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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