Wednesday Wonders: Chroniosuchus

Good tidings and well-wishes!

I’ve recently realized that my blog has just hosted three straight ‘Wednesday Wonders’ entries featuring synapsids. While everyone loves a good obscure dinocephalian, xenarthran, or artiodactyl, I think that I’ll give the class a rest for a while. As I’ve said elsewhere, when compared to fossilized reptiles (and in particular everyone’s favorite group of ancient chicken relatives), prehistoric amphibians receive very little attention from both the media and academia in comparison. This is truly unfortunate, given the fact that these animals have undergone a fascinating evolutionary history through their 365 million year reign and especially because of the growing amount of disturbing evidence which reveals beyond any shadow of doubt that modern amphibians are suffering enormous amounts of fatigue and decline on a global scale. For these (and other) reasons, I’m ecstatic to hear of Robert Carroll’s new book “The Rise Of Amphibians: 365 Million Years Of Evolution“. Like many of you (I’m sure), I’ll be purchasing my copy in the near future (hopefully after the price goes down a wee bit on Amazon). Until then, however, I’ll satisfy my amphibious needs by discussing an interesting genus known as Chroniosuchus.

Chroniosuchus sp.

As always, it’s necessary to begin with a bit of phylogenetic information regarding this week’s animal before moving on to the ‘fun stuff’. The Chroniosuchus species were anthracosaurs which inhabited modern Russia from the latest Permian to the middle Triassic. Anthracosaurs were once considered to have belonged to the obselete “Labyrinthodontia” order. However, recent revisions concerning the cladistic relationships of early amphibians has placed the family Anthracosauridae within the superorder Reptiliomorpha and specifically inside of the order Embolomeri. It’s been suggested that anthracosaurs were closely akin to the order Seymouriamorpha  due to several shared anatomcal characteristics, though many authors have questioned this and attributed their similarities to congruent evolution. In “The Age Of Dinosaurs In Russia And Mongolia”, Igor Novikov, Mikhail Shishkin and Valerii Golubev write:

“Anthracosaurs were crocodile-like piscivorous amphibians with a rather deep elongated skull from 50 to 500 mm long. During their history, they retained a resemblance to the most primitive temnospondyls in such characters as the extent of the lacrimal bone up to the naris(or septomaxilla), presence of an intertemporal [(a bar of bone separating the upper and lower temporal fenestrae in reptiles)] in most forms, movable basipterygoid articulation (between braincase and upper jaw), narrow interpterygoid vacuities,pterygoids with median [(as in towards the animal’s midline)] contact anteriorly, a single concave occipital condoyle, occipital exposure of the opisthotic bone, and, usually, absence of retroarticular process on the lower jaw. Peculiar for anthracosaurs also is the clear demarcation [(boundary)] and loose joint between the skull roof and cheek, a condition inherited from rhipididtian fishes.”

Other features sported by anthracosaurs as noted by these authors were a lack of contact between the exoccipital and the skull roof, notably long tabular horns, comparatively narrow vomers, the presence of five digits on the forelimbs (as opposed to four in temnospondyls), and a gastrocentrous (reptile-like) spinal condition in many genera.

Chroniosuchus was, shockingly, a member of the Chroniosuchia suborder, which is divided into two families: the Chroniosuchidae and the Bystrowianidae. According to Novikov et al., “They are readily distinguished from older groups by a row of dermal plates over the vertebral column, conspicuously ball-shaped [intercentrum] in most forms, fenestration (presence of holes) of the skull roof (at least in the Chroniosuchidae) and some advanced characters in the skull roof pattern.”

Chroniosuchus sp. reconstruction courtesy of Wikipedia.

Unlike many earlier and contemporary amphibians, Chroniosuchids lacked a well-developed lateral line system, which indicates that these creatures likely inhabited a largely terrestrial or semiaquatic niche. The teeth of these animals were rather conical and pointed and typically displayed a slight infold at their bases. Chroniosuchids traditionally displayed additional “tusk-like teeth on their vomers, pairs or groups of small tusks on palatines and ectopterygoids.” (Novikov et al.)

However, one of the group’s most striking features was a row of sculptured osteoderm plates which ran down their backs each of which was connected (by a ligament-like structure) to its underlying vertebra via the neural arch. Adding to the intricacies accompanying the spinal cords of these beasts was the fact that their pleurocentra were amphiocoelous (concave on the anterior and posterior ends) and were, in many forms, stutured to the neural arches, and often indistinguishably so.

As for Chroniosuchus itself, there isn’t much else worth commenting on other than that two species are known: C. paradoxus and C. levis, each of which dwelled in the upper Permian (their more derived ancestors and cousins went on to inhabit the Triassic). Hopefully further research will reveal a great deal more about these and other fascinating amphibians!

May the fossil record continue to enchant us all!

UPDATE: Michael of “The Life Of Madygen” has informed me that he has been working on chroniosuchians of late and that the group is currently the subject of several research efforts. To view his remarks, check out the comment section below.