As I mentioned in another post on placodonts, these armored, vaguely turtle-like archosauromorphans can get pretty odd. Most investigation of placodont biology has been superficial, which is to say exterior examination or at the least histological work on the limb bones of various specimens, or of the armor, or through the odd, oblate and flat teeth they all possess. Placodonts have secondarily closed the laterotemporal fenestra, but retain a large open supratemporal fenestra. They have robust jaws, and the coronoid process is both large, tall but also anteriorly placed, increasing the leverage. Like some mammals and even some dinosaurs, the dentition can expand medial and caudal to the rostralmost adductor muscles, which allows maximal power at the posterior flattened palatal teeth.
But placodonts are notable for several other things, including dermal armor and a turtle-like body plan in the cyamodontoid radiation (which includes Henodus chelyops).
However, while a lot of work on placodonts has been done to describe their general anatomy, and even external braincase anatomy (with work also on endocasts), little work has been done to develop a deeper knowledge of the cranial structure due to the tight interlocking of various elements, and the obscurity of the lateral braincase to examination via the plate of the “cheek” fromed from various suspensorial and circumorbital bones. James Neenan and Torsten Scheyer (2012) have recently presented work on the braincase of the basal placodont “placodontoid” Placodus gigas, and in the circumstance were also able to examine the inner ear, which resolves certainly several issues, such as likely cranial attitude and orientation, as well as pegs this animal among marine-going reptiles.
First, Neenan and Scheyer recovered an inner ear with distinct semicircular canals (A, above), and following work by others associating the orientation of the lateral semicircular canal, suggested that the skull would be oriented around 20° from the horizontal, shown in Placodus gigas specimen UMO BT 13, above: A-C, D1. In D, the red line represents the horizontal from which the skull is declined, adjusted to the distal quadrate and placed in the articular cotylus; in D2, the jaw is oriented is relation to my arguments of “neutral” orientation for mandibles where the two coordinates are the vertex of the articular cotylus and the posteriormost point of occlusion of the jaw to the skull (here at the distalmost flat tooth) — grey outlines represent silhouettes of the skull. The orange outline in C represents an hypothesis for how much flesh might surround the skull and jaw, and thus a comparison to the skull itself and how “skin-wrapped” reconstructions may miss the mark in reasonable levels of soft-tissue.
Placodonts presents dorsally positions orbital fenestrae in broad skulls, which effectively reduces the lateral exposure of the eye. This has been suggested to support a bathyal habitus for placodonts, but with orientation of the head downward to such a degree as Neenan and Scheyer argue, facial attitude is fairly forward-oriented, suggesting instead that the eyes are well-suited for binocular vision. This may further suggest that placodonts (the whole lot of ‘em) were visual foragers. This is not to suggest that olfaction or other forms of chemosenses were not utilized: the relative size of the nasal vestibulum, as well as the size of the olfactory bulbs and exit of cranial nerve
21 (olfactory nerve) [edit: damn, that was a horrible mistake: changing cn "2" to "1"], cannot be recovered due to lack of preservation of the anterior sphenoid bones in the specimens examined by Neenan and Scheyer. This tells us that we may simply not have the information required to assess the question of olfaction. However, due to the size of the nasopharyngeal canal (extending over 50% of the skull’s length) it may be possible that olfaction was utilized; I suggest, though, that visual prey detection, predatory avoidance, and spatial foraging was primarily visual.
Head attitude has implications for other placodonts. Neenan and Scheyer assessed the most basal known placodonts, but the cyamodontoids (representing the rest of Placodontia) show turtle-like “psuedoshells” with torsos much, much broader than they are deep — and in Henodus chelyops, this reaches an extreme. Placodonts typically have short limbs for their size, so that unlike other aquatic archosauriforms, a primary aquatic propulsive limb is absent in comparison to body size and morphology: due to the “shell,” placodonts probably did not exhibit lateral undulation, and thus locomotion was probably tail/limb-based, yet the limbs remain small, suggesting limited foraging requirements while in water. This further suggests that placodonts may have, like marine iguanas or such, spent a good deal of their time on land, but foraged in water. I know, I’m stretching things here.
A comparison of placodont body types:
Placodus gigas (Agassiz, 1843) with semi-mobile torso and dermal armor in the form of tubercles;
Psephoderma alpinus (von Meyer, 1858), with more rigid torso developed into a two-part “shell” possibly permitting some lumbar flexibility; and
Henodus chelyops (von Huene, 1938), with a very broad and likely rigid torso and “psuedoshell” and proportionally shortest limbs.
I’ve chosen to reconstruct Psephoderma alpinus with a narrow, pointed rostrum, similar perhaps to aetosaurs, but without much to base this on. I am unsure how much, if much at all, soft-tisse might alter the gross appearance of the head. In wither lateral or dorsal view, the snout seems very odd in its “spike-like” rostrum. I’ve added in sauropsidan “lips,” but have limited their extent to preserve the rostrum. I suspect, but cannot confirm, that the snout was adapted to crevice probing or, as in some shorebirds, prying open valved molluscs or pulling out ammonites and other shelled animals. Low mobility of the spine and short limbs suggest that pursuit predation was likely limited. Henodus chelyops, on the other hand, is another matter.
There’d be a lot to say on this guy; things that, say, Darren Naish said over at Tetrapod Zoology or in his Geology Today review in 2004 has already said. But there is more, including a suggestion that, rather than a long bank of filter-feeding features of the jaws, Henodus chelyops was a suction feeder. Features alluding to this are numerous, including well-developed hyoid apparatus, a broad, flat rostral “face,” and a high vaulting of the palate with short-deep jaws with rostral insertions on the mandible for the mandibular adductor musculature which would lead to a sort of “pseudocheek” and provide a large oral cavity. It is quite likely then, like the matamata Chelus fimbriatus, it exhibited ram-suction feeding, but then also like the extant walrus Odobenus rosmarus or the vaguely walrus-like cetacean Odobenocetops peruvianus (de Muizon, 1993) would have been a hunter of buried, sessile prey. An absence of strong crushing teeth (limited to a small pair of elongated, flat teeth on each jaw) suggests indeed that little oral processing occurred, and that food was swallowed directly — even if it was strained from the water through baleen-like plates fixed within grooves in upper and lower jaws.
These reconstructions offer an odd appearance, one in which the wizened, “hideous” face is augmented by substantial soft-tissue and covered the sides of the jaw. This is an untested hypothesis, however, and is not to be taken at face value. [*chortle*]
Placodonts represent a small clade (~8 “genera”) which nonetheless represent a diverse — and in a short time during the Triassic, very plastic and adaptive — group, ranging from the marine but nonpelagic and near-coast or lagoonal bathyal feeders, and I suspect may have been primarily terrestrial with primary aquatic foragers. Diedrich (2011) suggested that most, if not all placodonts were algal feeders, either scraping the foodstuffs from the seafloor or surfaces, or by consuming suspended “weed,” with broad comparison to manatees. I commented on this hypothesis in the previous post, and will not continue further here. One must wonder, had these taxa persisted into the Jurassic, they might have diversified further; yet, sadly, it seems that many other aquatic sauropsidans, chief among them the sauropterygians (ichthyosaurs and plesiosaurus: elasmoosaurs, pliosaurs) merely outcompeted them, or that their food resources, perhaps protected by the Tethys Sea, diminished as the Tethys opened up with the initial break up of Pangaea. Nonetheless, these taxa represent an interesting study of a group that were chiefly durophages, produced several oddities, and pose very interesting questions.
Agassiz, L. 1843. Recherches Sur Les Poissons Fossiles [Study of the Fossil Fish]. Tome I (livr. 18). (Imprimerie de Petitpierre, Neuchatel.)
Diedrich, C. G. 2011. Fossil Middle Triassic “sea cows” — placodont reptiles as macroalgae feeders along the north-western Tethys coastline with Pangaea and in the Germanic Basin. Natural Science 3(1):9-27. [PDF]
Naish, D. 2004. Fossils explained 48: Placodonts. Geology Today 20(4):153-158.
Neenan, J. M. & Scheyer, T. M. 2012. The braincase and inner ear of Placodus gigas (Sauropterygia, Placodontia)—a new reconstruction based on micro-computed tomographic data. Journal of Vertebrate Paleontology 32(6):1350-1357.
Rieppel, O. 2001. The cranial anatomy of Placochelys placodonta Jaekel, 1902, and a review of the Cyamodontoidea (Reptilia, Placodonta). Fieldiana Geology (new series) 45:1-104.
von Huene, F. 1932. Die fossile Reptile-Ordnung Saurischia, ihre Entwicklung und Geschichte [The fossil reptile order Saurischia, their development and history]. Monographien zur Geologische und Paläontologische (parts I, II; series I) 4:1-361.
von Meyer, C. E. H. 1858. Psephoderma alpinum, aus dem Dachsteinkalke der Alpen [Psephoderma alpinum, from the Dachstein Limestone of the Alps]. Paläontographica 6:246-252, pl.xxix