The Dog Days of Dissorophids, Week 1: Cacops

We're really in the dog days of summer up here in Toronto (remind me not to move somewhere with humid summers for my next job), so I'm putting a spin on this for the next couple weeks with a series I'm calling "The Dog Days of Dissorophids," where I talk about my actual dissertation study system for once instead of waxing poetic about metoposaurids. This may or may not be foreshadowing an in press publication as well...

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What's a dissorophid?
To kick off this thematic series of blog posts, we first have to discuss what a dissorophid is. Dissorophids are part of a clade called Dissorophoidea. It's very easy to mix up 'dissorophid' and 'dissorophoid,' including when you've added both of them to your computer's dictionary. Usually what I tell people is that 'dissorophoid' has more letters, so it is the more inclusive (more all-encompassing) taxonomic group. Dissorophids show up in the fossil record in the Late Carboniferous when temnospondyls start really taking off (along with other tetrapod groups). They stick around until the late Permian, but they really hit their peak in the early Permian of North America - they're pretty sparse in other places for some reason. One of their most distinctive features (among other things), is the presence of osteoderms (bony armour plates) along the backbone. This isn't necessarily protective armour in the same way that some other animals have very extensive coverings for protection (e.g., crocs, ankylosaur dinosaurs), and some work (Dilkes & Brown, 2007; Dilkes, 2009) has suggested that based on the association with only the backbone, dissorophid armour was for stiffening the backbone to prevent too much swaying when walking (inefficient on land). Osteoderms are very diverse among dissorophids, a theme that I'll revisit throughout this topical series. Dissorophids were definitely terrestrial (the total opposite of my favourite metoposaurids) and probably ran around terrorizing smaller tetrapods. Dissorophids aren't gigantic - most of them had skulls in the 5-10 cm range, which would put their bodies around 15-40 cm - but that's still larger than a lot of modern amphibians!

Photographs of Cacops aspidephorus showing the osteoderms (marked by 'e' in this figure by Dilkes & Brown, 2007).

Mounted skeleton of Cacops aspidephorus at the Field Museum in Chicago. Photo: Bryan Gee

A brief history of Cacops
Cacops is a fairly old (in a historical sense) dissorophid, particularly for North America. It was described in 1910 by the American palaeontologist Samuel W. Williston in a paper creatively titled "Cacops, Desmospondylus; new genera of Permian vertebrates" ("Desmospondylus" was some form of reptile that was almost immediately recognized to be the same animal as the already described stem amniote Seymouria the following year). The first species was Cacops aspidephorus, named from a dense bonebed from the early Permian of Texas that was aptly named the Cacops Bonebed. You can find the skeleton of this animal on display at the Field Museum in Chicago alongside other early Permian temnospondyls. For 99 years, this was the only known species of Cacops (not that unusual). Then in 2009, a second species from the early Permian Richards Spur locality in Oklahoma was named by my advisor, Robert R. Reisz, Canadian palaeontologist Jason Anderson (University of Calgary), and German palaeontologist Rainer Schoch (Staatliches Museum für Naturkunde Stuttgart): Cacops morrisi. A third species was named in 2012, also from Richards Spur, by German palaeontologist Nadia Fröbisch (Museum für Naturkunde) and Robert: Cacops woehri​.

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Photographs of two skulls of Cacops morrisi in right lateral and dorsal profiles. Scale bar = 1 cm. The larger specimen on the left is the holotype. Figure from Reisz et al. (2009).

How do you pronounce that name?
I'm honestly not sure - I hear both 'cake-ops' and 'cak-ops.' I'm sure someone who reads this blog will know based on the purported etymology of the name ('blind face').

Here's an awesome reconstruction of Cacops about to chow down on a captorhinid reptile that my buddy Brian Engh did back in 2017.

What makes Cacops special?
​There's a lot of dissorophids, so many of them are distinguished by unique combinations of fairly esoteric anatomical features, rather than a large number of singular unique features (autapomorphies). All species of Cacops have long skulls, with the back end being proportionately short. They have a pretty large otic notch (where the ear would have been) with a fairly unique architecture. Their osteoderms are short anteroposteriorly (front-back) and transversely (side-to-side). 

One other attribute (not diagnostic but special) is that we know of quite a lot of Cacops material. As the name suggests, the Cacops Bonebed was extremely productive, although it is unfortunately now underwater due to artificial damming of Lake Kemp, and there is only one other published specimen of Cacops aspidephorus not from that locality, so the future prospects in Texas seem rather slim... The Richards Spur site has continued to produce on the other hand. Fragmentary remains were first identified at the site by the late American palaeontologist John Bolt (e.g., Bolt, 1977) and later by Robert and Canadian palaeontologist Corwin Sullivan (Sullivan et al., 2000), but there was nothing that suggested that it was a different species from C. aspidephorus. Fast forward to today, and we're up to five complete skulls of Cacops morrisi and about 0.75 skulls worth of Cacops woehri​ (half of one skull and a back end of another) (Reisz et al., 2009; Fröbisch & Reisz, 2012; Fröbisch et al., 2015). Considering that many dissorophids are known from a holotype skull and maybe one or two more fragmentary specimens, this is pretty substantial! Why does having a lot matter? It gives palaeontologists a better idea of variation, for example that which occurs during development of the animal. If you only have one, and the animal is long-dead, you can't exactly make many inferences about how it grew. We have a bunch of variably sized Cacops ​that help us characterize growth and thus use it as a reference for other dissorophids that might have had similar development but that don't have enough specimens at the moment to know. 

Reconstruction of Cacops aspidephorus by Williston (1914).

The first photograph I took of the two Cacops blocks fitting together at like 8 PM in my lab in March 2017. Fittingly, it's sitting on Bob Carroll's 1964 study of dissorophids. Photo: Bryan Gee.

A tale of two dissorophids
When I moved to Canada, one of the first things I started working on was this pair of blocks, each with a complete skull of Cacops morrisi. One was articulated with the front half of the body, and the other had a nearly complete backbone lying on top of it. This was some of my first exposure to the Richards Spur locality, and these specimens were AMAZING. I spent the first couple months of my PhD studying these (in parallel with metoposaurid projects of course), during the course of which I did a little bit of prep on each of them. Lo and behold, there was actually more Cacops in each block. The first block (we'll call it block A) with the articulated skeleton had a snout on it, and the second block (block B) with the full backbone had part of a postorbital (behind the eye) region. Cool, no problem, add it into the description. It was late one night (like 8 PM, so not that late, but the sun sets at 5 in March up here, so it was uncomfortably dark outside), and I was putting together the last touches on my manuscript. I wrote the sentence, "block A has a complete skull and a partial skull with the pre-orbital region, and block B has a complete skull and a partial skull with the postorbital region." My thoughts then proceeded in the following fashion:

• "Haha what a coincidence, the front half of one skull on this one, and the back half of another skull on this other one."
• "Wait...."
• *double checks that each block has its own specimen number* (they did)
• "No way..."
• *sticks blocks together in a perfect fit* -> MIND EXPLOSION
• *goes to senior PhD student* "Mark....these fit together right?"
• *MIND EXPLOSION for the next 12 hours*

I honestly thought this was some kind of set up - that everybody else had known all along that these blocks fit together and it was some weird test to see whether I could put it together (literally and figuratively). It was not apparently The punchline? Robert got those blocks from different people, and he didn't know they fit together either! Although the fit is tight between blocks (i.e. there wasn't a lot lost on the edges of the blocks), it was only with the exposure of the skull that's broken in half between the two blocks that there was a clear connection; there wasn't really much other fossil spanning the break that would have keyed us into these blocks fitting together. Again, they had distinct specimen numbers too, which usually I've not questioned the reliability of...

Given all of this, the next day, we pulled out all of the large blocks we had from Richards Spur and tried to jigsaw them together. We ended up finding one more fit that comprised a large headless non-Cacops dissorophid, again made of two blocks with different specimen numbers and that came from different people! You can read about that specimen (which also has a Cacops ​butt preserved on it) in Gee & Reisz (2018b in Fossil Record).

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Nice cleaned-up photo of the Cacops block (figure from Gee & Reisz, 2018a). Scale bar = 3 cm.

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Refs

• Bolt JR. 1977. Cacops (Amphibia, Labyrinthodontia) from the Fort Sill Locality, Lower Permian of Oklahoma. Fieldiana: Geology 51:235-249. [link]
• Dilkes DW. 2009. Comparison and biomechanical interpretations of the vertebrae and osteoderms of Cacops aspidephorus and Dissorophus multicinctus (Temnospondyli, Dissorophidae). Journal of Vertebrate Paleontology 29(4):1013-1021. doi: 10.1671/039.029.0410
• Dilkes D, Brown LE. 2007. Biomechanics of the vertebrae and associated osteoderms of the Early Permian amphibian Cacops aspidephorus. Journal of Zoology 271(4):396-407. doi: 10.1111/j.1469-7998.2006.00221.x
• Fröbisch NB, Reisz RR. 2012. A new species of dissorophid (Cacops woehri) from the Lower Permian Dolese quarry, near Richards Spur, Oklahoma. Journal of Vertebrate Paleontology 32(1):35-44.​ doi: 10.1080/02724634.2012.633586
• Fröbisch NB, Brar A, Reisz RR. 2015. New specimen of Cacops woehri indicates differences in the ontogenetic trajectories among cacopine dissorophids. Fossil Record 18(1):73-80. doi: 10.5194/fr-18-73-2015
• Gee BM, Reisz RR. 2018a. Cranial and postcranial anatomy of Cacops morrisi, a eucacopine dissorophid from the early Permian of Oklahoma. Journal of Vertebrate Paleontology 38(2):e1433186. doi: 10.1080/02724634.2018.1433186
• Gee BM, Reisz RR. 2018b. Postcrania of large dissorophid temnospondyls from Richards Spur, Oklahoma. Fossil Record 21(1):79-91. doi: 10.5194/fr-21-79-2018
• Reisz RR, Schoch RR, Anderson JS. 2009. The armoured dissorophid Cacops from the Early Permian of Oklahoma and the exploitation of the terrestrial realm by amphibians. Naturwissenschaften 96(7):789. doi: 10.1007/s00114-009-0533-x
• Sullivan C, Reisz RR, May WJ. 2000. Large dissorophoid skeletal elements from the Lower Permian Richards Spur fissures, Oklahoma, and their paleoecological implications. Journal of Vertebrate Paleontology 20(3):456-461. doi: 10.1671/0272-4634(2000)020[0456:LDSEFT]2.0.CO;2
• Williston SW. 1910. Cacops, Desmospondylus; new genera of Permian vertebrates. Bulletin of the Geological Society of America 21(1):249-284. doi: 10.1130/GSAB-21-249
• Williston SW. 1914. Water reptiles of the past and present. University of Chicago Press; 251 p. [link]