Wednesday 10 December 2014

Sharks and Rays of the Eocene Canadian High Arctic.

Palaeontological studies of the Arctic during the Early-to-Middle Eocene have revealed a world in which the ice-free Arctic Ocean was surrounded by lush warm-temperate rainforests, inhabited by creatures such as Alligators, Turtles and Hippo-like Mammals. The fauna of the Arctic Ocean itself is less well known, however, though Shark’s teeth and Fish otoliths (mineralized tissues from the ears of Fish and some marine invertebrates) from Ellesmere Island have been since at least the 1970s, and from Banks Island since the 1980s.

In a paper published in the Journal of Vertebrate Paleontology on 4 November 2014, Aspen Padilla of the University of Colorado Museum of NaturalHistory at the University of Colorado at Boulder and the Chemeketa CommunityCollege Library, Jaelyn Eberle of the University of Colorado Museum of Natural History and the Department of Geological Sciences at the University of Colorado at Boulder, Michael Gottfried of the Department of Geological Sciences and Museum at Michigan State University, Arthur Sweet of the Geological Survey of Canada (Calgary) at Natural Resources Canada and Howard Hutchison of the Universityof California Museum of Paleontology describe the results of a study in which sediments from the Eureka Sound Formation collected from near the Muskox and Eames rivers, within the boundaries of Aulavik National Park on Banks Island were sieved to yield Sharks teeth. These sediments are judged to be Early-to-Middle Eocene in age, spanning the Early Eocene Climatic Optimum. Sharks produce large numbers of teeth throughout their lives, regularly shedding and replacing teeth. This means that they produce very large numbers of tooth fossils that can be used in biostratigraphy (dating rocks by their fossil content) and palaeoenvironemental reconstructions, even though Shark macrofossils (body fossils) are very rare.

Map of Arctic Canada showing locations of Eocene shark localities within Aulavik National Park, northern Banks Island, NWT (inset). L McConnaughey in Padilla et al. (2014).

The first Shark taxa recorded is Striatolamia macrota, a form of Sand Tiger Shark previously recorded from Eocene sediments in a variety of locations. This is one of the most abundant species present, with thousands of isolated teeth found.

Teeth of Striatolamia macrotafrom northern Banks Island.(A–C) an anterior tooth in lingual (A), labial (B), and profile (C) views; (D, E) lower lateral tooth in lingual (D) and labial (E) views; (F, G) upperlateral tooth in lingual (F) and labial (G) views; (H, I) posterior tooth in lingual (H) and labial (I) views. Padilla et al. (2014).

The second species recorded is an unnamed species of Carcharias (anther Sand Tiger Shark), referred to as Carcharias sp. A. This species is also represented by thousands of isolated teeth. Members of the genus Carcharias are still extant today, and inhabit warm-temperate and tropical seas.

Teeth of Carcharias sp. A from northern Banks Island. (A–C) anterior tooth in lingual (A), labial (B), and profile (C) views; (D, E)lateral tooth in lingual (D) and labial (E) views (A–E share a scale); (F, G) lateral tooth in lingual (F) and labial (G) views; (H, I) posterior tooth in lingual (H) and labial (I) views. Padilla et al. (2014).

The third species recorded is a second species of Carcharias, referred to a Carcharias sp. B. Specimens assigned to species B have week striations, which are absent in species A, and the largest teeth of species B are notably smaller than the largest teeth of species A. This species is also represented by thousands of isolated teeth.

Teeth of Carcharias sp. B from northern Banks Island. (A–C) Anterior tooth in lingual (A), labial (B), and profile (C) views; (D, E) lateral tooth in lingual (D) and labial (E) views. Padilla et al. (2014).

The fourth species recorded is Odontaspis winkleri, a fourth species of Sand Tiger Shark, though in this case only represented by two isolated teeth. The genus Odontaspis first appeared in the Late Cretaceous, and is still extant, with a modern species found in tropical and warm temperate waters in the Atlantic, Indian and Pacific Oceans and the Mediterranean Sea.

 Lower symphysealtooth of Odontaspis winkleri in lingual(A), labial (B), and profile (C) views. Padilla et al. (2014).

The fifth species recorded is Physogaleus secundus, an extinct relative of Tiger and Sharpnose Sharks (but not Sand Tiger Sharks, which are not closely related to Tiger Sharks). This species is recorded from 22 isolated teeth; this being the furthest north the genus Physogaleus has been recorded.

Anterolateral tooth of Physogaleus secundus in lingual (D) and labial (E)views. Padilla et al. (2014).

The seventh species recorded is a single tooth also placed in the genus Physogaleus, but recorded as cf. Physogaleus americanus; that is to say Padilla et al. are confident that the tooth come from a Shark of the genus Physogaleus, and that they most closely resemble the tooth of Physogaleus americanus, but that they are not completely confident of this. Physogaleus americanus is known from the Late Palaeocene and Early Eocene of Mississippi, though the Banks island tooth is considerably larger than the largest teeth recorded from Mississippi, at 6.1 mm, compared to an average of 3 mm for the Mississippi teeth.

Lateral toothof Physogaleus cf. Physogaleus americanus in lingual (H)and labial (I) views. Padilla et al. (2014).

The eighth species recorded is a single tooth from an unknown Myliobatid Ray, Myliobatis sp., the genus that includes modern Eagle Rays, and which is now found in warm-temperate to tropical waters across the globe.

Upper median tooth of Myliobatis sp. in occlusal (J), side (K), and basal (L) views. Padilla et al. (2014).

The Shark faunal assemblage from the Early Eocene of Banks Island conforms to the predicted climatic model, being made up entirely of species found in warm temperate, or warmer, waters today. However it is unusually poor in species for such an assemblage, with just a tiny handful of specimens that do not come from the most abundant three species. This is typical of modern polar species assemblages, which tend to be dominated by one or two abundant species, but unusual in warmer waters, where a more diverse community is typical.

There does not seem to be any good reason why a warm Arctic fauna would be dominated by a low number of species in the way modern cold arctic faunas are. Instead Padilla et al. propose an alternative hypothesis for this low species diversity, which is independent of the latitude of the site. Modern Sand Tiger Sharks often enter brackish (low salinity) estuarine and delta waters to feed, as to Eagle Rays, but most other Sharks prefer fully saline waters. The deposits on Banks Island appear consistent with bar formations around the outer margin of a delta, an environment where fresh and saline water often mix, producing a low salinity environment, and Padilla et al. suggest that this may be the cause of the low species diversity seen there.

See also…

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 The remains of a variety of non-marine Vertebrates have been recovered from Ellesmere and Axel Heiberg Islands in the eastern Canadian High Arctic from the 1970s onwards. These vertebrate assemblages date from early – middle Eocene (53–50 million years ago), when the islands were close to their current positions within the...

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