A tribute to Robert L. Carroll (1938-2020)

Photo of Bob Carroll from his 2003 festschrift, original from his wife, Anna Di Turi.

I never got the chance to meet Bob Carroll, a giant in vertebrate paleontology and arguably the most influential and foundational Canadian paleontologist. Bob was best known among the broader paleo community for his landmark textbook "Vertebrate Paleontology and Evolution", which was first published in 1987 as a follow up to Romer's seminal 1966 text and which was used by perhaps tens of thousands of students in paleontology courses around the world. He was a prolific publisher on early tetrapods, including temnospondyls, 'microsaurs,' and other Paleozoic tetrapods from North America and Europe, and much of his work laid the foundations for ongoing work in the 21st century. By the time I moved to Canada for my PhD, Bob's health was in decline for a number of years, and last week, he unfortunately passed away due to COVID-related complications, the latest revered member of our community to pass within the past few months (John Bolt, Jenny Clack). ​I wanted to dedicate this week's blog post to his legacy with a specific focus on many of the groups that he worked on that I find myself working on these days (at varying degrees of enthusiasm) . 

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My indirect ties to Bob are two-fold: (1) he was the PhD supervisor of my own advisor, Robert Reisz (as well as that of more or less all of the famous living Canadian paleontologists); and (2) he was one of the preeminent workers of both dissorophids and 'microsaurs' at a time when both really surged in notoriety for their proposed relationships to modern amphibians. I've read dozens of Bob's papers over the course of my dissertation and draw considerably on them for much of my research, so Many people who knew Bob personally have written beautifully about it (Twitter tributes collated here), including in the 2003 festschrift for him, and I won't try to re-summarize them here. Those with access to the Canadian Journal of Earth Sciences can find a brief summary of Bob's life from the festschrift here, otherwise there is conveniently a publicly accessible archived version of the same PDF here. The link for the entire festschrift with regular journal articles (subscription-only) is here.

The dissorophoid revolution

Prior to the mid-20th century, dissorophoids were "just another temnospondyl group," so to speak. Various representatives had long been known from different parts of the world, ranging from the terrestrial Acheloma/Trematops, Cacops, and Dissorophus from the Texas red beds of North America to aquatic branchiosaurids and micromelerpetids with remarkable quality and detail of preservation from former lakes of western Europe. However, their relationship to modern amphibians could not have been more understated. Virtually all aspects of the origin of modern amphibians, including how closely related the three clades (anurans, caudates, gymnophionans) truly were and the extinct group to which they were most closely allied remained hotly debated and without much consensus (Parsons & Williams, 1963 is a good historical review). While temnospondyls were long recognized as a possible ancestor, there were (are) many other groups generically termed 'amphibians,' and among the temnospondyls, there was no clear consensus as to which clade might hold the key to the flourishing of the modern lineages. As remains largely true now, the record of crown lissamphibians remains very patchy, and the timing of their origin remains largely unconstrained such that back in the 1950s, basically any temnospondyl known to humanity could have been fair game. For reference, the oldest frog, Czatkobatrachus is from the Early Triassic and was named in 1998, and the oldest caecilian, Eocaecilia is from the early Jurassic, by which point almost all temnospondyls were extinct, and was named in 1993.

The greatest credit for pointing the finger at dissorophoids as probable stem lissamphibians belongs to another luminary in vert paleo, John Bolt, who passed late last year. Like Carroll, Bolt also worked on dissorophoids for his doctoral thesis in the late 60s, and Bolt was the one who named Doleserpeton, the first temnospondyl to feature pedicellate, bicuspid teeth, long a feature considered to be unique to lissamphibians (Bolt, 1969, 1977). Nonetheless, the fruits of Bob's own dissertation have emerged as one of the most seminal works on dissorophoids (Carroll, 1964a), even if some of them emerged prior to the broader understanding of the import of dissorophoids for lissamphibian origins (before they were "cool"). "Early evolution of the dissorophid amphibians," about 80 pages of graphically detailed description includes what is still the most complete description of the amphibamiforms Amphibamus and Tersomius and descriptions of several new dissorophid taxa: Broilellus brevis, Brevidorsum profundum, Dissorophus angustus, and Conjunctio multidens (you can freely read this paper here). Even though Bob moved on from dissorophids soon after, this work would go on to become foundational (165 citations to date) for what became a remarkable renaissance in the study of dissorophoids (particularly dissorophids) just in the following two decades (e.g., Berman & Berman, 1975; Berman et al., 1981, 1985; Bolt, 1974a, 1974b, 1974c, 1977a, 1977b, 1979; Daly, 1983; DeMar, 1966a, 1966b, 1967, 1968; Eaton, 1973; Gubin, 1980; Olson, 1965, 1972; Vaughn, 1969, 1971)! I've myself have cited this work in many of my own dissorophoid studies (at least five off the top of my head).

Illustrations of the enigmatic Brevidorsum profundum from Texas, a taxon that not everyone even agrees is a dissorophid, from Carroll (1964).

Miscellaneous amphibian stuff

Unsurprising for someone long affiliated with the Redpath Museum, Bob published a fair bit on the Joggins temnospondyls (basically dendrerpetids; Carroll, 1967; Godfrey et al., 1987; Holmes et al., 1998). Much of this work advanced what remains a very confusing taxonomy of Joggins dendrerpetids (how many taxa are there really?) and includes some of the most complete and well-preserved specimens known from the famed stumps. Early in his career, Bob also published a paper naming a new species of the enigmatic Parioxys from Texas (Carroll, 1964b), which has recently begun to look an awful lot like a dissorophid (Schoch & Milner, 2014). He dabbled here and there in crown lissamphibians (e.g., Jenkins et al., 2007; Carroll & Zheng, 2012).  If you lump early tetrapods in as "amphibians," Bob also has a number of papers on broader tetrapod evolution (Carroll, 1992, 1993, 2001; Carroll et al., 2005). Lastly, Bob published a number of synthesis papers on amphibian evolution (Carroll, 1977, 2007; Carroll & Holmes, 1980, 2007; Schoch & Carroll, 2003), which arguably culminated in co-editing of a volume of the Amphibian biology series that focused on the paleontological record and origins of amphibians in 2000 and his book, "The Rise of Amphibians: 365 Million Years of Evolution," published in 2009.

'Microsaurs' and friends

Evidently temnospondyls weren't exciting enough for Bob because he moved on to other early tetrapods for the most part. Some of his most notable contributions were in the area of the eternally controversial 'microsaurs,' a group for which he provided both a great detail of anatomical descriptions and broader syntheses of their relationships to either modern amphibians or to reptiles. This even includes a paper that famed Alberta dinosaur expert Phil Currie was on back in the day (Carroll & Currie, 1975)! Carroll's works remain some of the most informative and up-to-date regarding the Joggins 'microsaurs,' (e.g., Carroll, 1963, 1966), a very confusing assemblage that was partially overhauled by myself and a large cohort of other Canadian early tetrapod workers in a forthcoming paper (Mann et al., in press), but he dabbled in 'microsaurs' really across their entire geographic and stratigraphic range (e.g., Carroll, 1990, 1991; Carroll & Baird, 1968; Carroll et al., 1991). Perhaps the most prominent among these contributions is the "holy grail" of 'microsaur' literature (aptly named "The Order Microsauria") that he co-authored with Pamela Gaskill in 1978 and that contains substantial amounts of new information (hardly a 'review' in any sense of it) and the Lepospondyli volume of the Handbuch der Paläoherpetologie [Handbook of Paleoherpetology] published in 1998. 

Illustration of an articulated skeleton of Batropetes fritschi from Carroll (1991).

Postcranial skeleton of the 'microsaur' Pantylus cordatus from Carroll (1968).

Illustrations of the type specimen of the brachystelechid 'microsaur' Quasicaecilia texana from Carroll (1990).

Holotype of Llistrofus pricei from Carroll & Gaskill (1978).

Particularly with respect to 'microsaurs,' Bob named a great many taxa, including but not limited to: the tuditanid Boii​, a taxon that periodically experiences a resurgence in the paleomeme community, the pantylid Trachystegos from Joggins, the gymnarthrids Cardiocephalus peabodyi from Richards Spur and Euryodus dalyae from South Grandfield, the hapsidopareiid Llistrofus, also from Richards Spur (the name is 'Fort Sill' backwards + -us), the possible hapsidopareiid Saxonerpeton from the Czech Republic, the ostodolepid Pelodosotis from Texas, the brachystelechid Quasicaecilia from Texas, and the enigmatic Utaherpeton (guess where that one's from). People familiar with my own research will of course know that I have published on material of both Llistrofus and Euryodus dalyae. It goes without saying that Bob's work is tremendously influential for anyone working on lepospondyls (he published on other groups as well), including the growing consensus that Lepospondyli is a  terribly polyphyetic grouping (e.g., Carroll & Chorn, 1995). Much of Bob's work on not-microsaur reptiles has also had a big impact on our interpretations of reptile origins and where 'microsaurs' fit into that (Carroll, 1964a, 1969a, 1969b, 1970, 1982). 

I definitely think that the paleomeme community needs to be aware of this animal. pic.twitter.com/VnmjqyyMI3

— Darren Naish (@TetZoo) March 14, 2020

Suffice it to say that Bob Carroll was an absolute legend and perhaps the single person most responsible for what is now a flourishing Canadian paleontological community with its own professional society (with a prize for best student talk named after Bob). His influence will be clearly felt for generations to come and his mark on the history of our discipline eternal.

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I'll be back next week for regular temno content and will make an earnest effort to keep it going weekly throughout this pandemic for the immediate future. Note that what is an extremely lengthy ref list for this week's blog is below.

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