Placodonts are by far one of the most interesting if less diverse clades of Triassic sauropterygian (a group including the far more diverse plesiosaurs and ichthyosaurs). These taxa include the relative basal Placodus and Paraplacodus, but the more interesting taxa are by far the Cyamodontoidea. Today, I take a look at one, Placochelys placodonta (Jaekel, 1902).
Based on a host of specimens, Placochelys placodonta’s name gives you a shortcut to its meaning: “plate-toothed, plate turtle.” It was broad, relatively shallow, and the teeth are extremely distinctive as flat, broad “plates.” It had only two mandibular teeth, three marginal upper teeth, and two large palatal teeth.
Among toothed placodonts, only Henodus chelyops has fewer teeth, with just one in each jaw for a total of four (Placochelys placodonta has 14) (Rieppel, 2001). As strange as placodonts go, Placochelys placodonta is not the oddest, although it comes close. Like other cyamodontoids, which comprise the bulk of placodonts, the teeth have been expanded broadly into hockey-puck-shaped “plates” with shallow and broad roots constricted at the margin between root and crown, and the rostrum is offset from the rest of the jaw margin as a thin spline. While this region is toothed in some cyamodontoids, it is not in the derived placochelyidans, including the shallower-skulled Psephoderma alpinus. A thin sheet of enamel covers each tooth, which is almost completely comprised of dentine, and while the rostral teeth show development of a lateral cusp around the rim, these are absent in the mesial teeth of the palate. This implies that, while the jaw may have had something of an “edge” to it, it was mostly peripheral and the large portion of the dental array is contained in the deep interior of the maw.
Placochelys placodonta has an extremely deep mandible, perhaps one of the deepest, owing largely to the extreme development of its coronoid process. This element, which is made of the dentary and surangular with a medial contribution of the coronoid bone in most placodonts, is made almost entirely of the coronoid in Placochelys placodonta.
This coronoid process is deeply etched with muscle attachment sites, and like many turtles, would have anchors the jaw closing adductor musculature both medially and laterally; in addition, the dentition is amazingly inset from the lateral jaw margin by clear lateral ridges on both upper and lower jaws, mimicking the inset buccal margins in mammals. This may imply that the jaws had a more rostral muscular set-up as in mammals, or a pseudomasseter as in birds such as parrots. Moreover, the rostrum is very narrow and does not contact its opposite on the lower jaw, implying that such soft tissues around the jaw may have contributed to the facial anatomy in a way that simply slapping “skin lines” around the head would not have possibly correlated to anything truly lifelike.
A lifelike Placochelys! While the restoration of the dermal armor follows what has been preserved, with very large nodes surrounded by a “matrix” of finer, granular tubercles, the limbs have been reconstructed as approximating sea-turtles with larger claws, and the preservation may not favor this precisely. Moreover, it has been proposed (Diedrich, 2010, 2011) that placodonts fed of seaweed and other types of shallow marine algae, which are non-silicaceous and plant-like, and thus required less teeth and more “sucking” power to consume. As a comparison, sea-plant feeders such as manatees and dugongs were considered to be comparable.
I will get around to the “sucking” hypothesis in a bit, when I discuss Henodus chelyops. There, I will attempt to evaluate (somewhat) the algal-feeding or sucking feeding hypotheses. Until then, note that large, broadened flat teeth typically only occur in animals that involve a large amount of mashing in their diet, effectively pulverizing plant food. “Suckers” and algal feeders, due to also reducing their dentition, tend to have a higher emphasis on certain jaw muscles, less on others, and their teeth tend to be either so underused for feeding they are “degenerate,” or are specialized for acquisition rather than processing. In manatees, for example, the teeth are low-crowned, broad, flat and high-cusped, teeth useful for tearing plant matter, but not highly effective at it (Dinocerata, Eocene stem-artiodactyls, have similar “primitive” dentition).
Something funny is going on in the skull of cyamodontoids, especially with the slender snout-tip, the possible presence of “cheeks,” and the reinforced “spout” of the mesial mandible. The jaw adductors were very, very large, and coupled with an expanded pterygoid flange of the palate developed extremely strong bite force. I am generally displeased with the “algal-feeding” hypothesis, but I’ll get to that down the road.
Previously: The teeth for the job. The Nitpicker.
Up next: We’re making cookies!
Update: As it turns out, we’re not making cookies … yet. The post is taking somewhat of a backseat, so instead, we’re gonna go in a less-culinarian fashion.
Up next: The strange case of Dr. Masiaka and Mr. Vicious.
Diedrich, C. G. 2010. Palaeoecology of Placodus gigas (Reptilia) and other placodonts — Middle Triassic macro-algae feeders in the Germanic Basin of central Europe — and evidence for convergent evolution with Sirenia. P3: Palaeogeography, Palaeoclimatology, Palaeoecology 285:287-306.
Diedrich, C. G. 2011. Fossil Middle Triassic “sea cows” — placodont reptiles as microalgae feeders along the north-western Tethys coastline with Pangaea and in the Germanic Basin. Natural Science 3(1):9-27.
Jaekel, O. 1902. Ueber Placochelys n. g. und ihre Bedeutung für die Stammesgeschichte der Schildkröten [On Placochelys, n. g., and its significance for the phylogeny of turtles]. Neues Jahrbuch für Mineralogie, Geologie und Palaontologie, Abhandlungen 1:127-144. [in German]
Rieppel, O. W. M. 1993. The cranial anatomy of Placochelys placodonta Jaekel, 1902, and a review of the Cyamodontoidea (Reptilia, Placodonta). Fieldiana, New Series 45:1-104.