Zoneait
Name: Zoneait
(Large tooth).
Phonetic: Zone-ate.
Named By: Eric W. Wilberg - 2015.
Classification: Chordata, Reptilia,
Crocodylomorpha, Thalattosuchia, Metriorhynchoidea.
Species: Z. nargorum (type).
Diet: Carnivore/Piscivore.
Size: Unavailable.
Known locations: USA, Oregon - Snowshoe
Formation, Weberg member.
Time period: Aalenian of the Jurassic.
Fossil representation: Partial skulls and post
cranial remains of several individuals.
Named
in 2015, Zoneait was an important discovery for
those interested in
marine
crocodiles. Zoneait represents a
transitional genus and
possible sister taxon of the Metriorhynchidae, the group that
features many famous genera of marine crocodiles such as Metriorhynchus
and Geosaurus.
In addition to this the Zoneait holotype fossils
were
found in fossil bearing deposits estimated to have come from the
Aalenian age of the Jurassic, meaning that at the time of the first
description of the genus, Zoneait is the oldest
marine crocodile that
we know about.
However
as is often the case, although Zoneait is the
oldest marine crocodile
at the time of its discovery, it is not the most primitive form known
to us. Both Teleidosaurus
and Eoneustes
are known from the Bajocian
stage of the Jurassic, and yet both are more primitive in form when
compared to Zoneait. Zoneait
has been stablished as a marine
crocodile upon the basis that all remains have so far been found in
what would have been a shallow marine environment, and the eyes were
also orientated to look out to the sides instead of above like in
modern day semi-aquatic crocodiles which ambush animals that are above
them. Passages for enlarged blood vessels have also been found on
Zoneait skulls, strongly suggesting that these
were for carrying
blood to a salt gland which would have extracted the unusually high
levels of salts from the blood which would have been absorbed by living
in a salt water environment.
Ultimately
though, Zoneait was not as well adapted to a
marine lifestyle as
later metriorhynchid crocodiles such as Tyrannoneustes
and
Plesiosuchus.
Key to this understanding is the forelimbs which are
not shaped into paddles like in true metriorhynchids. In addition the
forelimb only shows some shortening of the lower ulna (one of the
lower forelimb bones), while no reduction is apparent in the
humerus (upper arm limb bone). This tells us two things, one
being that forelimb reduction in marine crocodiles would have started
in the lower extremities of the limb, before eventually progressing
into the upper forelimbs of later descendants. Second is that Zoneait
might not have spent as much time in the water as true
metriorhynchids, and possibly spent more time on the coastlines to
rest. It is for these reasons that Zoneait has
been placed within the
Metriorhynchoidea, making the genus a metriorhynchoid. Here Zoneait
is treated as a sister taxon to the metriorhynchids of the
Metriorhynchidae, evolving alongside their ancestors, though
possibly not being ancestral to them. Only future research and
discoveries will allow us to be sure.
Further reading
- A new metriorhynchoid (Crocodylomorpha, Thalattosuchia) from
the Middle Jurassic of Oregon and the evolutionary timing of marine
adaptations in thalattosuchian crocodylomorphs. - Journal of
Vertebrate Paleontology 35 (2): e902846. - Eric W.
Wilberg - 2015.
----------------------------------------------------------------------------
Random favourites
 |
 |
 |
 |