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Wednesday, 11 October 2017

The appearance and lifestyle of Thalassodromeus sethi, supercrested pterosaur

Thalassodromeus sethi, a juvenile Mirischia asymmetrica, and half a spinosaurid hang out in Cretaceous Brazil. The spinosaurid wants to go home.
One of my favourite pterosaurs is the Brazilian thalassodromid Thalassodromeus sethi: a large (4-5 m wingspan) Cretaceous azhdarchoid known only from a broken skull and cranial fragments of disputed affinity (Kellner and Campos 2002; Veldmeijer et al. 2005; Martill and Naish 2006). Characterised by an especially large bony cranial crest, toothless jaws and a robust skull construction, Thalassodromeus gained fame (and it's name, which translates to 'sea runner') from a presumed habit of skim-feeding. Long-time readers or pterosaur aficionados will know that multiple studies have suggested pterosaurian skim-feeding was unlikely on anatomical grounds (we discussed this most recently here and here) and was especially improbable for large species on account of the huge energy demands of ploughing large, blunt jaws through water (e.g. Humphries et al. 2007). A lack of skim-feeding habits does not make Thalassodromeus any less interesting however: it's a large, charismatic animal with a heavy dose of pterosaur weirdness, so there's still plenty to like. I recently had reason to overhaul the Thalassodromeus painting from my 2013 book (above) and took the opportunity to revisit my understanding of this animal's anatomy. The process had me fall for Thalassodromeus' cresty charms all over again, and I've taken this as impetus to share the love here.

The continuing puzzle of the Thalassodromeus skull

Thalassodromeus sethi skull elements as figured in Witton (2013). Note how the holotype skull is a giant jigsaw with well- and ill-fitting elements. The little (drawn) jaw to the left is no longer referred to Thalassodromeus, but is now the holotype of the dsungaripterid Banguela oberlii. This photo composite was created using photographs provided by the excellent Andre Veldmeijer and Erno Endenburg. 
The holotype skull of Thalassodromeus is pretty well preserved as pterosaur fossils go, but isn't quite as exceptional as it first appears (above). Though three dimensionally preserved and uncrushed, it's suffered damage in several areas and is broken into multiple pieces, some of which are ill-fitting with the rest of the skull or are missing entirely. It's a jigsaw puzzle which is complete enough to get the general picture of the skull shape, but some large areas remain open to interpretation. Pterosaur literature records that different bits of this specimen were once scattered across American research institutions and we have to hope that some of the last missing elements are still in a drawer somewhere, waiting to be reunited with the rest of the skull.

That the shape of the Thalassodromeus skull is somewhat ambiguous is evident by our history of T. sethi skull reconstructions (below). The first reconstruction - published in Kellner and Campos (2002) - is a little odd in that it shows a downturned, irregular upper jaw with a straight mandible. It also features 'classic' structures that we've come to know and love in this species: that badass 'V'-shaped chunk missing out of the back of the crest, a boss-like structure on the upper jaw, and a partly hooked mandibular tip. This reconstruction has always looked a little odd to me because I'm not sure how the animal is meant to close its mouth. A second reconstruction, which I presented in my 2013 book, was similar to the first except for showing both jaws as straight, without a downturned upper jaw. My logic was that Thalassodromeus should look something like the better known thalassodromid Tupuxuara, which has entirely straight jaws. Later, Headden and Campos (2015) presented a third interpretation, where the mandible was bent down at the base of the mandibular symphysis. Jaime Headden's (as far as I know unpublished) skull reconstruction hints at further differences from previous reconstructions, including a lack of that cool 'V' notch in the back of the crest.

Select T. sethi skull reconstructions, with my latest take at the bottom right. All three agree on some aspects of basic morphology, but there's not quite enough data to eliminate some possibilities of jaw and crest shape. Note that the 2017 skull outline is pretty conservative - the crest may have been longer and taller.
Which of these, if any, is correct? We await a comprehensive description of the skull to fully augment our understanding of T. sethi anatomy but, based on published information, it's likely that some of our earlier interpretations were erroneous. The gnarly crest shape drawn by Kellner and Campos (2002) probably takes damaged margins and missing elements too literally - this includes that awesome-looking V-shaped notch at the end, which is likely just another chunk of missing crest (this is certainly reported by colleagues who've examined the skull first hand). There's also no obvious reason why the mandible should be restored with an upturned tip. This interpretation was at least partly fuelled by an upturned jaw tip once referred to Thalassodromeus (Veldmeijer et al. 2005), but this specimen has since been considered a new genus of dsunagripterid pterosaur (Headden and Campos 2015).

It's also looking possible that - as indicated by Headden and Campos (2015) - both sets of Thalassodromeus jaws were downturned. It's difficult to be confident about any jaw reconstruction in this animal because these regions are not well represented in the holotype skull, but preserved elements of the upper and lower jaw margins imply a subtle downturn at the base of the rostrum and mandibular symphysis (and no, this isn't an effect of distortion or damage). Either Thalassodromeus had some sort of wibbly jaw shape or else it had a downturned jaw similar to azhdarchoids such as Tapejaridae* and Caupedactylus**. Whether this is convergence or further evidence of a close relationship between thalassodromids and tapejarids depends on your take on azhdarchoid interrelationships - this is still an area of disagreement that would benefit from dedicated investigation.

*of which thalassodromids - or thalassodromines - may, or may not be, a subdivision of. Ah, pterosaur phylogeny...

**I'm as confident as I can be that Caupedactylus is synonymous with my own "Tupuxuara" deliradamus. I should really write this up one day...

But hey, evidence for facial tissues and life appearance!

Thalassodromeus has some interesting features which allow us to reconstruct some aspects of its facial anatomy in detail, even in lieu of soft-tissue preservation. The crest of Thalassodromeus is marked by very conspicuous neurovascular grooves which were linked to a thermoregulatory function by Kellner and Campos (2002). They look pretty near identical to the sorts of branching grooves you find under bird beaks however (below), and my suspicion is that they're not a specialisation for controlling body temperature but simply a correlate for a keratinous sheath (Hieronymus et al. 2009). Similar grooves are seen on crestless parts of pterosaur jaws (the holotype of Serradraco sagittirostris has some especially obvious ones, for instance - see Rigal et al. 2017) as well as under the keratinous horns and beaks of animals everywhere. We don't need to imagine a unique function for these grooves just because they're on a big pterosaur crest, they're a standard variant of tetrapod skull anatomy.

Branching neurovascular networks on the Thalassodromeus crest - this is the region above the eye and posterior end of the nasoantorbital fenestra. Note the conspicuous groove crossing across the photo - this is the boundary between the premaxilla and underlying skull bones. From Kellner and Campos (2002).
Keratinous sheaths can have sharp margins which leave signature textures on the underlying skull. Bony steps or 'lips' can mark the transition to another tissue type, or a groove may form where one sheath plate abuts another. Both are evident in bird species which have beaks composed of multiple plates instead of a single keratinous covering (below), and we can look for similar features in fossil skulls to make predictions about life appearance. In Thalassodromeus we see a deep groove running along the boundary between the large premaxillary bone (the bone which makes up the jaw tip and top region of the entire crest) and the frontoparietal region (the base of the crest from the eye region backwards). Correlates for keratinous sheaths occur on both sides of this groove, so there's a chance that the crest covering was a compound structure composed of two abutting sheaths rather than one continuous one. If so, we might have been able to see this join on the live animal, just as we see the joins on the beaks of certain birds.

Gannet (Morus bassanus) skull with keratinous sheaths removed. Note the branching neurovascular impressions and deep grooves that mark the position of keratinous sheaths - we would predict a compound beak from these textures if we only knew gannets from fossils.
Can we test this idea? We could chop up our super-rare Thalassodromeus specimens to see if  histological data matches the surface texture interpretation (it's not only bone surface texture which records epidermal types - see Hieronymus et al. 2009) but I'll wager that most folks don't want the Thalassodromeus holotype carved up any more than it already is. Happily, there are other lines of data that might help us out. The first is the presence of the crest groove itself. Pterosaur skulls are normally devoid of sutures between bones because, in adults, they fuse so solidly that all trace of the original bone outlines is obliterated. Thus, the presence of a conspicuous groove in a mature Thalassodromeus specimen indicates that something unusual was happening, and influence from facial tissues is a well-known phenomenon that could explain this feature.

Schematic take on thalassodromid crest growth, from Martill and Naish (2006). The crest doesn't begin fully formed in juveniles, with the premaxillae (dark shading) having to overgrow the rest of the skull. Fun fact: my first ever PR palaeoart, now 11 years old, was to publicise this study.

A second line of support stems from studies into thalassodromid crest growth (Martill and Naish 2006). The "upper" (or premaxillary) component of the thalassodromid crest does not cover the skull in juveniles: rather, it has to overgrow the skull as the animal ages (above and below). Keratin sheaths are difficult to modify once formed because they're thick and inert (Goss 2012), so it's likely that parts of the premaxillary sheaths formed in juveniles migrated with the bone over the skull, meeting their counterparts at the skull posterior in later life. If the sheaths couldn't join once they met because they couldn't be modified or resorbed, they probably continued to grow as a compound cover, explaining the retention of an obvious groove between the two crest-forming bones. I find this idea pretty neat. Features like grooves on beaks or crests are nuances of animal appearance that are mostly lost to time but are important to characterising the appearance of living species. The idea that Thalassodromeus (and probably thalassodromids) had this feature makes them that little bit more real. Painting the images for this post certainly felt a little more like painting an animal than illustrating a hypothesis, just because of this detail.

Thalassodromid crest growth and compound keratinous sheathing, modelled by T. sethi. Note how the juvenile has an obvious 'two part' crest composition, and that the front/upper part (the premaxilla) sits on top of the posterior (frontoparietal) elements. With enough time, they form the monster-sized crest we know from big thalassodromid specimens. See Martill and Naish (2006) for more details.

Skull mechanics and lifestyle

It would be remiss to write about Thalassodromeus without mentioning its robust skull construction. The skull is proportionally wide, has especially deep jaws, a partly sealed orbit region, and the mandibular symphysis has a robust 'teardrop' cross section instead some flimsy crest. Its robustness is especially obvious when compared to the skull of the otherwise similar Tupuxuara (below), which has more typically open and airy pterosaurian cranial architecture. Thalassodromeus thus has a skull which looks like it could take a little more punishment than that of an average pterosaur, and this correlates nicely with observations that the regions for jaw adductor muscles are expanded on both the skull and lower jaw (Witton 2013; Pêgas and Kellner 2015). It's unsurprising that foraging hypotheses for Thalassodromeus have favoured forceful feeding habits such as skim-feeding (Kellner and Campos 2002) or being a predator of small-to-medium animals in terrestrial settings (Witton 2013).

Tupuxuara leonardii skull and mandible - looking pretty slender compared to the star of this post.
The possibility of downturned jaws in Thalassodromeus becomes especially interesting in light of its robust skull. Long, curving bones are a biomechanical paradox because they're weaker in compressive loading than a straight equivalent. This is, in part, because applying loads directly to both ends of a curved bone induces bending stresses even though the bone is not being bent in a traditional fashion. This is why big, slow animals tend to have straighter limb bones than smaller ones: they benefit from the increased strength of straight shafts, and they load their limbs in compression virtually all the time. From this perspective, the curved jaw of Thalassodromeus might seem like a disadvantage, being weaker under compression than that of a straight jawed animal. If striking violently at prey head on, the straight jawed species might be less likely to go home with a broken jaw.

However, curved bones are superior to straight bones at handling unpredictable, dynamic stresses. Curvature introduces predictability to stress distribution throughout a bone shaft, so they behave more reliably under a variety of loading regimes, be it compression, bending or twisting. A bone which responds to stress in the same way no matter how you deform it is easier to manage behaviourally, and to optimise mechanically, than a straight bone, and loss of raw strength created by bone curvature can be compensated for by modifying cross sections, shaft diameters and internal reinforcement (Bertram and Biewner 1988). These attributes have not been ignored by evolution and, in fact, most animal limb bones are curved to some degree to take advantage of these effects (Bertram and Biewner 1988). The superior compressive performance of a straight bone may not be as advantageous as the reliability and potential all-round stress resistance of a curved variant so, in simple terms, if you're planning some crazy stunts with your long bones, you want curved bone shafts, not straight ones.

A curved jaw thus complements the strong skull and jaw muscles of Thalassodromeus. If Thalassodromeus used foraging mechanics which were forceful or violent - such as catching big or powerful prey types, or using its beak to batter or tear at other animals - a curved beak may have served it well. This jaw shape - assuming we've interpreted it correctly, remember - could be further evidence of foraging habits at the more explosive and exciting end of the pterosaur ecological spectrum. Exactly what Thalassodromeus did for a living remains unknown, but it's hard not to compare these cranial features with other ideas of robust, terrestrial azhdarchoid predators - maybe this 'large pterosaur predator' niche has a longer roster than we've traditionally thought.

Hypothesis B: spinosaurids were allergic to curved jaws. Hey, it could happen.
Thalassodromids and their azhdarchoid kin are exceptionally interesting animals and we could probably talk about them all day, but we'll have to stop there. Coming soon: pterosaurs from the other end of the pterodactyloid spectrum, or a return to the world of extinct mammals. Probably.

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