Between shark and ray: The evolutionary advantage of the sea angels
Threatened with extinction despite perfect adaptation

Date:
August 4, 2020
Source:
University of Vienna
Summary:
Angel sharks are sharks, but with their peculiarly flat body
they rather resemble rays. An international research team has now
investigated the origin of this body shape. The results illustrate
how these sharks evolved into highly specialized, exclusively
bottom-dwelling ambush predators and thus also contribute to a
better understanding of their threat from environmental changes.



FULL STORY ==========================================================================
The general picture of a shark is that of a fast and large ocean
predator. Some species, however, question this image -- for example
angel sharks. They have adapted to a life on the bottom of the oceans,
where they lie in wait for their prey. In order to be able to hide on
or in the sediment, the body of angel sharks became flattened in the
course of their evolution, making them very similar to rays, which are
closely related to sharks.


========================================================================== Flattened body as indication for a successful lifestyle The oldest
known complete fossils of angel sharks are about 160 million years
old and demonstrate that the flattened body was established early in
their evolution. This also indicates that these extinct angel sharks
already had a similar lifestyle as their extant relatives -- and that
this lifestyle obviously was very successful.

Angel sharks are found all over the world today, ranging from temperate
to tropical seas, but most of these species are threatened. In order
to understand the patterns and processes that led to their present low diversity and the possible consequences of their particular anatomy,
the team has studied the body shapes of angel sharks since their origins
using modern methods.

Today's species are very similar For this purpose, the skulls of extinct species from the late Jurassic period (about 160 million years ago)
and of present-day species were quantitatively analysed using X-ray
and CT images and prepared skulls employing geometric- morphometric
approaches. In doing so, the evolution of body shapes could be explained comparatively, independent of body size.

The results show that early angel sharks were different in their external shape, whereas modern species show a comparably lower variation in
shape. "Many of the living species are difficult to identify on the
basis of their skeletal anatomy and shape, which could be problematic
for species recognition," explains Faviel A. Lo'pez-Romero.

Angel sharks are well adapted, but react slowly to environmental
changes It has been shown that in living species the individual parts
of the skull skeleton are more closely integrated than in their extinct relatives. This led to a reduced variability in appearance during the
evolution of angel sharks.

"The effect of integrating different parts of the skull into individual,
highly interdependent modules can lead to a limited ability to evolve
in different forms, but at the same time increases the ability to
successfully adapt to specific environmental conditions," explains
Ju"rgen Kriwet.

In the case of the angel sharks, increasing geographical isolation
resulted in the development of different species with very similar
adaptations. "But modular integration also means that such animals are
no longer able to react quickly to environmental changes, which increases
their risk of extinction," concludes Ju"rgen Kriwet.


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


========================================================================== Journal Reference:
1. Faviel A. Lo'pez-Romero, Sebastian Stumpf, Cathrin Pfaff, Giuseppe
Marrama`, Zerina Johanson, Ju"rgen Kriwet. Evolutionary
trends of the conserved neurocranium shape in angel sharks
(Squatiniformes, Elasmobranchii). Scientific Reports, 2020; 10
(1) DOI: 10.1038/s41598- 020-69525-7 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200804111528.htm

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