Gerrothorax
Name:
Gerrothorax
(Plated chest).
Phonetic: Ger-ro-thor-ax.
Named By: Nilsson -1934.
Classification: Chordata, Amphibia,
Temnospondyli, Plagiosauroidea, Plagiosauridae.
Species: G. rhaeticus (type),
G.
pulcherrimus.
Diet: Carnivore.
Size: 1 meter long.
Known locations: Germany, Greenland, Sweden.
Time period: Norian of the Triassic.
Fossil representation: Many individuals.
The
body of Gerrothorax is flattened to the extreme
and is one of the
morphological features that indicate Gerrothorax
lived at the bottom of
aquatic habitats such as lakes and rivers. The eyes of Gerrothorax
are placed on top of the head in an orientation that really would only
provide upwards vision. Rather than moving around by sight,
Gerrothorax seems to have used this vision to
locate prey that was
swimming above.
Living
on the bottom would make it difficult for any animal to open its lower
jaw without scraping the bottom. Gerrothorax
seems to have avoided
this problem completely by tilting its head and opening its upper jaw
instead. This also infers a specialised hunting strategy using lures
to attract prey. For this purpose Gerrothorax
most probably had at
least camouflaged patterning across its body to allow it to blend in
with the bottom, something that would not only hide it from prey,
but also from possible predators. Once Gerrothorax
realised that
prey was available it would open its mouth, the inside of which was
probably camouflaged as well with the exception for a brightly coloured
lure. Gerrothorax could then use such a lure by
moving it about to
look like a worm which would then attract the attention of fish and
smaller amphibians that were themselves looking for prey. Once close
all Gerrothorax would have to do is bring its upper
jaw down to trap
its prey.
While
there are no fossils to confirm the presence of a lure in Gerrothorax,
this hunting strategy can be observed today in the alligator snapping
turtle (Macrochelys temminkii) which has a
brightly coloured
appendage that looks like a worm on the tip of its tongue. The turtle
lies completely motionless with the exception of wriggling its lure
until a fish swims towards its mouth with the intent of eating the
‘worm’. Once the fish come too close the turtle lunges forward
with its head and snaps its sharp jaws around the fish.
Another
interesting thing about Gerrothorax is the
retention of larval stage
gills. These gills were situated directly behind the head and
arranged into three pairs. There are known to be a permanent feature
of adult animals because they were actually supported by bone. It is
possible that the gills were sometimes placed behind the broad head to
both protect them from damage as well as hide them from view. The
retention of these gills, a feature known as neoteny, also means
that Gerrothorax probably spent most if not all of
its life within the
water.
Needless
to say that the gills allowed for a greater amount of oxygen to be
absorbed from the water, by doing so Gerrothorax
may have been able
to survive in water with lower oxygen content. They may have also
helped Gerrothorax to attain it large one meter
length, as even
though amphibians absorb oxygen through their skin, and Gerrothorax
did have a proportionately large skin surface to body mass ratio, the
lower body would have been in regular contact with the bottom of its
environment. It’s possible that this contact restricted the even flow
of water over its body, as well as the presence of features for the
purpose of camouflage and protection that hampered its ability to
process oxygen from above, necessitating the presence of external
gills.
The
retention of juvenile stage gills can also be seen in the much smaller
amphibian Microbachis
from the Permian, which itself resembles a
modern day axolotl.
Further reading
- Gerrothorax pulcherrimus from the Upper Triassic
Fleming Fjord
Formation of East Greenland and a reassessment of head lifting in
temnospondyl feeding. - Journal of Vertebrate Paleontology 28 (4):
935–950. - Farish A. Jenkins Jr, Neil H. Shubin, Stephen M. Gatesy
& Anne Warren - 2008.
- Braincase, palatoquadrate and ear region of the plagiosaurid
Gerrothorax pulcherrimus from the Middle Triassic of
Germany. -
Palaeontology 55 (1): 35–50. - F. Witzmann, R. R. Schoch, A. Hilger
& N. Kardjilov - 2011.
- Cranial morphology of the plagiosaurid Gerrothorax
pulcherrimus as an
extreme example of evolutionary stasis. - Lethaia vol45, issue 3, pp
371-385 - Rainer R. Schoch & Florian Witzmann - 2011.
- Reconstruction of cranial and hyobranchial muscles in the Triassic
temnospondyl Gerrothorax provides evidence for akinetic suction
feeding. - Journal of Morphology. 274 (5): 525–542. - Florian Witzmann
& Rainer R. Schoch - 2012.
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