Episode 1: Diminutive, adaptable atoposaurids

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Welcome to The Wealden Crocodyliformes Trilogy! As in, three whole posts dedicated to the major types of Wealden Crocodyliformes! Yeah! Woot! Let's all have a pear!

Right, let's back up a bit. Hopefully, it’s well known that modern crocodilians represent only a tiny fraction of crocodile-line archosaur diversity. Crocodyliformes, the major group of crocodile-like archosaurs which ultimately begat our modern crocodile fauna, was ancestrally much more morphologically and ecologically diverse than its modern representatives. This is true to the point that the common labelling of crocodilians as ‘living fossils’ is only true in a very loose sense. Superficially crocodile-like Crocodyliformes have certainly been around for well over 100 million years, but even some of their close relatives were very different beasts. Despite this - hopefully – widely known fact, reconstructions of ancient Crocodyliformes, or even articles about them, are not exactly commonplace. Running through the major types of crocodyliform once found in Lower Cretaceous Britain, specifically those from the Wealden Supergroup, gives an opportunity to at least scratch the surface of Mesozoic crocodyliform diversity. Wealden Crocodyliformes are diverse, with 11 species identified in a recent review (Salisbury and Naish 2011). As with many Wealden groups, the long research history of these animals (Wealden crocodyliform teeth were first found in the 1820s) does not mean they have been 'done' by palaeontologists - there remains much to learn about Wealden crocodylomorph palaeobiology. Indeed, at some point in this trilogy, we'll cover a cool, new and in-press discovery - more on that later, as I've already said too much.

A key point to note in each of these articles is the major anatomical differences these animals have from modern crocodiles, meaning that not only their lifestyle but also appearance would contrast from anything we associate with Crocodyliformes today. This is despite the Wealden Crocodyliformes not, by any stretch of the imagination, representing the most 'extreme' bauplans offered up by the croc-line archosaurs. This message should ring particularly loudly for artists who simply place modern crocs in the Mesozoic (a crime I'm as guilty of as anyone). All right, enough preamble already – let’s meet our first subject, the resourceful, tiny atoposaurid crocodyliform, Theriosuchus.

Neat things come in small packages
Our fist Wealden crocodyliform is a far cry from the role of large, voracious apex predator we often think of when crocodiles are mentioned. At only 550 mm long, Theriosuchus was tiny compared to most Wealden Crocodyliformes and likely risked predation from even moderately-sized contemporary predators. Theriosuchus is a long lived genus known across Europe and Asia from Late Jurassic – Late Cretaceous deposits and several species are known. Wealden fossils of this animal – a partial skull from the Wessex Formation and isolated teeth from the Ashdown and Wadhurst Clay Formations of East Sussex – are too fragmentary to refer to any existing species, or permit identification of a Wealden-specific one. For the time being then, the Wealden Theriosuchus is simply referred to as Theriosuchus sp.

The Lower Cretaceous, Wealden atoposaurid, Theriosuchus sp., prancing about in pursuit of locusts. 

Theriosuchus belongs to Atoposauridae, a group of neosuchian Crocodyliformes which are seem closely related to the group containing modern crocs, Eusuchia. Atoposaurids possess several 'derived' eusuchian features in their nasal and vertebral regions, hinting at a possible close relationship with this group (fun fact: classic 'eusuchian' features actually evolve repeatedly in ancient Crocodyliformes, which can confound taxonomic assessments of fragmentary fossil crocodyliform material - more on this later). The most distinguishing feature of atoposaurids is their size. Even when fully grown, no atoposaur exceeds one metre in length. They are correspondingly sometimes labelled as 'dwarf’ species, but this label is not an entirely accurate. So-called ‘dwarf’ species are not uncommon (elephants, deer, many lizards and crocodilians are just some lineages containing 'famous' dwarfs) but - by definition - they must be descended from closely related, larger relatives. All currently known atoposaurs are small, so they cannot be said to have reduced their size from their ancestral condition. Thus, they are not true ‘dwarf’ Crocodyliformes, just small ones.

Although unquestionably crocodile-like, the life appearance of Theriosuchus probably wasn't strongly comparable with any modern animal. Broadly, atoposaurids recall attributes of crocodiles and long-legged lizards, but their relatively svelte skeletons and long limbs are also vaguely reminiscent of some small mammals. Their skulls are rather low and short, thanks to an unusually abbreviated and broad snout which tapers into a rounded muzzle. Unlike many Crocodyliformes, atoposaur nasal openings remain separated and placed on the lateral margins of the snout tip, rather than being combined into a single opening on the upper snout surface. Their eye sockets are proportionally large but an opening in the skull above and behind the eye, the upper temporal fenestra, is rather reduced. This suggests that at least some of their jaw muscles were not as large as the famously enormous jaw muscles of modern crocodiles. Atoposaurid body proportions are rather typical of Crocodyliformes with short necks, tubular trunks and a tail of moderate length, but the limbs of Theriosuchus and most other atoposaurids are rather longer and more gracile than we've come to expect from croc-line archosaurs (this is not universal across the group: other atoposaurids have rather squatter limb proportions). Two rows of square or rectangular, keeled osteoderms extended along the neck, back and tail, becoming slightly more prominent on the tail. Most are pretty flat, so atoposaurid backs would look considerably less ornate than those of modern crocodilians. In Theriosuchus at least, the dorsal osteoderms possess ‘peg and groove’ joints which locked each osteoderm into its neighbour, forming a relatively immobile bony sheet along the back. This feature is not common to all atoposaurids, but is found in some other Crocodyliformes - albeit not modern ones. It's thought that this locking mechanism provided more than just reinforcement of  the armour along the animals back, also helping to resist bending and twisting movements in the torso when the animal walked or ran. Additional osteoderms occur beneath the tail and neck.

Theriosuchus pusillus, one of the best known members of this genus, from the lower Cretaceous Lulworth Formation, UK. Image from Owen (1878), borrowed from Wikimedia Commons.

Like many Crocodyliformes, the teeth of Theriosuchus are deceptively complex. The teeth lining the jaw tip are rather conical with slight striations and carinae, while those behind become rather lance-shaped - longer than wide, with a pointed apex. The carinae of these teeth are rather coarser than those at the jaw tip. In some Theriosuchus species, the teeth at the back of the mouth are compressed into blade-like structures with particularly coarse serrations. Two peaks in tooth size can be seen in atoposaurid jaws, the first occurring with a large conical tooth which forms a ‘psuedocanine’, and the second (smaller) peak among the anterior lance-like dentition. A notch in the side of the upper jaw means that the ‘pseudocanines’ were probably visible even when the mouth was closed. Put together, the dentition of Theriosuchus was multifunctional and ideally suited to processing soft prey items: the anterior teeth could pierce and stab; the lance-shaped teeth could crush and cut, and the blade-like teeth (if present) could shear and rip into soft-food.

Raccoon-crocs?
The dentition of Theriosuchus suggest that, like other atoposaurids, it may have been primarily carnivorous, likely foraging for small vertebrates, arthropods and carrion. As in modern crocodiles, their diverse teeth may have also permitted ingestion of nutritious plant matter. Unlike modern crocodiles however, several features of atoposaurid anatomy suggest they found much of their prey away from aquatic settings. Relatively long limbs increased their stride distances, allowing for potentially rapid locomotion, while their interlocked osteoderms likely reduced the strain of walking and running on the trunk skeleton. The latter is likely true of all Crocodyliformes with interlocking osteoderms, but Theriosuchus was also a small, lightweight creature, thus reducing strain on its trunk even further when walking. Put together, the combination of small body size and a reinforced back may have allowed Theriosuchus to sustain long periods of walking and running compared to other Crocodyliformes. The general rarity of atoposaurid fossils compared to those of other Crocodyliformes - both in the Wealden and elsewhere - lends some support to this idea: animals which spend a lot of time in water generally have a higher preservation potential than those which don’t, simply because their remains are that much closer to environments where sediments are likely to accumulate and bury them.

Aquatic behaviour for Theriosuchus cannot ruled out, however. While the osteoderm bracing system likely limited their torso flexibility - thus somewhat impeding the ability for rapid and manoeuvrable swimming - the retention of a powerful, flexible tail and low body shape probably still permitted fair swimming performance. Thus, it is quite possible that terrestrial food sources were supplemented with diminutive fish and other small aquatic prey from time to time. Atoposaurids like Theriosuchus may be best regarded as very adaptable, generalised species which, if we were forced to crowbar them into a modern niche, may be most equivalent to small, semiaquatic mammalian carnivorans – raccoons, otters and so forth. Their generalised diet means that Theriosuchus probably competed for food with lizards and amphibians more than their fellow Crocodyliformes, and perhaps their ability to forage on land and in water, in concert with low body sizes, gave Theriosuchus an edge in a crowded Wealden ecosystem filled with many aquatic and terrestrial predators. Thus, while many Wealden animals were probably relatively restricted to specific foraging habitats and prey types, Theriosuchus could forage freely in both settings, resourcefully enjoying whatever morsels it could wrap its tiny jaws around.

The crocodyliformes we'll meet in the next instalment of The Wealden Crocodyliformes Trilogy are not quite as ecologically generalised as Theriosuchus - aspects of their size, dentition or proportions dictate that they had to commit to at least some lifestyle specifics. To see what they are, and how they fit more broadly into Wealden palaeoecology, you'll have to come back for Episode 2...

References

• Owen, R. (1879). Monograph on the fossil Reptilia of the Wealden and Purbeck Formations. Supplement IX, Crocodilia (Goniopholis, Brachydectes, Nannosuchus, Theriosuchus, and Nuthetes)". Palaeontographical Society of London Monograph 33: 1–19.
• Salisbury, S. W. & Naish, D. (2011). Crocodilians. In: Batten, D. J. (ed.) English Wealden Fossils. The Palaeontological Association (London), pp. 305-369.