The role of diet in the rise of modern shark communities

Date:
October 6, 2021
Source:
Uppsala University
Summary:
The availability of prey and the ability to adapt to changing
environments played key roles in the evolution of sharks. A new
study, in which over 3,000 shark teeth were analyzed, provides
new insight into how modern shark communities were established.



FULL STORY ==========================================================================
The availability of prey and the ability to adapt to changing environments played key roles in the evolution of sharks. A new study, in which over
3,000 shark teeth were analysed, provides new insight into how modern
shark communities were established. The results are published in the
journal Current Biology.


==========================================================================
The new research investigated the diversity of shark teeth from the
final stages of the Mesozoic (approximately 80 million years ago) to
the present day.

In particular, it sought to understand why there are only 15 species
of mackerel sharks living today (scientific name: Lamniformes), whereas
there are more than 290 species of ground sharks (Carcharhiniformes).

"The modern-day imbalance in diversity between mackerel and ground sharks
has, until now, lacked a deep-time ecological perspective. However,
if we look to the past, we see the opposite pattern: Lamniforms were
more diverse than Carcharhiniforms," says study leader Dr Mohamad Bazzi, previously a researcher at Uppsala University but now at the University
of Zurich.

The evolution of sharks is a story told by their teeth. This is because
most of what remains behind for palaeontologists to work on are shed
teeth. As a result, researchers must seek innovative approaches to help
us fill in the many gaps in knowledge relating to their past.

"Tooth morphology is a more direct proxy for the living habits of sharks
and our study is the first attempt to explore how diet impacted lamniform
and carcharhiniform sharks over the past 83 million years. By measuring
the association between tooth shapes and diets from living sharks,
we built a foundation for interpreting changes in diet in the fossil
record," says Dr Bazzi.

The research team compiled an extensive database of over 3,000 shark teeth
and then evaluated their shape. To attain a measure of shape diversity
(or disparity as it is technically called) the study relied on a method
known as geometric morphometrics, which is a type of mathematical shape analysis.



==========================================================================
To understand the role played by diet in shaping the diversity of sharks,
the team assembled previously published records of shark diets attained
from stomach contents.

Many Cretaceous Lamniformes had specialised diets for eating marine
reptiles and, following their loss at the end of the age of dinosaurs,
these Lamniformes went extinct. On the other hand, Lamniformes with
more generalised diets and Carcharhiniformes survived the extinction
event. Carcharhiniformes particularly benefited from the diversification
of bony fish following the extinction event, and the spread of coral
reefs about 56 million years ago.

As it turns out, the same extinction that killed off the non-bird
dinosaurs may have set the stage for the modern shark community.

A similar event may have occurred only 5 million years ago to the largest
shark ever to have existed, the lamniform megalodon (scientific name:
Otodus megalodon).

Megalodon fossils are mostly from the Miocene (which spanned from
approximately 23 to 5 million years ago). During this period, lamniform
sharks had record-low tooth disparity. Megalodon likely specialised in
eating the large whales of that time and so this low disparity again
suggests that specialised diets among top predatory sharks may place
them at a greater risk of extinction.

"Here, we have a good example of the important biological insights
offered by studying fossils," says Dr Nicola's Campione, co-author and
member of the University of New England's Palaeoscience Research Centre.

"Our research demonstrates that living Lamniformes are the result
of repeated extinction events, perhaps resulting from a tendency to
specialise. Dietary specialisation, on top of environmental changes,
were likely major factors behind the previously mentioned imbalance
between Lamniformes and Carcharhiniformes," says Dr Campione.

"We now have evidence that the availability of prey and the ability of
sharks to adapt to changing environments played an important role in
their past evolution. These factors underpinned the modern diversity
of sharks and, critically, will likely decide their survival into the
future," says Dr Bazzi.

========================================================================== Story Source: Materials provided by Uppsala_University. Original written
by Linda Koffmar.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Mohamad Bazzi, Nicola's E. Campione, Benjamin P. Kear, Catalina
Pimiento,
Per E. Ahlberg. Feeding ecology has shaped the evolution of modern
sharks. Current Biology, 2021; DOI: 10.1016/j.cub.2021.09.028 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211006104426.htm

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