodonsaurus. The idea of a sheep-sized prehistoric frog seems outlandish in the 21st century, it was an entirely sensible interpretation of Owen's take on the available fossil material, from the proportions of the body to the upright limbs. I find the capturing of the "L. pachygnathus" jawline and dentition especially commendable.

Mastodonsaurus jageri, the 'real' "Labyrinthodon", striking at the rhynchosaur Fodonyx spenceri. Far from being an oversize frog, M. jageri occupies anatomical space somewhere between a salamander and alligator.
We now know that Mastodonsaurus resembled a giant salamander more than a frog, though in truth no living amphibian is a close analogue for this often giant Triassic form. A large, flattish head dominates a long, slender body with reduced limbs. The skull is covered with sculpted and textured bones somewhat reminiscent of crocodylian skull surfaces, and detailed investigation suggests this records a tight, tough facial skin (Witzmann 2009). Also like crocodylians, Mastodonsaurus eye sockets are situated on the top of the skull, not the sides as depicted at Crystal Palace. This was a peculiar decision from Hawkins, given that good Mastodonsaurus skulls were known in the early 1800s (e.g. Plieninger 1844), and that Owen knew about them (1854). Hawkins older illustrations and draft Labyrinthodon model (which was presumably shown to Owen) also show flatter heads. Do the fleshy-faced, side-eyed Crystal Palace amphibians reflect Hawkins paying more attention to frogs than Mastodonsaurus fossils? It may, as there are several other examples of Hawkins' models overriding fossil data with extant animal form, as we'll see throughout this review. Alternatively, were they errors? A misguided revision suggested by Owen or someone else? Was Hawkins simply following the illustrations of others, such as that presented in Owen's (1854) guide? Whatever the cause, this is a clear example of Hawkins not using fossil data where he could have done, in contrast to his sometimes exacting reproductions of anatomy in other areas.

Ichthyosaurs

In terms of scientific credibility, Hawkins’ three ichthyosaur statues have probably held up best of all his non-mammalian sculptures. This undoubtedly pertains to ichthyosaur skeletons being entirely known from very early in palaeontological history, as well as their familiar whale- or fish-like form. I consider them a good measure of Hawkins’ skill as a palaeoartist because it puts him on a more equal footing with modern practitioners, and suggests that when he had comprehensive datasets and suitable modern analogues he was able to produce very reasonable interpretations of fossil forms. It was largely a lack of information, not poor knowledge of anatomy and zoology, that lead to the inexactitude of the Crystal Palace models. There are three species of ichthyosaur on display, each distinguished by size and proportions, and once all considered different taxa of Ichthyosaurus. In modern parlance, they are Ichthyosaurus communis (the mid-sized ichthyosaur model), Temnodontosaurus platydon (the largest) and Leptonectes tenuirostris (smallest).

The Crystal Palace icthyosaurians in various states of visibility and repair. Top, Leptonectes tenuirostris as photographed in 2018 (I don't have any good photos of this model on account of it being hard to access and, when I was able to see it properly, the site was overgrown with lush vegetation); middle, Ichthyosaurus communis in 2018; bottom, Temnodontosaurus platydon (another 2013 photo, but this reflects the current state of the model - notice the contrast with the restored sculptures).
Much about Owen’s views on ichthyosaurs, and much of how Hawkins rendered them, remains accurate today. Owen (1854) specifically mentions the presence of smooth, scale-less skin, predicts some sort of fin at the end of their tails (identified in by Owen in 1840(a), though he was uncertain of the shape) and large eyes in these animals. We still restore ichthyosaurs in this way, albeit with some additional guidance and confidence from fossilised ichthyosaur body outlines and soft-tissues (e.g. Lindgren et al. 2018). Major additions to post-Crystal Palace reconstructions include the presence of a dorsal fin, tall and crescent-shaped tail fins, and more generous allocations of soft-tissue across the body as befitting fully marine, whale-like creatures. The presence of large eyes was then, as now, deduced not from large eye sockets but from the enormous scleral bones found in fossil ichthyosaur skulls. Owen’s statement that the function of these bones was supporting and protecting the eye is also correct, although it’s unlikely that the sclerotic ring was conspicuous in life as Hawkins depicted it. Plenty of living animals have large sclerotic rings, but they are hidden beneath eyelids and other anatomy.

An unusual property of the Crystal Palace models is the skin on their flippers, which has a very obvious scaly appearance. This reflects the Owenian hypothesis that the bones of the flippers were somehow reflected in the overlying skin scales (Owen 1841b), which Hawkins faithfully reproduced on his models. This seems unlikely given what we now know of fossilised ichthyosaur skin and the relationships between bone texture and skin anatomy. Hawkins was not solely guided by fossils in his restorations however, with details of the ichthyosaur faces reflecting whales and dolphins. This is particularly evidenced by the dolphin-like grooves and lips along their jaws, and seems entirely reasonable given what we know of ichthyosaurian skulls and the relationship between jaw bone surfaces and facial features.

A modern restoration of Temnodontosaurus eurycephalus (the larger, strand-feeding species) and Ichthyosaurus breviceps (the prey animals). In many ways, not so different from Hawkins' take.
Two aspects of the models date them firmly to palaeontology's early years. The first is that they are meant to be crawling around in shallow water, not swimming along the water surface (Owen 1854). This reflects a now long-abandoned view these reptiles could come ashore to sleep or for reproductive purposes. Secondly, all three ichthyosaur models have a great degree of flexibility in their tails, which is no longer considered plausible. The three sculpted species likely had varied capacity for tail flexion in life, with two species (Temnodontosaurus and Leptonectes) probably having more flexible tails than the relatively thunniform ('tuna-like') Ichthyosaurus communis. None had an ability to attain the eel-like tail shape reflected in the Crystal Palace models, however. This was not a mistake unique to Hawkins, but a fairly typical way of restoring ichthyosaurs in th early 1800s.

Plesiosaurians

Complete plesiosaurian skeletons allowed Hawkins to reconstruct them in a generally credible light, though the results were not as precedent as his takes on ichthyosaurs. To be fair, plesiosaurians are not as intuitive to reconstruct as ichthyosaurs and many aspects of their anatomy and functionality are debated even today. Making three-dimensional plesiosaurian sculptures just decades after their fossils were found was no mean feat, and Hawkins' models no less credible than other mid-19th century takes on these animals. Three models were created and, though similar, they have varying proportions and sizes on account of representing three species. Today, we classify these as Plesiosaurus dolichodeirus, “Plesiosaurusmacrocephalus, and Thalassiodracon hawkinsi.

Plesiosaurians of Crystal Palace. These are all restored to their former glory now (as per the top image) but, as with the ichthyosaur images above, I'm forced to use older, shoddier photos for two models because of all that fantastic greenery. Top, “Plesiosaurus” macrocephalus in 2018; middle, Thalassiodracon hawkinsi (2013 - note missing flipper, now replaced); bottom, Plesiosaurus dolichodeirus (2013). I've taken these identifications from McCarthy and Gilbert (1994), but I'm not sure they're correct. Surely the middle is the short-necked macrocephalus? Owen's guide is not entirely clear on the matter, unfortunately.
The proportions of Hawkins plesiosaurs are not exact to fossil data, a fact especially obvious for "P." macrocephalus, which lacks its characteristically large skull. They capture the main characteristics of plesiosaurians however, with “P”. macrocephalus - undersized head aside - being especially pleasing with its robust, deep tail and powerful-looking shoulders (see this blog post for a run down on plesiosaurian life appearance). These attributes make it the most ‘modern looking’ of all three sculptures. Additional fine details include the eyes being angled upwards, and might the obvious teeth reflect a suggestion that they were permanently visible? I’m not sure where we are regarding ideas about plesiosaurian facial tissues, but it’s not unreasonable to assume liplessness, or at least lipless regions, for some or most plesiosaurians (and it seems near certain for some taxa, like pliosaurids). The presence of smooth skin does not entirely align with what we now know of plesiosaurian anatomy, but it’s not a bad inference given that their scales were actually tiny - just millimetres across (Frey et al. 2017). It’s difficult to imagine how the materials available to Hawkins and his team could have been crafted to show such fine detail even if such data was available to them.

There are several major differences between Hawkinsian plesiosaurians and our modern takes. The most obvious of these is their thin, highly flexed necks, which recall those of long-necked birds or snakes even down the obvious neck/skull junction. It is highly unlikely that plesiosaurians could bend their necks as depicted at Crystal Palace, nor do their neck vertebrae imply a light covering of musculature (Noè et al. 2017). Today, we assume plesiosaurians were capable of a reasonable degree of neck flexion, but perhaps only to the extent of forming broad arcs, not multiple tight curves. The Crystal Palace plesiosaurians also have slender, flexible bodies, more like those of lizards (to which they were often compared in early palaeontological literature) than their actual stiffened, barrel-shaped torsos. The P. dolichodeirus and Thalassiodracon hawkinsi models are particularly afflicted with this issue, and their long, flexible tails accentuate their lithe forms further. We can perhaps rationalise this by the holotypes of these plesiosaurs having relatively narrow torsos (a taphonomic influence is probable in both cases) as well as the prevalent early-19th century idea that plesiosaurians were more closely related to lizards than other marine reptiles ("Plesiosaurus", roughly translated, means "allied to lizards" - Owen 1854). Unbeknown to Hawkins and Owen, we would eventually find soft-tissue outlines of plesiosaurians showing substantial soft-tissue around their tails, perhaps reflecting hindlimb musculature (assuming they anchored some major leg muscles on their tails, as is the case for most reptiles) as well as body-contouring fatty tissues (Frey et al. 2017). Their flippers were also augmented with soft-tissue expansions, something Owen knew about for ichthyosaurs, but would not be apparent for plesiosaurians until the late 19th century.

George Scharf's illustration of "P. macrocepahlus", featured in Owen (1840b). The skull is obviously very large in this species, but Hawkins did not capture this in his model.
Further contrast with modern plesiosaur reconstrusions concerns the attitude and flexion of the statues' flippers. They are shown as having ample fore and aft motion as well as obvious elbow and knee joints. This was pretty typical of plesiosaurian art in the 19th century, and probably reflected Victorian assumptions of a turtle-like locomotory capacity in these reptiles. Today, we regard plesiosaur flippers as having more limited flexion. They had no joints along their length, and forward and backward motions were the most limiting axes of their shoulder and hip articulations (e.g. Carpenter et al. 2010; Liu et al. 2015). These properties have bearing on another difference: the portrayal of all three plesiosaurians as crawling in shallow water. It’s near certain that plesiosaurians would struggle to move around out of water (see this blog post for details), and evidence that they gave live birth negates the need for land-based behaviour (O'Keefe and Chiappe 2011). But for Hawkins, Owen and other 19th century scholars, who still regarded even ichthyosaurs as using land-based reproduction, plesiosaurians crawling around on land would have seemed reasonable.

Plesiosaurus dolichodeirus as we know it today: not a million miles off the Crystal Palace reconstruction, but significantly different in several aspects.

That's all for now, but we'll soon move on to teleosaurids, pterosaurs and - the star attractions: dinosaurs! Remember to check out the Friends of Crystal Palace Dinosaurs website if you haven't already, and please consider getting involved with supporting these fantastic, significant models if you can.

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