Researchers discover first dinosaur era crabfully preserved in amber20 October 2021

Cretapsara athanata: The first crab in amber from thedinosaur era. Credit: Xiao Jia (Longyin Amber Museum)

Fossils trapped in amber provide a uniquesnapshot of the anatomy, biology, and ecology ofextinct organisms. The most common fossils foundin amber, which is formed from resin exuded fromtree bark, are land-dwelling animals, mainlyinsects. But on very rare occasions scientistsdiscover amber housing an aquatic organism.

In a study published October 20 in ScienceAdvances an international team of researchersdescribe the first crab from the Cretaceousdinosaur era preserved in amber. The study usedmicro CT to examine and describe Cretapsaraathanata, the oldest modern-looking crab(approximately 100 million years old) and the mostcomplete fossil crab ever discovered. It is rivalledin completeness by the mysterious Callichimaeraperplexa, a very distant relative nicknamed theplatypus of the crab world. Callichimaera'sstunning preservation included soft tissues anddelicate parts that rarely fossilize. Both Cretapsaraand Callichimaera are new branches in the crabtree of life that lived during the Cretaceous Crab

Revolution, a period when crabs diversifiedworldwide and the first modern groups originatedwhile many others disappeared.

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

Artistic reconstruction of Cretapsara athanata: Theimmortal Cretaceous spirit of the clouds and waters.Credit: Artwork by Franz Anthony, courtesy of JavierLuque (Harvard University).

The crab fossil record extends back into the earlyJurassic, more than 200 million years ago.Unfortunately, fossils of nonmarine crabs aresparse and largely restricted to bits and pieces ofthe animals carapace—claws and legs found insedimentary rocks. That is until now with thediscovery of Cretapsara athanata. "The specimen is

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spectacular, it is one of a kind. It's absolutelycomplete and is not missing a single hair on thebody, which is remarkable," said lead author JavierLuque, postdoctoral researcher in the Departmentof Organismic and Evolutionary Biology, HarvardUniversity.

A group of scientists led by co-lead author LidaXing, China University of Geosciences, Beijing,made micro CT scans of the fossil, which is housedin the Longyin Amber Museum in Yunnan, China.The scans created a full three-dimensionalreconstruction of the exquisite preservation of theanimal allowing Luque, Xing, and their team to seethe complete body of the animal including delicatetissues, like the antennae and mouthparts linedwith fine hairs. Shockingly they discovered theanimal also had gills.

C. athanata Luque gen. et sp. nov., a modern-lookingeubrachyuran crab in Burmese amber. (A to D) HolotypeLYAM-9. (A) Whole amber sample with crab inclusion inventral view. (B) Close-up of ventral carapace. (C) Wholeamber sample with crab inclusion in dorsal view. (D)Close-up of dorsal carapace. White arrows in (B) and (D)indicate the detached left fifth leg or pereopod. Credit:Javier Luque and Lida Xing

"The more we studied the fossil, the more werealized that this animal was very special in manyways," said Luque. Cretapsara is remarkablymodern-looking—superficially resembling someshore crabs found today—unlike most crabs during

the mid-Cretaceous era which looked quite differentfrom modern crabs. Yet, the animal was entombedin Cretaceous amber and the presence of well-developed gills indicated an aquatic to semi-aquaticanimal. Aquatic animals are rarely preserved in treeresins that become amber. Crabs previously foundin amber are by the handful and belong to a livinggroup of tropical land and tree-dwelling crabsknown as Sesarmidae from the Miocene (15 millionyears ago). How then, the researchers asked, did a100 million year old aquatic animal becomepreserved in tree amber, which normally housesland-dwelling specimens?

Gills allow aquatic animals to breathe in water. Butcrabs have successfully and independentlyconquered land, brackish water, and fresh water atleast twelve times since the dinosaur era. In doingso their gills evolved to include lung-like tissueallowing them to breathe both in and out of thewater. Cretapsara however, had no lung tissue,only well-developed gills indicating the animal wasnot completely land dwelling. "Now we were dealingwith an animal that is likely not marine, but also notfully terrestrial," Luque said. "In the fossil record,nonmarine crabs evolved 50 million years ago, butthis animal is twice that age."

3D mesh of C. athanata Luque gen. et sp. nov. holotypeLYAM-9. (A to E) 3D mesh extracted from reconstructedmicro-CT data in VGSTUDIO MAX, remeshed inMeshLab, and visualized using Autodesk Maya: (A)dorsal, (B) ventral, (C) right lateral, (D) oblique postero-dorsal, (E) oblique antero-ventral views, showing theclaws of equal size and four pairs of slender legs similarin shape and size, with P5 slightly smaller than the other

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legs. (F and G) Details of the dorsal (F) and ventral (G)carapace, showing details of the large eyes and orbits,small antennae, and a small, acute outer orbital spine[(F) thick arrow], two small anterolateral spines (F, thinarrows), a posterolateral margin bearing at least foursmall and equidistant tubercles (F, small arrows), straightposterior margin, slender coxae of the pereopods, atypical heterotreme eubrachyuran sternum (G), and areduced and folded pleon with the first pleonites dorsallyexposed. Left fifth pereopod digitally reattached. bcg,branchiocardiac groove; ca, carpus; cg, cervical groove;cx, coxa; da, dactylus; ib, ischiobasis; ma, manus or palmof claw; P1, claws or chelipeds; P2 to P5, pereopods orwalking legs 2 to 5; po, pollex or fixed finger chelipedpropodus; pr, propodus. Credit: Elizabeth Clark andJavier Luque. Used in journal.

The team's phylogenetic studies show thatcarcinization (the evolution of true crab-lookingforms) had actually already occurred in the mostrecent common ancestor shared by all moderncrabs more than 100 million years ago. Cretapsarabridges the gap in the fossil record and confirmsthat crabs actually invaded land and fresh waterduring the dinosaur era, not during the mammalera, pushing the evolution of nonmarine crabsmuch further back in time.

The researchers hypothesize that Cretapsara,measuring at five millimeters in leg span, was ajuvenile crab of a freshwater to amphibiousspecies. Or, that the animal is perhaps a semi-terrestrial juvenile crab migrating onto land fromwater as occurs to the iconic Christmas Island redcrabs where land dwelling mother crabs releasetheir babies into the ocean, which later swarm outof the water back onto land. Theyfurther hypothesize that like the crabs found inamber from the Miocene, Cretapsara could havebeen a tree climber. "These Miocene crabs are trulymodern looking crabs and, as their extant relatives,they live in trees in little ponds of water," saidLuque, "these arboreal crabs can get trapped intree resin today, but would it explain why Cretapsara is preserved in amber?"

Luque's research is centered on understanding whythings evolve into crabs, and their evolution anddiversification over time leading to the modernforms seen today. "This study is pushing the timing

of origin of many of these groups back in time.Every fossil we discover challenges ourpreconceptions about the time and place of originof several organisms, often making us look furtherback in time," Luque said.

The study is part of a National Science Foundationfunded project with Luque, Professor Javier Ortega-Hernández and postdoctoral researcher JoannaWolfe, both in the Department of Organismic andEvolutionary Biology, Harvard University, andProfessor Heather Bracken-Grissom, FloridaInternational University.

The researchers chose the name Cretapsaraathanata, which means the immortal Cretaceousspirit of the clouds and waters, to honor theCretaceous, during which this crab lived, andApsara, a spirit of the clouds and waters in Southand Southeast Asian mythology. The species nameis based on "athanatos", immortal, referring to itslifelike preservation as if 'frozen in time' in the timecapsule that is amber.

More information: Crab in amber reveals anearly colonization of non-marine environmentsduring the Cretaceous, Science Advances (2021). science.org/doi/10.1126/sciadv.abj5689

Author's Statement: The studied fossil, deposited inthe Longyin Amber Museum (LYAM), YunnanProvince, China, comes from a batch of commercial"raw" (dull, unpolished) amber pieces collected bylocal miners and sold to a vendor at an amberjewelry market in Myitkyina on May 12, 2015. Thepolished piece containing the fossil studied wasacquired by LYAM from the vendor's mineral storein Tengchong, China, on 10 August 2015. Weacknowledge the existence of a sociopoliticalconflict in northern Myanmar and have limited ourresearch to material predating the 2017 resumptionof hostilities in the region. We hope that conductingresearch on specimens collected before the conflictand acknowledging the situation in the Kachin Statewill serve to raise awareness of the current conflictin Myanmar and the human cost behind it.

Provided by Harvard University

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APA citation: Researchers discover first dinosaur era crab fully preserved in amber (2021, October 20)retrieved 24 November 2021 from https://phys.org/news/2021-10-dinosaur-era-crab-fully-amber.html

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