During last week's cold snap, I covered the temnospondyls from the South Pole. This week is covering the temnospondyls from the North Pole! Particularly appropriate after this part of North America got buried by a giant snowstorm last night. As with Antarctica, the modern-day Arctic isn't exactly the most hospitable area for amphibians (although there are a few species that live there today), but it was evidently a much nicer place to live 250 million years ago. The temnospondyl record in the Arctic is primarily confined to Greenland and Norway (and that's all I'm covering here), which were also home to a substantial number of important fish and early tetrapod taxa (e.g., Acanthostega from Greenland) during earlier geologic periods. Some of the northern regions of Russia are of a comparable paleolatitude but will be covered in future post(s) once I track down more literature / translations. P.S. some of this week's images are not exactly high-quality because they're phone pictures of print copies that are too fragile to be scanned / my university's closed until noon today anyway, so I don't have a scanner even if I wanted one. Svalbard temnospondylsThe Norwegian temnospondyls come from the Svalbard archipelago, specifically from the island of Spitsbergen (the largest in the archipelago). Considering that it's not a very big geographic region, there is a surprising amount of temnospondyl diversity, most from the Early Triassic, that is well-documented from Svalbard. There is some disparity in the quality of locality and stratigraphic data for these temnospondyls, but most are from the Vikinghøgda Formation (Early Triassic). Whether there is an actual loss of diversity in the later parts of the Triassic (fitting a general trend of temnospondyl decline throughout the Mesozoic) or just a sampling bias based on lack of exposures or work in upper sections isn't totally clear. Good summary articles for Norwegian temnos:
?Brachyopoidea: Boreosaurus thorslundi (Nilsson, 1943) This taxon is represented by a badly preserved jaw, a large portion of which is a cast of internal spaces, with few details on the precise morphology and nature of the specimen. Nilsson stated that it looked most like that seen in Dvinosaurus and "primitive Brachyopids" (p. 34 therein), but really not much can be made of it beyond that. Kear et al. (2016) list it as a brachyopoid, citing the review by Warren & Marsicano (2000), but the latter study considered to be an indeterminate temnospondyl. It has also been suggested to be a rhytidosteid (Shishkin & Valilov, 1985; Hewison, 1996).
Capitosauroidea: Sassenisaurus spitzbergensis Wiman, 1914 This taxon was initially described as a species of Cyclotosaurus before being given its own genus. It is represented only by the tip of the snout, although it has had a fair bit of other material (mostly isolated jaws) referred to it. It has been previously considered to be congeneric with Wetlugasaurus (Sennikov, 1981) and taxonomically indeterminate (Cox & Smith, 1973; Damiani, 2001).
Comparison of cranial reconstructions of different Scandinavian temnospondyls. (a) Aphaneramma rostratum; (b) Tertrema acuta; (c) Platystega depressa; (d) Lyrocephaliscus euri; and (e) l Cyclotosaurus sp. Greenland (Table 2): (f) Wetlugasaurus groenlandicus; (g) Stoschiosaurus nielseni; (h) Luzocephalus kochi; and (i) Gerrothorax pulcherrimus. (figure and caption [in part] from Kear et al., 2016). Cyclotosauridae: Cyclotosaurus sp. The documentation of Cyclotosaurus, a widely dispersed taxon (also found in Greenland [see below], Poland, Germany, and Thailand) is based on Kear et al.'s (2016) reappraisal of "Capitosaurus polaris," which is a cool name but an invalid taxon. This is in accord with previous workers's suspicions (e.g., Swinton, 1927; Nilsson, 1942; Damiani, 2001). Plagiosauridae: cf. Plagiosternum sp. This documentation is all based on anecdotal evidence, the reference fossil having been left in the field on an expedition to Bear Island in 1948 upon determination that it was too difficult to collect. It was first noted by Lowry (1949) and again by Panchen (1959) and was evidently quite large (2.6 m over the preserved length of the skeleton and with a 70 cm wide skull). Photographs were apparently taken of the specimen but only one was ever published, and there is very little description of the specimen. The association with Plagiosternum sp. was first made by Panchen and has since been propagated (Cox & Smith, 1973; Kear et al., 2016). The specimen was inadvertently rediscovered nearly 40 years after its initial discovery, being linked to past expeditions by the presence of a sardine can and subsequently verified against the original photos. This final report (Dore & Wandas, 1985) indicated that the specimen was being prepared for collection, but it seems probable that this was unsuccessful for some reason since it has never resurfaced in the primary literature. Plagiosauridae: cf. Plagiosuchus sp. Cox & Smith (1973) more or less made a single indirect reference to a purported occurrence of Plagiosuchus at Spitsbergen based on the nature of the pustular ornamentation, but otherwise made no description or figures of the specimen(s) in question. Greenland temnospondylsTemnospondyls from Greenland more or less come exclusively from the east coast, often in near-coastal localities, a pattern that holds true for other tetrapod clades. Good summary articles for Greenland temnos:
Capitosauridae: Selenocara groenlandicus Bjerring, 1997 [validity contested] This taxon is based on material that was previously referred to Wetlugasaurus by Säve-Söderbergh (1935), a temnospondyl that was originally discovered in Russia. There remains some controversy over whether it is actually a distinct genus - Damiani (2001) questioned whether the few differences were sufficient, while Novikov (2016) maintained the validity and erected a new species of Selenocara. In general, specimens assigned to this genus are not often discussed, and having never seen the specimens myself, my usage of the name here should not be considered an endorsement or support of its validity (I am more of a taxonomic lumper anyway). ?Capitosauroidea: Luzocephalus kochi Bjerring, 1999 [validity sort of contested?] Here's a second confusing and unresolved temnospondyl from Greenland. Luzocephalus was originally named Lyrocephalus, for which there were four species, L. euri, L. rapax, L. johanssoni, and L. kochi. The name was changed for the first time in 1961 (Kuhn) when it was realized that Lyrocephalus is preoccupied by a modern lizard, and the temnospondyl then became Lyrocephaliscus. Lyrocephaliscus euri (the same as the one above from Svalbard) is still a valid (and less-contentious) taxon that belongs to the Trematosauridae. The other three species were thought to be synonymous with each other (under Lyrocephaliscus kochi) and to be congeneric (within the same genus) as the lydekkerinid Luzocephalus blomi from Russia (Shishkin, 1980) (thus making it Luzocephalus kochi). THEN, Bjerring (1999) separated L. kochi from Luzocephalus, arguing that it could be sufficiently (and substantially) differentiated from Luzocephalus blomi, meriting a new genus, Aquiloniferus, (and reiterating that L. rapax, L. johanssoni, and L. kochi were all the same taxon, thus making it Aquiloniferus kochi). There has been very little work done on either Aquiloniferus and most other lydekkerinids since, and even mentions of it appear very rarely in the literature (only 3 hits on Google Scholar). Use of the previous name, Luzocephalus kochi, is also rare, to the extent that it is unclear whether Aquiloniferus is widely accepted as a valid genus. As above, I'm not taking any particular stance on this matter.
Tupilakosauridae: Tupilakosaurus heilmani (Nielsen, 1954) This taxon was originally described based on a partial skeleton embedded in a large concretion, which mostly obscured details of the skull, on which most taxa are described and differentiated, so understandably not that much is known about it. The skull was subsequently acid-prepped to dissolve the matrix encrusting the dorsal surfaces (Nielsen, 1967) but is quite incomplete. The vertebrae were (and still are) relatively unusual for temnospondyls, being short and amphicoelous (concave on both ends). Based largely on this observation, Nielsen suggested (though with a fair bit of uncertainty), that Tupilakosaurus or something like it was a close relative of the ichthyosaurs, which are well known to be entirely unrelated marine reptiles. Tupilakosaurids were quite enigmatic back in the mid-20th century, but they are better known now and are certainly temnospondyls of no close affinities to ichthyosaurs. Their vertebral anatomy, characterized by a condition called diplospondyly, where two centra are positioned beneath each neural arch (and spine), is found in other vertebrates (e.g., embolomeres) but is otherwise unknown within Temnospondyli outside of one brachyopid (Warren et al., 2011).
Plagiosauridae: Gerrothorax pulcherrimus Fraas, 1913 Gerrothorax is one of the best-studied European and Scandinavian temnospondyls, being known from solid records in both Greenland and Germany. It has a distinctly short yet broad skull and is the first temnospondyl to be thoroughly explored to assess whether long-standing hypotheses of unusual mouth opening ("head lifting" in a technical sense; "the toilet seat mechanism" in a colloquial sense) in flat-skulled temnospondyls held any weight (Jenkins et al., 2008; Witzmann & Schoch, 2013), which has been widely extended to other Mesozoic temnospondyls. It is also the best exemplar of evolutionary stasis in temnospondyls (Schoch & Witzmann, 2012), as it persists for over 35 million years with minimal changes in its anatomy, an astounding longevity for any tetrapod. Both of these will be covered in future blog posts. What's it all mean? As with the Antarctic temnospondyls, inferences of physiological constraints and correspondent preferences for certain environments suggest that the Arctic temnospondyls resided in regions that were relatively warm and conducive to ectothermic animals, hardly comparable to the modern-day conditions. An interesting observation is that while Greenland is today considered to be part of North America because it separated from Europe with the opening of the north Atlantic before it separated from the main part of North America, the temnospondyls of the region are quite comparable to those in Europe (e.g., those of Svalbard), more so than those in North America. Marzola et al. (2017) provide some good commentary on why this pattern occurs, namely that Greenland and Europe were of a comparably high paleolatitude compared to the near-equatorial basins in the southwestern U.S. Environmental disparity and geography probably thus played a major role as barriers to a more homogenous community across North America at large during the time period. This makes a lot of sense when you consider the changes in temnospondyl communities in western North America. In the early part of the Triassic, we have a lot of the same diversity here that's seen in other geographic regions (e.g., cyclotosaurids, brachyopids, trematosaurians). This is suggestive of the greater physical connectivity of the landmasses and the subsequently higher dispersal ability at the time. By the time we get to the Late Triassic, metoposaurids are more or less the only large temnospondyls found across North America, whereas other groups (particularly brachyopoids) continue to persist on most other continents. Most Mesozoic temnospondyls are overgeneralized as large, aquatic fish-eaters, but there had to be some degree of resource partitioning among co-occurring ones to avoid direct competition. This, as well as physiological differences that are hard to test (e.g., salinity tolerance) may have contributed to some of the patterns that we see as well. Up next week: 'labyrinthodonts' and labyrinthine infolding Refs
David Marjanović
2/4/2019 12:56:28 pm
"An interesting observation is that while Greenland is today considered to be part of North America because it separated from Europe with the opening of the north Atlantic before it separated from the main part of North America, the temnospondyls of the region are quite comparable to those in Europe (e.g., those of Svalbard), more so than those in North America." Comments are closed.
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