Large bodies helped extinct marine reptiles with long necks swim, new
study finds

Date:
April 28, 2022
Source:
University of Bristol
Summary:
Scientists have discovered that body size is more important than
body shape in determining the energy economy of swimming for
aquatic animals.



FULL STORY ========================================================================== Scientists at the University of Bristol have discovered that body size
is more important than body shape in determining the energy economy of
swimming for aquatic animals.


==========================================================================
This study, published today in Communications Biology, shows that big
bodies help overcome the excess drag produced by extreme morphology,
debunking a long- standing idea that there is an optimal body shape for
low drag.

One important finding of this research is that the large necks of extinct elasmosaurs did add extra drag, but this was compensated by the evolution
of large bodies.

Tetrapods or 'four-limbed vertebrates', have repeatedly returned to the
oceans over the last 250 million years, and they come in many shapes and
sizes, ranging from streamlined modern whales over 25 meters in length,
to extinct plesiosaurs, with four flippers and extraordinarily long necks,
and even extinct fish-shaped ichthyosaurs.

Dolphins and ichthyosaurs have similar body shapes, adapted for moving
fast through water producing low resistance or drag. On the other hand, plesiosaurs, who lived side by side with the ichthyosaurs in the Mesozoic
Era, had entirely different bodies. Their enormous four flippers which
they used to fly underwater, and variable neck lengths, have no parallel amongst living animals.

Some elasmosaurs had really extreme proportions, with necks up to 20 feet
(6 metres) long. These necks likely helped them to snap up quick-moving
fish, but were also believed to make them slower.

Until now, it has not been clear how shape and size influenced the energy demands of swimming in these diverse marine animals. Palaeobiologist
Dr Susana Gutarra Di'az of Bristol's School of Earth Sciences and the
National History Museum of London who led the research, explained: "To
test our hypotheses, we created various 3D models and performed computer
flow simulations of plesiosaurs, ichthyosaurs and cetaceans. These
experiments are performed on the computer, but they are like water
tank experiments." Dr Colin Palmer, an engineer involved in the project
said: "We showed that although plesiosaurs did experience more drag than ichthyosaurs or whales of equal mass because of their unique body shape,
these differences were relatively minor. We found that when size is taken
into account, the differences between groups became much less than the
shape differences. We also show that the ratio of body length to diameter, which is widely used to classify these aquatic animals as more or less efficient, is not a good indicator of low drag." Dr Gutarra Di'az said,
"We were also particularly interested in the necks of elasmosaurs and so,
we created hypothetical 3D models of plesiosaurs with various lengths of
necks. Simulations of these models reveal that past a certain point, the
neck adds extra drag, which potentially would make swimming costly. This 'optimal' neck limit lies around twice the length of the trunk of the
animal." Dr Benjamin Moon, another collaborator and expert on marine
reptiles, continued: "When we examined a large sample of plesiosaurs
modelled on really well preserved fossils at their real sizes, it turns
out that most plesiosaurs had necks below this high-drag threshold,
within which neck can get longer or shorter without increasing drag. But
more interestingly, we showed that plesiosaurs with extremely long necks
also had evolved very large torsos, and this compensated for the extra
drag!" Dr Tom Stubbs, another co-author summarised: "This study shows
that, in contrast with prevailing popular knowledge, very long necked plesiosaurs were not necessarily slower swimmers than ichthyosaurs and
whales, and this is in part thanks to their large bodies. We found that
in elasmosaurs, neck proportions changed really fast. This confirms that
long necks were advantageous for elasmosaurs in hunting, but they could
not exploit this adaptation until they became large enough to offset the
cost of high drag on their bodies." Professor Mike Benton, also part
of the research, commented: "Our research suggests that large aquatic
animals can afford to have crazy shapes, as in the elasmosaurs. But
there are limits: body sizes cannot get indefinitely large, as there are
some constraints to very large sizes as well. The maximum neck lengths
we observe, seem to balance benefits in hunting versus the costs of
growing and maintaining such a long neck. In other words, the necks of
these extraordinary creatures evolved in balance with the overall body
size to keep friction to a minimum."

========================================================================== Story Source: Materials provided by University_of_Bristol. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Susana Gutarra, Thomas L. Stubbs, Benjamin C. Moon, Colin Palmer,
Michael
J. Benton. Large size in aquatic tetrapods compensates for high
drag caused by extreme body proportions. Communications Biology,
2022; 5 (1) DOI: 10.1038/s42003-022-03322-y ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/04/220428125419.htm

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