The nesting of Atopodentatus unicus

The Triassic is known for its basal and bizarre reptiles, including Henodus, Tanystropheus, Longisquama, Lotosaurus and Hyperodapedon, among dozens of others.

Figure 1. Click to enlarge. Atopodentatus unicus, early Middle Triassic sister to Claudiosaurus and Largocephalosaurus.

Today we add Atopodentatus (Cheng et al. 2014, early Middle Triassic, WIBM SPC V 1107, Figs. 1-4) with a highly specialized feeding adaptation resembling that of a right whale.

Figure 2. DGS tracings of the second and third specimens of Atopodentatus in several views.

From the abstract:
“Its dentition is fence or comb-like and bears more than 175 pleurodont teeth in each ramus of the upper and lower jaws, tooth crown is needle-like distally and blade-shaped proximally; its rostrum strongly bends downward and the anterior end of its mandible expands both dorsally and ventrally to form a shovel-headed structure; and its ungual phalanges are hoof-shaped. The specializations of the jaws and dentition indicate that the reptile may have been adapted to a way of bottom-filter feeding in water. It is obvious that such delicate teeth are not strong enough to catch prey, but were probably used as a barrier to filter microorganisms or benthic invertebrates such as sea
worms. These were collected by the specialized jaws, which may have functioned as a shovel or pushdozer (the mandible) and a grasper or scratcher (the rostrum). Our preliminary analysis suggests that the new reptile might be more closely related to the Sauropterygia than to other marine reptiles.”

Figure 3. Anterior view of Atopodentatus rostrum.This is the right half of the rostrum as seen in anterior view. Rotate the image 90 degrees clockwise to get an accurate depiction. 

The most bizarre aspect of Atopodentatus is the anterior view of the rostrum. Unlike any other tetrapod, the premaxillae are medially split and filled with teeth that face one another. But these pseudo-jaws were immobile. These would have filtered sea water like the lateral needle-like teeth did.

Nice illustration here.

Figure 5. Atopodentatus compared to more primitive sister taxa, Adelosaurus and Claudiosaurus to scale.

The second most bizarre aspect of Atopodentatus is the extreme bend in the maxilla. As the mandible closed the tip raked up the ventrally directed portion of the maxilla.

Figure 4. Original nesting of Atopodentatus (yellow).

Originally Atopodentatus nested at the base of the Sauropterygia. In the large reptile tree Atopodentatus nests in roughly the same node, but between Claudiosaurus and the Saurosphargidae. It is the most basal taxon in the Enaliosauria to have elongated transverse processes. We’ll discuss other aspects of this bizarre aquatic reptile in future posts.

Figure 5 Subset of the large reptile tree: the marine younginiformes, including the Enaliosauria (Sauropterygia + Mesosauria + Thalattosauria + Ichthyosauria)

References
Cheng L, Chen XH,Shang QH and Wu XC 2014. A new marine reptile from the Triassic of China, with a highly specialized feeding adaptation. Naturwissenschaften. doi:10.1007/s00114-014-1148-4.