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Friday, 22 August 2014

Scleromochlus taylori: more than just 'the early ornithodiran'

The Triassic ornithodiran Scleromochlus taylori depicted as a nocturnal desert-specialist with filamentous insulation, fuzzy feet for purchase on drifting sands and a saltatorial means of locomotion. 

Like actors with one famous character, fossil taxa can become typecast to specific ‘roles’ in palaeontological discussions. One fact of their palaeobiological significance is entrenched so deeply that they are seldom mentioned outside of this context. Examples include Archaeopteryx as the first bird, Mei as the cute sleeping dinosaur, and Darwinopterus as the bridge between major stages of pterosaur evolution. Packaging these animals into simple factoids obscures much of their other interesting palaeobiology, so we rarely hear about their other remarkable features.

Step forth Scleromochlus taylori, a small Triassic archosaur from the Upper Triassic (Late Carnian) of Scotland. For 100 years Scleromochlus has been implicated as a relative of pterosaurs (e.g. Huene 1914; Padian 1984; Gauthier 1986; Sereno 1991; Bennett 1996; Hone and Benton 2008; Brusatte et al. 2010, Nesbitt 2010) or, at very least, an ornithodiran representing a very early stage of stem-bird evolution (Benton 1999; Hone and Benton 2008).* This is about all we ever hear about Scleromochlus, however: nothing more than a milestone in the evolution of pterosaurs or dinosaurs. I'm guilty of it too: in my own book, Pterosaurs (Witton 2013), Scleromochlus just formed an anchor for discussing ornithodiran evolution. Undoubtedly, this needs correcting: Scleromochlus is a unique and interesting animal in its own right, and one fully worthy of detailed discussion. To relieve my shame, I'm going to attempt such a discussion here. Just for fun, I'm going to write it in the same style as a Pterosaurs chapter.

*You can't mention Scleromochlus on the internet without someone pointing out that its status as an ornithodrian has not been tested in analyses containing non-archosaur archosauromorphs. This is true enough, but - at least within the current limits of testing - its ornithodiran status is not controversial, having been recovered in at least six different analyses (e.g. Gauthier 1986; Sereno 1991; Bennett 1996; Hone and Benton 2008; Brusatte et al. 2010) and sharing several unique characteristics with Pterosauria (Padian 1984). Hence, we're following convention here.

Select line drawings of Scleromochlus taylori fossils from Benton (1999). There are two specimens here, showing dorsal and ventral views. The specimen on the right is the holotype, and the left shows two associated individuals. Note the banded scales crossing the vertebrae of the larger individual.
Although represented by at least seven specimens from the Lossiemouth Sandstone Formation, no Scleromochlus is well preserved (Benton 1999). Most specimens comprise shallow sediment molds rather than actual bones, and none are complete. But we should consider ourselves lucky we know of this animal at all: the delicate, 180 mm long bodies of Scleromochlus occur in sandstone deposits representing an ancient, wind-blown desert with 20 m high dunes. Such deposits are often devoid of fossil remains, but the Lossiemouth Sandstones actually preserve a diverse reptile fauna (Benton and Walker 1985). Still, it’s remarkable that the tiny bones of these reptiles preserved at all in these harsh conditions and in relatively coarse (fine - medium) sands - the grains preserving Scleromochlus are each as large as Scleromochlus teeth. As is typical of Lossiemouth Sandstone specimens, most Scleromochlus fossils are more-or-less articulated and many appear to have been crouching at death. With little indication of sun-cracking or scavenging, their remains clearly represent animals which were buried alive or buried shortly after death, probably by sandstorms or dune collapses (Benton and Walker 1985). Although likely complete when buried, no specimens have survived intact to the present. Cross-scaling elements from different specimens has permitted a reasonable insight into Scleromochlus anatomy all the same (Benton 1999). Some details remain murky however, and disagreement persists over precise bone lengths and skull bone attitudes (Sereno 1991, Benton 1999; Padian 2008). This is perhaps expected, given that Scleromochlus remains are interpreted via low-angle light and plaster or plastic peels of the skeleton molds. Bennett (1996) sums up working on Scleromochlus as "low-angle illumination [is used] to examine and interpret molds and peels, but in my experience a considerable amount of imagination is necessary as well".


Specific details aside, palaeontologists are happy to say that the basic bauplan of Scleromochlus resembles a small lizard with enormous hindlimbs (below). The skull has a low lateral profile but is rather triangular in dorsal aspect, with a blunt muzzle and widened posterior. So far as can be seen, the orbit is by far the largest opening in the skull, making the reduced nares look even smaller by comparison. The temporal fenestrae - as illustrated by Benton (1999) - are fairly sized, although their full margins aren't clear in any specimen. These sit above a posteriorly lengthened retroarticular process on an otherwise fairly unremarkable lower jaw. Each jaw seems to house 15/16 teeth, which are apparently isodont and - so far as can be seen - relatively small and lanceolate. The lizard-like visage of Scleromochlus is further enhanced by its short neck, which contrasts with later ornithodirans. The tail appears rather short too, being about as long as the snout-vent length.

Reconstructed skeleton of Scleromochlus taylori from Witton (2013), a modified version of the skeletal in Benton (1999).

The limbs of Scleromochlus are where a lizard-like visage starts to unstick. The forelimb bones are long and slender, and capped with tiny hands. The fingers are poorly known, but the tiny metacarpals suggest they were rather diminutive and unlikely of any use for standing or walking, a hypothesis supported by the dichotomy in fore- and hindlimb length. Even less lizard-like are the hindlimbs, which are extremely long - about half the length of the entire animal - and end with a narrow foot with tightly bound metatarsals. Both the forelimbs and pelvis appear relatively small compared to the legs, though neither is atypically small for the length of the animal. The fifth toe appears to have been lost, the only remnant being a short, pointed metatarsal. Scleromochlus hindlimb arthrology betrays a parasagittal posture akin to that of dinosaurs and pterosaurs - the suite of characteristics associated with this is one clue that Scleromochlus is closely related to these clades (Bennett 1996; Benton 1999; Hone and Benton 2008).

Thin, transversely-banded scutes(?) covered the dorsal surface of the Scleromochlus torso, extending from at least the shoulders to the posterior pelvic region (indicated in the fossil illustrations, above). These indicate that Scleromochlus was at least partly scaled, although whether this represents the entire integument is not clear. It is increasingly apparent that scraps of fossil skin do not tell whole stories about ornithodiran integuments, as more and more specimens with extensive skin preservation present 'mosaics' of scales, naked skin and various kinds of filaments (demonstrated in pterosaurs, theropods and ornithischians; e.g. Bakhurina and Unwin 1994; Chiappe and Göhlich 2010; Godefroit et al. 2014). Scleromochlus may have been covered in scales, but it is equally likely that it had fuzz-like filaments in places. There are several reasons for this. Firstly, it belongs within a phylogenetic bracket where filaments are the ancestral condition or, at very least, scales were prone to developing filamentous morphologies. Secondly, virtually all models of archosaur evolution recover Scleromochlus as sister taxon to a fuzzy clade - pterosaurs, so there is good 'phylogenetic proximity' for fuzz. Thirdly, insulating integuments are common - if not ubiquitous - in small, active (see below) desert-dwelling animals. Thus, while the overall  integument of Scleromochlus remains mysterious, a mosaic of filaments and scales is not an unreasonable suggestion. In the reconstruction here, Scleromochlus is shown as rather fuzzy all over (see below for rationale), with filaments poking through its scaly back as they do on opossum tails and armadillo hide.

Please provide your own 'boing' sound effects.


Scleromochlus has long been recognised as a sprightly, cursorial or saltatorial biped because of its elongate, parasagittal hindlimbs (e.g. Woodward 1907; Huene 1914, Walker 1961; Padian 1984; Benton and Walker 1985; Benton 1999; Witton 2013). It has also been considered an arboreal glider with Sharovipteryx-like hindlimb membranes, as well as an aquatic diver, but few obvious adaptations to these lifestyles are found on its skeleton (Benton and Walker 1985). Cursorial features of Scleromochlus include lengthening of the distal hindlimb, reduction of the lateral pedal digits and narrowing of the metatarsal, and it is generally considered to have assumed a digitigrade stance, at least when moving at speed. Several features indicate that Scleromochlus was a saltator rather than a running creature: a relatively small but strong pelvis, short trunk skeleton, and a pronounced intercondylar groove at the end of the femur, which likely reflects a large quadriceps femoris tendon (Benton and Walker 1985). Saltation is an energy-efficient means of locomotion which has frequently evolved in desert-living species - extant examples include desert rodents, jerboas and kangaroos - and Scleromochlus has been favourably compared with such animals on several occasions (Walker 1961; Benton and Walker 1985; Benton 1999). Saltation may seem unusual means for a reptile to move, but other Triassic ornithodirans may have also locomoted in this way (Sereno and Arcucci 1994). Indeed, the powerful leaping and bounding abilities of early ornithodirans has been tied to the evolution of pterosaur flight (Bennett 1997; Witton 2013). 

Lifestyle and palaeoecology

It is difficult to say exactly what Scleromochlus ate because its teeth are poorly known, but a generalised diet of insects and other small prey seems mostly likely given the shape of its teeth and jaws (Benton and Walker 1985; Benton 1999). The wide skull and enlarged retroarticular process may have provided space for large and powerful jaw muscles, allowing Scleromochlus to make short work of tough insect carapaces. The association of crouching, articulated Scleromochlus skeletons (see line drawings, above), along with the recovery of multiple specimens (actually 5% of the Lossiemouth Sandstone fauna - Benton and Walker 1985), hints at some degree gregarious behaviour. It is difficult to imagine how the two associated individuals shown above were preserved in such a way unless they were alongside each other when they died - huddled together against whatever catastrophe buried them. Was coupling or group living 'normal' behaviour in Scleromochlus? Statistically, the odds of rare fossil specimens like those of Scleromochlus preserving unusual, 'one in a million' types of behaviour are low, so we might take the co-preservation of two animals as being representative of 'average' or typical behaviour in this species.

For those of you now weeping about tiny, panicked pairs of Scleromochlus dying in huddled balls of fear, here's a speculative reconstruction baby Scleromochlus to cheer you up. Using back of the envelope calculations of lizard egg mass and size, I predict this gangly hatchling was 50-60 mm long. The image is deliberately displayed at this size to stress the tiny proportions: on my 'standard issue' laptop screen, it's about life-size. Click to embiggen.

The likely saltatorial locomotion of Scleromochlus may not be their only adaptation to desert life. Their metatarals are rather flattened posteriorly (Benton and Walker 1985), permitting sitting or squatting on plantigrade feet without sinking into sand. Their nares are small, and flanges from the back of the skull cover the tympanic region (the location of the ear opening), both adaptations common among modern xerocoles to prevent moisture loss and minimise irritation from wind-blown sands (Benton and Walker 1985). Their orbits, by contrast, are very large, and may reflect another common response to desert life - nocturnality. Tiny animals like Scleromochlus rapidly overheat under a desert sun, but foraging at night negates that risk. Of course, desert temperatures plummet once the sun sets, but a layer of filaments (if present) may have countered this. Perhaps groups of Scleromochlus spent their days under shelter - rocks or vegetation - before venturing out at night to forage for insects. This strategy also helps avoid predators, of which the Lossiemouth Sandstone Formation has its fair share: early dinosaurs and nimble pseudosuchians are likely predators of Scleromochlus (Benton and Walker 1985). Hypothetical filaments of Scleromochlus may have had further uses in desert life, including enhancing their grip - and therefore agility - on sandy substrates, as seen in some modern saltatorial desert species. Likewise, covering or filling nose and ear openings with long scales or fur is another feature common to desert species, enhancing resistance to evaporation and airborne sand. The desert habitat of this early ornithodiran presents several intriguing reasons for the development of filamentous structures, which is obviously of interest when considering the origins of fuzz in Ornithodira more broadly.

And that, in a way, brings us full circle: back to considering Scleromochlus anatomy in the context of wider Ornithodira. Still, I'm sure we can all agree Scleromochlus is actually a very interesting animal in its own right, and definitely worthy of escaping typecasting as 'the early ornithodiran'.


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  1. One large disconfirmation to the saltatorial lifestyle as was proposed is the absence of a large moment for the ankle extensor, which reduces necessary leverage for hopping or ricocheting. Instead, as in some other "rabbit-like" reptiles such as Lagosuchus/Marasuchus and Lagerpeton (named for this very allusion, in fact) was the absence of a distinct calcaneal heel. Similarly, Saltoposuchus, the so-called "hopping croc," was named on the premise of its extremely long legs, and it should have similar if not more attributes indicating saltation, but as a croc with clearly hypercursorial adaptations we typically assume otherwise. Frog-like leaping is counter-indicated on grounds of the muscle adaptations in frogs as compared to say reptiles, so without the musculoskeletal features of other extant leapers I am hesitant to agree with this idea. It should also be noted that many birds have similar attributes of their limbs, can hop, but are not considered saltators.

    1. Not sure I agree with you, for two reasons:

      1) Large calcanea are common across all mammals from rats to elephants. They may be larger in some mammalian saltators - seemingly not all - but ossified 'heels' do not define mammalian saltators. Rather, some saltators elaborate the standard mammalian structure to their needs. As you know, reptiles ancestrally lack such ankles and, as you point out, lots of birds hop along just fine without them. Other saltators - frogs - also lack mammal-like ankle extensors. With birds indicating that the ornithodiran hindlimb is capable of saltation-like activity without a modified ankle, other modifications to the hindlimb - outlined in the article and below - might be sufficient to develop habitual saltation without adopting an entirely mammal-like approach.

      2) Even without a large ankle extensor, there are still a suite of features suggestive of saltation in Scleromochlus. Some of these - such as the pelvis size to limb length ratio, the atypically large groove in the distal femur - seem more consistent with saltation than cursoriality. They do not exclude the latter as means of transport, but known cursors lack these features, while known saltators do. So, even if we chalk the lack of a large ankle extensor up as a problem for the saltator hypothesis (which it probably isn't, for reasons mentioned above), the weight of evidence still supports it because other Scleromochlus anatomies are unnecessary for cursoriality, but consistent with saltation.

    2. Is there any quant stuff on this coming forward?

    3. Is there any quant stuff on this coming forward?

    4. Not any that I know of. Given some of the disagreements over Scleromochlus detailed anatomy, as well as the poor quality of the specimens, I suspect we could only perform fairly basic quantified assessments of their functional morphology.

  2. Strange, it really seems to resemble the modern Australian Antechinomys. I wonder if the short neck might also be related to a hopping or saltatorial lifestyle, as shortened necks with sometimes fused cervical vertebrae are common in living hopping or bounding mammals.