#teleosauroidea

[arminreindl]

Indosinosuchus the freshwater Teleosaur

Ignoring the fact that I skipped out on posting about my recent deepdives on Armadillosuchus and Odobenocetops (besides that one figure I made) on Tumblr, let me catch up by talking about Indosinosuchus.

Indosinosuchus (Indochinese Crocodile, after the Indochinese Tectonic Block) is a type of teleosaurid from the Late Jurassic Phu Kradung Formation of China. First of all the Phu Kradung is a fascinating formation with a long and somewhat convoluted dating history. For quite some time researchers argued whether or not its Late Jurassic (based on fauna) or Early Cretaceous (based on pollen). Recent papers seem to argue that both can be true, with the lower parts being Jurassic and the upper Cretaceous, which also accounts for a faunal turnover visible throughout the record. Related to Indosinosuchus, we have classically Jurassic teleosaurs in the lower levels and pholidosaurids in the upper.

artwork by Kmonvish Lawan and Sakka Weerataweemat

Whats far more interesting however is that the Phu Kradung Formation is continental in nature, preserving floodplanes dominated by lakes and rivers. This is quite the unusual habitat for a teleosaur, which are traditionally marine animals, living in the coastal or even open waters of the Tethys. In fact the only other known example of a freshwater teleosaur is an unnamed animal from China that sometimes clades closely to Indosinosuchus.

Indosinosuchus is known from a total of 10 fossil individuals, largely consistsing of skulls and mandibles with the occasional postcrania, which were all found in a single locality and have been determined to represent at least two different species: I. potamosiamensis (Thai River Indochinese Crocodile) and I. kalasinensis (Indochinese Crocodile from the Kala Province).

Now overall, Indosinosuchus had a pretty long and narrow snout compared to most modern crocodilians. However, among teleosaurs, this animal was actually among the more robust genera. Even more so when you consider that within teleosaurs, it falls within the teleosaurid family, not the overall more robust machimosaurids that would eventually become durophages and macropredators, cracking turtle shells.

Analysing the biomechanics of Indosinosuchus did show some cool things. The snout was pretty average at resisting stress and force, at least for teleosaurs, but the bite force was respectably high. The force that could be transferred to the tip of the snout in particular is among the highest in this group and the maximum force it could excert was the single highest among all teleosaurs.

The two species however begin to differ in how fast they could close its jaws. I. potamosiamensis follows what would be expected. A teleosaur with a massive bite but only closing its jaws slowly as in teleosaurs with similar mechanical advantage values. I. kalasinesis however could close its jaws surprisingly fast, something you'd otherwise expect from animals with a much lower bite force that needed to strike a lot more swiftly in order to catch prey.

While we don't quite know the precise ecology of the two species, scientists could deduce a few things. For one, obviously the difference in the jaw closing/opening speed is a good sign that they had somewhat different lifestyles, which could explain why two so similar species coexisted at the same time in the same place. Secondly, although its bite force was notably high, it does not appear like Indosinosuchus went after especially large prey. Sure, it could bite hard and had a relatively robust rostrum, but its teeth were not particularily specialised like in Machimosaurus. So it has been suggested that it was a generalist, not a specialist as shown in this figure by Johnson, Young and Brusatte.

Last but certainly not least is the way these animals died. It does seem rather odd that 10 specimens all seemingly died in the same place at the same time, with most preserving the skull and not much else. Well there may be a reason. One clue stems from the most complete non-skull we have, which is a remarkably well preserved spinal column preserved in articulation, but missing the skull. So the idea is that the carcass may have floated in rather still waters (like an oxbow lake) and the weight of the head would eventually separate the two pieces. The spine must have been burried rather quickly afterwards so that it wouldn't just fall apart. Well the hypothesis continues that this could have been during a drought, when water levels were gradually receeding. We know that modern crocodiles will amass in great numbers during droughts until bodies of waters are nothing but mud and crocs, so it makes sense here. The full drying out of their sanctuary would then contribute to preservation and we even have evidence that they spend a minimum of 8 weeks in the dry thanks to the feeding traces of scavenging beetle larvae.

There is also a second hypothesis that suggests that the bodies may have been washed on land during a flashflood, but I personally think the drought hypothesis is a lot more interesting. Plus I managed to get @knuppitalism-with-ue to draw something akin to the later, with an unfortunate group of Indosinosuchus being stuck in a dried up lakebed as a local mamenchisaur approaches in the search for water.

Obligatory link to the Wikipedia page post-overhaul: Indosinosuchus - Wikipedia and the before and after, wondering if I should do the same with Armadillosuchus and Odobenocetops or if I should just come back once I got the next project done (Doratodon)

[arminreindl]

Two more croc-line archosaurs to dig into with this one and both are super fascinating.

Obviously the star of this piece is our friend in the foreground, Plesiosuchus manselii, the largest of the fully marine metriorhynchids (the skull of which being the first one shown in the image to the right, taken from Young et al. 2012)

More broadly speaking, metriorhynchids are a fascinating group of early croc-relatives that stand out for having returned completely back into the water, developing a fluked tail, tiny little reduced forelimps, flipper like hindlimbs and smooth skin entirely lacking in the osteoderm armour we so strongly associate with pseudosuchians.

Plesiosuchus specifically is the largest of them. Close to 7 meters in length, it's basically in the same range as Liopleurodon (although smaller than several of the pliosaurs we know from the Kimmeridge Clay like that very decorative skeleton). Plesiosuchus actually wasn't the only metriorhynchid from the formation either. We also know of the small Cricosaurus, the name-giving Metriorhynchus and most interestingly the large-bodied Dakosaurus and Torvoneustes, which were likely to reach lengths of 4.5 to 4.7 meters respectively. At least with metriorhynchids this diversity can be explained in the different animals having very different preferences. Torvoneustes is interpreted as a durophage, feeding on hard-shelled prey. Dakosaurus is characterized by slicing dentition, potentially able to cut apart large prey with tooth wear possibly suggesting sharks or suction feeding habits. Plesiosaurus meanwhile would be able to take on large prey simply due to its great size and large gape but may have been limited by the size of its own head.

The other marine "croc" in the image is Bathysuchus, which may be described as a cousin to the metriorhynchids.

Remember Machimosaurus from the Guimarota locality that Josch drew a few weeks back? Yeah its related to that one. A teleosauroid to be precise, which are the sister group to metriorhynchoids and appear much more croc-like in their appearance despite still being a long way off from crocodiles in the strict sense. Teleosauroids may appear like what you'd think of as basal to metriorhynchoids, but they simply did their own thing after diverging, thus retaining this more ancestral bauplan with more developed limbs, more croc-like skulls and maintaining their armour plating (keep that in mind).

Bathysuchus is kind of a weird one tho. Why? Because from what we can tell Bathysuchus seems to have tried to become more like its metriorhynchoid cousins. It's known from deeper waters, its osteoderms were much more reduced and if the closely related Aelodon is anything to go from then its quite likely that this guy also started reducing the size of its forelimbs. In short, adaptations to a more pelagic lifestyle that moves away from the coastal habitat other teleosauroids seem to have preferred. Who knows, maybe its possible that in some other timeline these guys might have become something akin to metriorhynchoids like what happened with mosasaurs.

Unlike metriorhynchoids, teleosauroids were way less common in the Kimmeridge Clay, hell, to my knowledge Bathysuchus is the only one of them, a pretender among the other pelagic pseudosuchians.

Result from the Kimmeridge Clay #paleostream!

We were only able to scratch the surface, but I think you get the idea ;)

[saritapaleo]

Patreon request for @/rome.and.stuff (Instagram handle) - Turnersuchus hingleyae

Described just last year, the Early Jurassic Turnersuchus is the most basal known member of the thalattosuchians: sea-dwelling crocodylomorphs. It existed before the two major thalattosuchian groups, the paddle-limbed Metriorhynchoidea and the long-snouted Teleosauroidea, split, and it may be the ancestor of both. While only parts of the skull are known, its shape suggests a narrow snout that was not as long as that of later thalattosuchians. Only one osteoderm is known, and it is oval with a slight keel down the middle, believed to have been attached to the tail. As humble as this small pseudosuchian was, it helped us fill in the major gaps of our understanding of thalattosuchia, and finally gave us an idea of their origin. Previously, all we knew was that the highly-derived metriorhynchoids and the more crocodile-like teleosauroids were related, lived alongside each other, and seemingly appeared in the Jurassic. While not as well known as the icthyosaurs, plesiosaurs, and mosasaurs, thalattosuchians were very successful sea crocodylomorphs until around the Early Cretaceous, and they owe their success to this little pseudosuchian who decided to return to the sea.

Found in the Charmouth Mudstone Formation of England, Turnersuchus would have lived alongside a variety of ammonites like Phricodoceras and Gemmellaroceras, as well as icthyosaurs like Leptonectes. It likely fed on the small fish that lived here. As it was not as highly adapted for ocean life as its descendants, it may have spent some time basking on land, where it could have come across early dimorphodontid pterosaurs and the armored Scelidosaurus.

[typhlonectes]

A new early diverging thalattosuchian (Crocodylomorpha) from the Early Jurassic (Pliensbachian) of Dorset, U.K. and implications for the origin and evolution of the group

Eric W. Wilberg,Pedro L. Godoy,Elizabeth F. Griffiths,Alan H. Turner &Roger B. J. Benson

ABSTRACT

Among archosaurs, thalattosuchian crocodylomorphs experienced the most extensive adaptations to the marine realm. Despite significant attention, the phylogenetic position of the group remains uncertain. Thalattosuchians are either the sister-group to Crocodyliformes, basal mesoeucrocodylians, or nest among longirostrine neosuchians. 

The earliest definite thalattosuchians are Toarcian, and already possess many synapomorphies of the group. All phylogenetic hypotheses imply a ghost lineage extending at least to the Sinemurian, and a lack of older or more plesiomorphic forms may contribute to the uncertain phylogenetic placement of the group. 

Here we describe a new species, Turnersuchus hingleyae, gen. et sp. nov., from the early Pliensbachian Belemnite Marl Member of the Charmouth Mudstone Formation (Dorset, U.K.). The specimen includes partially articulated cranial, mandibular, axial, and appendicular elements. It can be attributed to Thalattosuchia based on the following features: distinct fossa on the posterolateral corner of the squamosal; broad ventrolateral process of the otoccipital covering the dorsal surface of the quadrate; large supratemporal fenestrae lacking a flattened skull table; broadly exposed prootic; orbital process of quadrate lacking bony attachment with the braincase. This specimen represents the earliest thalattosuchian currently known from diagnostic material. 

Phylogenetic analyses of two published datasets recover Turnersuchus as the earliest diverging thalattosuchian, and sister to Teleosauroidea + Metriorhynchoidea. Bayesian tip-dating analyses suggest a Rhaetian or Sinemurian divergence of Thalattosuchia from other crocodylomorphs, depending on topology, with confidence intervals spanning from the Norian to the Pliensbachian. 

The new specimen extends the fossil record of Thalattosuchia, but the time-scaling analyses demonstrate that a significant ghost lineage remains.

Read the paper here:

https://www.tandfonline.com/doi/abs/10.1080/02724634.2022.2161909