We the north: a brief history of Canadian temnospondyls

In honour of Canada Day, this week's blog post (delayed post since I'm in the field right now) covers the few (but notable) Canadian temnospondyls. Note that this is a brief, not a comprehensive history, considering there's quite a lot of articles spanning over 150 years and also that I wrote this in between a conference and packing for fieldwork (P.S. I'm in the field now, sorry about any errors or typos!).

A (relatively) long time ago...
....Richard Owen, yes the same one who coined the Dinosauria and named some of its first members, created a name for some fragmentary remains of a temnospondyl from the Joggins deposits of Nova Scotia. This temnospondyl, one of the better known Paleozoic ones, was named Dendrerpeton acadianum, the genus name being given for the unusual preservation of Joggins tetrapods in large stumps and the species name being given for the indigenous name for the Nova Scotia region. 
   As an aside, the actual original description of D. acadianum is pretty weird. The name first appears in the title of an article by Charles Lyell (yes, the famous geologist one) and JW Dawson in volume 9 of the Quarterly Journal of the Geological Society of London on page 58 (and is called a reptile). A few figures of material of D. acadianum were also presented. This article was followed, in the same volume, by some descriptions of other "reptilian remains" by Jeffries Wyman. This article was in turn followed by the article by Owen to which the naming of D. acadianum is accorded, which doesn't have any actual figures; all of the figures are in Lyell & Dawson's plates. It all gets even more confusing because Lyell & Dawson's article credits the name to both Owen and Wyman, but really it was only Owen's article that named the animal, even if Wyman facilitated the examination of the specimens of D. acadianum ​by Owen.

As often happens, there were several new species or genera named after Dendrerpeton acadianum that are in fact the same as D. acadianum; one example is Dendrerpeton oweni, which was named by Dawson in 1863. Confusion over whether the animal was a reptile or an amphibian (looking neither like modern amphibians nor like the large aquatic temnospondyls of Europe like Archegosaurus) sometimes contributed to misidentification and erroneous referral of materials. It's important to note that a lot of this material was very fragmentary and not necessarily diagnostic; Owen's original description of D. acadianum is no exception. The fragmentary and compressed nature of many Joggins specimens is one of the greatest roadblocks to a better understanding of this important Carboniferous ecosystem. Most of the historic taxa have since been synonymized with Dendrerpeton acadianum (e.g.,  Platystegos loricatum). 

The next substantial work to address Dendrerpeton was the review of Joggins amphibians by Margaret Steen in 1934. Steen provided thorough figures and descriptions of numerous specimens, many of which were previously unfigured or only generically figured. I won't go into too much detail here because a lot of it is the gory descriptive work and taxonomy that bores a lot of people, but it is really a great piece of work with a lot of good information in it that is still useful 85 years later. Steen redescribed a number of previously described taxa and updated their diagnoses and also named a few new ones, such as Dendrysekos helogenes, which is an important taxon but sparsely represented in the literature for reasons that will be explained in short below. 

Drawings of a specimen of Dendrerpeton acadianum by Steen (1934).

Reconstruction of Dendrerpeton acadianum by Steen (1934).

Reconstruction of Dendrerpeton acadianum by Carroll (1967).

More recent workers have tended to further lump Joggins temnospondyls under Dendrepreton acadianum. ​Carroll (1967) synonymized both Dendrysekos helogenes and Dendryazousa dikella (a pretty wild name if you ask me), taxa named by Steen, with Dendrerpeton acadianum. Milner (1996) re-established Dendrysekos helogenes as a valid species but of Dendrerpeton and also identified a new species, Dendrerpeton confusum. The resurrection of "Dendrysekos" helogenes as a distinct species of Dendrerpeton was based on the description of a relatively complete and undistorted skull by Godrey et al. (1987). That specimen (see figure to right) was placed within Dendrerpeton acadianum following Carroll's taxonomy but was then subsequently identified as a specimen of Dendrerpeton helogenes.  

This was even further complicated by description of an even better specimen (see figure to left) by Holmes et al. (1998), which is essentially a complete skeleton. These authors did not necessarily reject Milner's (1996) interpretation of temnospondyl diversity at Joggins, but they noted that their specimen did not cluster with any of the three that Milner proposed were valid (Dendrerpeton acadianum, Dendrerpeton confusum, and Dendrerpeton helogenes) when various measurement ratios were assessed. As such, they referred their new specimen to D. acadianum because it was the species most commonly referred to in the literature (not a particularly compelling case per say and not really a conservative approach as claimed). Schoch & Milner (2014) were the last to revise the taxonomy, resurrecting Dendrysekos helogenes as both a distinct species and genus and referring the specimens of Godrey et al. (1987) and Holmes et al. (1998) to that taxon.

Reconstruction of Dendrerpeton acadianum by Godfrey et al. (1987) based on a specimen now referred to Dendrysekos helogenes.

Reconstruction of Dendrerpeton acadianum by Holmes et al. (1998) based on a specimen now referred to Dendrysekos helogenes.

The taxonomy of Joggins temnospondyls is not a trivial issue. For one, the Joggins tetrapods include some of the oldest occurrences of various crown clades (e.g., temnospondyls) in continental North America. Secondly, whether there is a single rather morphologically variable taxon or a number of more morphologically constrained taxa is important for understanding the diversity and evolutionary history of temnospondyls at Joggins. The last point is that Dendrerpeton acadianum has long been utilized as the outgroup for many temnospondyl studies, but this has usually been based on the descriptions of the above two specimens that are now considered to belong to Dendrysekos helogenes ​(particularly the one described by Holmes et al., 1998). This is why more recent analyses now list D. helogenes as the outgroup, even though the coding and information remain the same as when it was listed as D. acadianum​. Another look at the Joggins temnospondyls is certainly warranted, and a number of researchers, particularly in Canada (e.g., Jason Anderson, Hillary Maddin, Arjan Mann, and Jason Pardo) are diving back into collections to reassess the taxonomy of a lot of the Joggins tetrapods.

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Outside of Joggins, temnospondyls are quite rare. This is large in part due to an absence of many terrestrial deposits from the Permian, when temnospondyls were abundant across North America (amongst other regions). The only other temnospondyl I'll mention in this post is near and dear to my heart - a metoposaurid! The specimen is a natural mould of a partial skull of a small-bodied individual. It was first noted in a conference abstract by Don Baird and Paul Olsen back in 1983, mentioned again by Baird (1986), and then photographed by Hunt (1993). The best photograph to date is that published by Sues & Baird (2015) in their synopsis of the Fundy Basin. It is known from the Carnian-age Wolfville Formation and has been previously assigned to "Metoposaurus" bakeri, a taxon from the Carnian of Texas whose generic placement remains somewhat complicated because it cannot be properly referred to Metoposaurus (lacrimal excluded from orbit), but neither can it be referred to either Anaschisma or Apachesaurus, the other two North American genera. Unfortunately, material of metoposaurids seems quite scarce in the region, which is unfortunate as it might be able to give us some important insights into how the North American and Moroccan metoposaurids relate.

Photograph of the natural mould of the skull of "Metoposaurus" bakeri from Nova Scotia (from Sues & Olsen, 2015).

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Refs

• Baird D. 1986. Some Upper Triassic reptiles, footprints, and amphibian from New Jersey. The Mosasaur 3:125-153.
• Baird D, Olsen PE. 1983. Late Triassic herpetofauna from the Wolfville Fm. of the Minas Basin (Fundy Basin) Nova Scotia, Canada. Geological Society of America, Abstracts with Program 15:122.
• Carroll RL. 1967. Labyrinthodonts from the Joggins Formation. Journal of Paleontology 41:111-142. [link]
• Dawson JW. 1863. Notice of a New Species of Dendrerpeton, and of the Dermal Coverings of certain Carboniferous Reptiles. Quarterly Journal of the Geological Society 19(1-2):469-473. doi: 10.1144/GSL.JGS.1863.019.01-02.42
• Godfrey SJ, Fiorillo AR, Carroll RL. 1987. A newly discovered skull of the temnospondyl amphibian Dendrerpeton acadianum Owen. Canadian Journal of Earth Sciences 24(4):796-805. doi: 10.1139/e87-077
• Holmes RB, Carroll RL, Reisz RR. 1998. The first articulated skeleton of Dendrerpeton acadianum (Temnospondyli, Dendrerpetontidae) from the Lower Pennsylvanian locality of Joggins, Nova Scotia, and a review of its relationships. Journal of Vertebrate Paleontology 18(1):64-79. doi: ​10.1080/02724634.1998.10011034
• Hunt AP. 1993. Revision of the Metoposauridae (Amphibia: Temnospondyli) and description of a new genus from western North America. Museum of Northern Arizona Bulletin 59:67-97.
• Lyell C, Dawson JW. 1853. On the remains of a reptile (Dendrerpeton acadianum, Wyman and Owen) and of a land shell discovered in the interior of an erect fossil tree in the coal measures of Nova Scotia. Quarterly Journal of the Geological Society 9(1-2):58-67. doi: 10.1144/GSL.JGS.1853.009.01-02.20
• Milner A. 1996. A revision of the temnospondyl amphibians from the Upper Carboniferous of Joggins, Nova Scotia. Special Papers in Palaeontology 52:81-103.
• Owen R. 1853. Notes on the above-described fossil remains. Quarterly Journal of the Geological Society 9(1-2):66-67. doi: 10.1144/GSL.JGS.1853.009.01-02.20
• Schoch RR, Milner AR. 2014. Handbook of Paleoherpetology: Temnospondyli I (Part 3A2). München: Verlag Dr. Friedrich Pfeil.
  
• Steen MC. 1934. The Amphibian Fauna from the South Joggins. Nova Scotia. Proceedings of the Zoological Society of London 104(3):465-504. doi: 10.1111/j.1096-3642.1934.tb01644.x
• Sues H-D, Olsen PE. 2015. Stratigraphic and temporal context and faunal diversity of Permian-Jurassic continental tetrapod assemblages from the Fundy rift basin, eastern Canada. Atlantic Geology ​51:139-205. [PDF]
• ​Wyman J. 1853. Notes on the reptilian remains. Quarterly Journal of the Geological Society 9(1-2):64-66. doi: 10.1144/GSL.JGS.1853.009.01-02.20