Snakes are one of the
most successful groups of living vertebrates, with over 3000 living
species found in all but the very coldest environments on Earth.
Modern snakes exhibit a wide range of habitat preferences, from
totally marine species to species found entirely in the treetops, but
it is thought that the earliest members of the group were either
burrowers of swimmers, lifestyles which have led to limb-reduction
and loss in a variety of other groups.
The most reliable way to
assess the lifestyle of a modern Snake is to examine the
trunk-to-tail ratio, i.e. the number of trunk and tail vertebrae it
possesses. At first sight this would seem a useful tool to
palaeontologists studying the origin of the group, as vertebrae are
the most robust bones in the body of a Snake, with excellent
potential to enter the fossil record. However in order for this
analysis to be used all of the vertebrae of an individual snake have
to be preserved, which is a very rare occurrence, making most known
fossil Snakes useless for this purpose.
In a paper published in
the journal Science Advances on 27 November 2015, Hongyu Yi of the
School of Geosciences at the University of Edinburgh and the Divisionof Paleontology at the American Museum of Natural History and MarkNorell, also of the Division
of Paleontology at the American Museum of Natural History, discuss
the results of a study which examined the inner ear bones of
Dinilysia
patagonica,
a Cretaceous fossil Snake thought to have been closely related to the
last common ancestor of all living Snakes, though not actualy
ancestral to such Snakes itself, as well as those of a variety of
living Snakes, Lizards and Amphisbaenians (limbless Squamates which
lost their limbs separately to the Snakes).
Ear
bones are a potential good indicator of the habitat the medium in
which an animal is living as solid ground, liquid water and gaseous
air all reflect sound waves in different ways, so that in order to
achieve good hearing (and most Snakes have very good hearing) the
earbones of the Snake should reflect the environment in which it
lives.
Yi
and Norell found that in burrowing Squamates the vestibule is
enlarged and almost spherical, the formamen ovale enlarged and the
semicircular canals very slender. This was not seen in aquatic or
above-ground dwelling Squamates, though some ground dwelling Snakes
that burrow as a defense mechanism did have enlarged vestibules.
(A)
Snake skulls in right lateral view, showing that the inner ear
(orange) locates inside the braincase and opens to the stapes (blue)
in the middle ear. Ear and skull models are not to scale. (B) Inner
ear of Laticauda
colubrina,
an aquatic species. (C) Ptyas
mucosa,
terrestrial generalist (D) Xenopeltis
unicolor,
a burrowing species. Yi & Norell (2015).
The
inner ear of Dinilysia
patagonica was
found to conform closely to the burrowing form with an enlarged and
almost spherical vestibule, enlarged formamen ovale and slender
semicircular cannals. This suggests that this species was extremely
likely to have been a burrower, suggesting that this was the
ancestral ecological preference for Snakes. A cladistic analysis of
Snake evolution (computerised analysis of relationships within the
group based entirely upon shared common features rather than assumed
relationships) also suggested tha trait was ancestral within the
Snakes, with the earliest derived groups allexclusively or
predominantly burrowing and terrestrial or aquatic lifestyles
arrising as specializations within several groups. This held true
even when Mosasaurs, a group of Mesozoic Marine Reptiles thought to
have been closely related to Snakes, were added into the matrix.
The
braincase and inner ear of Dinilysia
patagonica.
(A) Braincase of Dinilysia
patagonica,
showing the right otic region in lateral view. (B) X-ray CT model of
Dinilysia
patagonica,
with the inner ear highlighted in blue. (C) Bony inner ear of
Dinilysia
patagonica.
FO, foramen ovale; LR, lagenar recess; SC, semicircular canal; V,
vestibule. Scale bars, 5 mm. Yi & Norell (2015).
Giant fossil Sea Snakes from the early Eocene of Morocco. The Sea Snake Palaeophis maghrebianus was first described by
Camille Arambourg from the early Eocene phosphate beds of Morocco in
1952, though like many fossil Snakes it has been known only from...
A fossil Snake from the Cretaceous of Brazil. Snakes are generally accepted to have appeared and diversified during
the Mesozoic, though their fossil record is not extensive. This is due
to their lightly mineralized, easily disarticulated skeletons, which...
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Giant fossil Sea Snakes from the early Eocene of Morocco. The Sea Snake Palaeophis maghrebianus was first described by
Camille Arambourg from the early Eocene phosphate beds of Morocco in
1952, though like many fossil Snakes it has been known only from...A fossil Snake from the Cretaceous of Brazil. Snakes are generally accepted to have appeared and diversified during
the Mesozoic, though their fossil record is not extensive. This is due
to their lightly mineralized, easily disarticulated skeletons, which...
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