First dinosaur era crab fully preserved in amber discovered

Date:
October 20, 2021
Source:
Harvard University, Department of Organismic and Evolutionary
Biology
Summary:
Researchers describe the first crab from the Cretaceous dinosaur
era preserved in amber. The study used micro CT to examine and
describe Cretapsara athanata, the oldest modern-looking crab
(approximately 100 million years old) and the most complete fossil
crab ever discovered.



FULL STORY ========================================================================== Fossils trapped in amber provide a unique snapshot of the anatomy,
biology, and ecology of extinct organisms. The most common fossils
found in amber, which is formed from resin exuded from tree bark,
are land-dwelling animals, mainly insects. But on very rare occasions scientists discover amber housing an aquatic organism.


==========================================================================
In a study published October 20 in Science Advances an international
team of researchers describe the first crab from the Cretaceous
dinosaur era preserved in amber. The study used micro CT to examine
and describe Cretapsara athanata, the oldest modern-looking crab
(approximately 100 million years old) and the most complete fossil
crab ever discovered. It is rivalled in completeness by the mysterious Callichimaera perplexa, a very distant relative nicknamed the platypus
of the crab world. Callichimaera's stunning preservation included soft
tissues and delicate parts that rarely fossilize. Both Cretapsara and Callichimaera are new branches in the crab tree of life that lived during
the Cretaceous Crab Revolution, a period when crabs diversified worldwide
and the first modern groups originated while many others disappeared.

True crabs, or Brachyura, are an iconic group of crustaceans whose
remarkable diversity of forms, species richness, and economic importance
have inspired celebrations and festivals worldwide. They've even earned
a special role in the pantheon of social media. True crabs are found
all around the world, from the depths of the oceans, to coral reefs,
beaches, rivers, caves, and even in trees as true crabs are among the
few animal groups that have conquered land and freshwater multiple times.

The crab fossil record extends back into the early Jurassic, more than
200 million years ago. Unfortunately, fossils of nonmarine crabs are
sparse and largely restricted to bits and pieces of the animals carapace
-- claws and legs found in sedimentary rocks. That is until now with
the discovery of Cretapsara athanata. "The specimen is spectacular, it
is one of a kind. It's absolutely complete and is not missing a single
hair on the body, which is remarkable," said lead author Javier Luque, postdoctoral researcher in the Department of Organismic and Evolutionary Biology, Harvard University.

A group of scientists led by co-lead author Lida Xing, China University
of Geosciences, Beijing, made micro CT scans of the fossil, which is
housed in the Longyin Amber Museum in Yunnan, China. The scans created
a full three- dimensional reconstruction of the exquisite preservation
of the animal allowing Luque, Xing, and their team to see the complete
body of the animal including delicate tissues, like the antennae and
mouthparts lined with fine hairs.

Shockingly they discovered the animal also had gills.

"The more we studied the fossil, the more we realized that this animal
was very special in many ways," said Luque. Cretapsara is remarkably modern-looking - - superficially resembling some shore crabs found today
-- unlike most crabs during the mid-Cretaceous era which looked quite
different from modern crabs.

Yet, the animal was entombed in Cretaceous amber and the presence of well- developed gills indicated an aquatic to semi-aquatic animal. Aquatic
animals are rarely preserved in tree resins that become amber. Crabs
previously found in amber are by the handful and belong to a living group
of tropical land and tree-dwelling crabs known as Sesarmidae from the
Miocene (15 million years ago). How then, the researchers asked, did
a 100 million year old aquatic animal become preserved in tree amber,
which normally houses land-dwelling specimens?


========================================================================== Gills allow aquatic animals to breathe in water. But crabs have
successfully and independently conquered land, brackish water, and fresh
water at least twelve times since the dinosaur era. In doing so their
gills evolved to include lung-like tissue allowing them to breathe both
in and out of the water.

Cretapsara however, had no lung tissue, only well-developed gills
indicating the animal was not completely land dwelling. "Now we were
dealing with an animal that is likely not marine, but also not fully terrestrial," Luque said.

"In the fossil record, nonmarine crabs evolved 50 million years ago,
but this animal is twice that age." The team's phylogenetic studies
show that carcinization (the evolution of true crab-looking forms) had
actually already occurred in the most recent common ancestor shared by
all modern crabs more than 100 million years ago. Cretapsara bridges
the gap in the fossil record and confirms that crabs actually invaded
land and fresh water during the dinosaur era, not during the mammal era, pushing the evolution of nonmarine crabs much further back in time.

The researchers hypothesize that Cretapsara, measuring at five millimeters
in leg span, was a juvenile crab of a freshwater to amphibious
species. Or, that the animal is perhaps a semi-terrestrial juvenile
crab migrating onto land from water as occurs to the iconic Christmas
Island red crabs where land dwelling mother crabs release their babies
into the ocean, which later swarm out of the water back onto land. They
further hypothesize that like the crabs found in amber from the Miocene, Cretapsara could have been a tree climber. "These Miocene crabs are
truly modern looking crabs and, as their extant relatives, they live in
trees in little ponds of water," said Luque, "these arboreal crabs can
get trapped in tree resin today, but would it explain why Cretapsara
is preserved in amber?" Luque's research is centered on understanding
why things evolve into crabs, and their evolution and diversification
over time leading to the modern forms seen today. "This study is pushing
the timing of origin of many of these groups back in time. Every fossil
we discover challenges our preconceptions about the time and place of
origin of several organisms, often making us look further back in time,"
Luque said.

The study is part of a National Science Foundation funded project with
Luque, Professor Javier Ortega-Herna'ndez and postdoctoral researcher
Joanna Wolfe, both in the Department of Organismic and Evolutionary
Biology, Harvard University, and Professor Heather Bracken-Grissom,
Florida International University.

The researchers chose the nameCretapsara athanata, which means the
immortal Cretaceous spirit of the clouds and waters, to honor the
Cretaceous, during which this crab lived, and Apsara, a spirit of the
clouds and waters in South and Southeast Asian mythology. The species name
is based on "athanatos," immortal, referring to its lifelike preservation
as if 'frozen in time' in the time capsule that is amber.

Author's Statement: The studied fossil, deposited in the Longyin Amber
Museum (LYAM), Yunnan Province, China, comes from a batch of commercial
"raw" (dull, unpolished) amber pieces collected by local miners and
sold to a vendor at an amber jewelry market in Myitkyina on May 12,
2015. The polished piece containing the fossil studied was acquired by
LYAM from the vendor's mineral store in Tengchong, China, on 10 August
2015. We acknowledge the existence of a sociopolitical conflict in
northern Myanmar and have limited our research to material predating the
2017 resumption of hostilities in the region. We hope that conducting
research on specimens collected before the conflict and acknowledging
the situation in the Kachin State will serve to raise awareness of the
current conflict in Myanmar and the human cost behind it.

========================================================================== Story Source: Materials provided by Harvard_University,_Department_of_Organismic_and
Evolutionary_Biology. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Javier Luque, Lida Xing, Derek E. G. Briggs, Elizabeth G. Clark,
Alex
Duque, Junbo Hui, Huijuan Mai and Ryan C. McKellar. Crab in amber
reveals an early colonization of nonmarine environments during
the Cretaceous.

Science Advances, 2021 DOI: 10.1126/sciadv.abj5689 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211020150253.htm

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