A edopoid that time forgot: Nigerpeton

Note: This was supposed to be out last week, but then the Kobe Bryant tragedy happened, and as a native Angeleno, that was a lot to deal with, so I put the post off to this week.

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The Land That Time Forgot is a classic 1918 trilogy (technically combined into a single volume in 1924 and itself part of the Caspak trilogy) in which a hardy bunch of Americans and Brits hijack a U-boat in the middle of WWI and find themselves in a weird island with a bunch of prehistoric creatures including various purported stages of human evolution living as distinct tribes. The trailer for the 1975 movie tells you that you can learn the "secrets of evolution." I have never seen the movie, but since I liked The Valley of Gwangi, which I watched on a mobile projector displayed on a bedsheet draped over the soccer mom van of the education department of the LACM, I am sure that it is of high cinematic quality in spite of its Rotten Tomatoes rating. These movies capture a certain affinity for the notion that pockets of animals thought to be long-extinct might exist, and indeed there are various cryptozoological monsters themed around this, like the mokele-mbembe, based on the notion of a sauropod-like beast living in the depths of the jungles of the Congo. For some reason, nobody's ever come up with a cryptozoological creation based on a temnospondyl, which seems like a real missed opportunity to put a 3-meter amphibian in the Mississippi River, and I am not proposing one now, although as I have told some people in person, I am amenable to the idea of donning a temnospondyl body suit and cruising around a body of water. 

Temnospondyls are not forgotten by time - if anything, they are just forgotten, period - but there are a few records of lineages where one taxon shows up way after its compatriots were thought to have kicked the can. We call these "relict taxa," a name obviously referring to the remnant of something much more common or widespread in earlier days (which were usually better days if you were a temnospondyl, back when the world was warm and humid and probably humming with crunchy invertebrates). Edopoids, already regarded as one of the more primitive temnospondyl groups, are one of the few for which a relict taxon is known: Nigerpeton, aptly named for Niger from which it is known.

A (really) brief history of study

Nigerpeton is the newest edopoid to be named and the only one named in the 21st century, so it doesn't have too much history. It was briefly described in 2005 by a team led by Chris Sidor (Nigerpeton ​is one of the few temnospondyls to make it into Nature) and was then re-described in greater detail the following year by a team led by Seb Steyer; this description flushes out details on the cranial anatomy and the scant information on the few postcranial elements that are known. The species name, N. ricqlesi honours Armand de Ricqlès, a prominent French paleontologist who has done a ton of pioneering histology work. Some additional cranial material was described by Chris in 2013. Nigerpeton​ would have been appreciably large, at least large enough to have given your dog a scare it won't forget, with the holotype skull measuring almost half a meter in length. 

Anatomy

Edopoids have some pretty weird anatomy already, but Nigerpeton in particular has a number of distinct autapomorphies. For example, it has two large openings at the tip of snout that you might think are the nostrils, but the nostrils are way far back (normal for edopoids) as a result of the enlargement of the premaxillae; the openings at the front are actually for accommodating the fangs from the front end of the lower jaws, a feature that otherwise only shows up in Mesozoic temnospondyls. Accompanying the enlarged premaxillae is a row of enlarged teeth adjacent to the marginal tooth row, which is matched by a corresponding row on the maxilla, which is expanded both inwards and outwards. These should not be confused with the normal palatal dentition, either the enlarged "fangs" of the vomer, palatine, and sometimes the ectopterygoid or with the row of teeth on the palate of similar size to the marginal teeth that runs along the maxilla in lots of Mesozoic temnospondyls. There are other Paleozoic temnospondyls with swelling of the maxilla, which often accommodates what we refer to as a 'caniniform region' where the marginal teeth in that region are much larger (e.g., Eryops, trematopids), but those taxa don't have an extra set of teeth.

Note: as pointed out in the comments by David, it's questionable whether there are actually openings at the tip of the snout for the mandibular fangs or if it's just a thinly ossified area that got weathered (suggested by Sidor, 2013). Chris suggested that it could be ontogenetically or intraspecifically variable and should be questionably regarded as an autapomorphy.

Reconstruction of the skull of Nigerpeton by Franck Limon Duparcmeur (figure 4 from Steyer et al., 2006).

Time warp

Before the discovery of Nigerpeton and its buddy Saharastega, who is of more uncertain placement (but suggested as the sister taxon to all edopoids), it was thought that edopoids were a classic Late Carboniferous to early Permian clade that kicked the bucket like a lot of other coeval temnospondyls. For example, most of the dissorophoids disappear, as do all but one eryopoid and all zatracheidids. These earlier temnos are replaced (though rarely are the replacements direct ecological substitutes) by stereospondylomorphs like the long-snouted archegosauroids. A lot of those "last-standers" show up in eastern Eurasia, primarily Russia and China, such as the eryopoid Clamorosaurus and the dissorophids Kamacops and Zygosaurus. As noted by Sidor et al. (2005), this puts a gap of around 40 million years between these African edopoids and the otherwise last surviving edopoids (in North America). Sidor and colleagues attributed the presence of the African taxa as relicts that had become isolated by climate change that produced more regional heterogeneity (basically, if you were a fairly water-dependent temno, you got stuck in one small habitable zone, which used to be much more extensive).

Time-calibrated phylogeny from Sidor et al. (2005) showing the huge temporal gap between Nigerpeton, Saharastega, and all other edopoids.

A fellow time-warper, the amphibamiform Micropholis from the Early Triassic of S. Africa and the only dissorophoid from either Gondwana or from the Mesozoic (figure from Schoch & Rubidge, 2005).

Although edopoids aren't known from Africa before the Late Permian, there aren't very many localities either - Africa's richest records come from the Late Permian and the Early Triassic, historically from the Karoo Basin in South Africa, although the Argana Basin in Morocco preserves a nice package of temnos going all the way to the Late Triassic. So they may have been there and been much more widespread generally before the Late Permian but then survived only in those more tropical near-equatorial zones with an increasingly unfavourable climate. Another good example of this is the amphibamiforms. They're almost exclusively restricted to the Late Carboniferous and early Permian of North America but then pop up in the Early Triassic of South Africa in the form of Micropholis​ - there is no clear record of amphibamiforms between the early Permian and the Early Triassic anywhere. Sampling biases (i.e. many areas preserve no fossils of a given time period and among those that do, some are more accessible or fossiliferous) are an important consideration in any interpretations of broader patterns through time and space.

Nonetheless, one apparent and ubiquitous pattern is that amphibians do not do well in drier habitats, which predisposes them to becoming isolated and thus endemic (localized) in their distribution. These network distributions that Chris has done a lot of work with (see right) show the overlap between major depositional regions at the genus and the family levels; yellow circles indicate a taxon shared by at least two regions, while beige circles are endemics. South Africa looks like the mother of all endemics (large beige tree), but as Chris points out, the Karoo Basin is one of the most extensively sampled basins, so many of its taxa may only appear endemic because they are somewhat rare and haven't yet been found elsewhere. Certainly food for thought in considering any claims of endemism in the fossil record. Morocco and Niger are way the heck away from the rest of Africa at both taxonomic levels, and the only endemics anywhere in Africa at the family level are amphibians. 

Up next week: The posts are probably going to get a little more sporadic; these Bayesian stats problem sets I have take a while... I'm still mulling over the next theme (edopoids was pretty random), but I'm thinking about talking about some classic temno-rich localities!

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Refs

• Schoch RR, Rubidge BS. 2005. The amphibamid Micropholis from the Lystrosaurus assemblage zone of South Africa. Journal of Vertebrate Paleontology 25(3): 502-522.
  
• Sidor CA. 2013. The vertebrate fauna of the Upper Permian of Niger–VIII. Nigerpeton ricqlesi (Temnospondyli: Cochleosauridae) and tetrapod biogeographic provinces. Comptes Rendus Palevol 12(7-8): 463-472. DOI: 10.1016/j.crpv.2013.05.005
• Sidor CA, O'Keefe FR, Damiani R, Steyer JS, Smith RM, Larsson HC, Sereno PC, Ide O, Maga A. 2005. Permian tetrapods from the Sahara show climate-controlled endemism in Pangaea. Nature 434(7035): 886-889. DOI: 10.1038/nature03393
• Steyer JS, Damiani R, Sidor CA, O'Keefe FR, Larsson HC, Maga A, Ide O. 2006. The vertebrate fauna of the Upper Permian of Niger. IV. Nigerpeton ricqlesi (Temnospondyli: Cochleosauridae), and the edopoid colonization of Gondwana. Journal of Vertebrate Paleontology 26(1): 18-28. DOI: 10.1671/0272-4634(2006)26[18:TVFOTU]2.0.CO;2

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