For the second week of the Dog Days of Dissorophids, I'm covering the eponymous Dissorophus! If you didn't catch last week's post with the first entry in this topical series, pop over there to read the little intro blurb on what a dissorophid is so you're up to date for this week's post. It is a veritable batrachian armadillo. -E.D. Cope on Dissorophus A brief history of Dissorophus Dissorophus was described even earlier than Cacops (1895 vs. 1910) and owes its name to the famed American palaeontologist Edward Drinker Cope (he of the Bone Wars), who deemed it "a batrachian armadillo" (the literal title of the paper naming Dissorophus). It was named for the first, and presently the only, species, D. multicinctus. Subsequent taxa named by Cope (two species of "Otocoelus" and "Dissorophus articulatus" are now synonymized with D. multicinctus. It may be of interest to readers that Otocoelus was originally described as an ancestor of turtles with a remarkable degree of convergence with Dissorophus (Cope, 1896). Longiscitula houghae, named by American palaeontologist Robert DeMar in 1966, was also synonymized with D. multicinctus (Milner, 2003). A second species of Dissorophus, D. angustus, was named by Canadian palaeontologist Robert Carroll in 1964. Schoch & Milner's (2014) encyclopaedia places this as a synonym of a closely related dissorophid, Broiliellus brevis, which was collected from the same site and named immediately before D. angustus in the same paper by Carroll.
Dissorophine-type osteoderms Within Dissorophidae are a number of subfamilies. Dissorophinae and Eucacopinae (classically Cacopinae) are the oldest, but there is also an Aspidosaurinae and a Platyhystricinae that Schoch & Milner (2014) recently defined. There are only two genera of dissorophines - Dissorophus (1 species) and Broiliellus (5 species). One of the defining features of dissorophines is their unique style of two series (external and internal) of osteoderms (although note that osteoderms are totally unknown in some species of Broiliellus, and others apparently only have one set of osteoderms, so maybe it's not really that defining). Eucacopines like Cacops also have two series, but they are different than in dissorophines. In dissorophines, the internal series (the one closer to the spines) remains unfused to the spine (they fuse in eucacopines) and has a prominent ventral flange that inserts between adjacent spines (eucacopines either lack ventral flanges on the internal series or the flange inserts into the spine to form the initial contact). You can see this ventral flange in #4-5 below. Dissorophines don't have any ventral flanges on the external osteoderms, so that series was mostly attached to the internal series by soft tissues. As a result, a number of dissorophine specimens, including the above one from Richards Spur, have clean separation of the internal series from the external series. In eucacopines, the external osteoderms have a ventral flange that inserts between adjacent internal osteoderms - this isn't a tight contact, but it's more complex than dissorophines (which have nothing). This changes some aspects of trunk flexibility, which is compared in Dilkes (2009). Photographs and digital renderings of some isolated osteoderms and neural spines of Dissorophus from Richards Spur. 1, photograph in dorsal view; 2, rendering in dorsal view; 3, rendering in ventral view; 4, rendering in left lateral view; 5, isolated rendering of the internal osteoderm; 6-7, isolated renderings of the neural spine. Figure from Gee et al. (2019). If you read last week's post, there was a fun story about me fitting two Cacops blocks together. Sorry, no such story for Dissorophus. There is quite a lot of Dissorophus at Richards Spur, but peculiarly, almost all of it is just isolated osteoderm fragments. The most complete skull material only consists of the back right corner of a skull, in stark contrast to the seven complete skulls we have of Cacops morrisi. I don't see too many cranial fragments of Dissorophus either (ornamentation and teeth make it easy to ID vs. Cacops), so it's not like the skulls just exploded and scattered to the four corners of the cave system. That being said, Dissorophus is a pretty well-known dissorophid, with a fair bit of material having been collected from the Texas red beds. Unlike Cacops, it is known from a fair number of localities, which makes the prospects for future work a little better. DeMar (1968) described essentially all parts of the skeleton from a number of different specimens, so it is arguably as well known as Cacops if not slightly better. Refs
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