Monday, 25 October 2021

Evolution News - Yet another of Those Pesky 'Non-Existent' Transitional Forms!

Cretapsara athanata Luque gen. et sp. nov., a modern-looking eubrachyuran crab in Burmese amber.(A to D) Holotype LYAM-9. (A) Whole amber sample with crab inclusion in ventral view. (B) Close-up of ventral carapace. (C) Whole amber 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.

Photos by L.X. Figure by J.L.
International team of researchers discover first dinosaur era crab fully preserved in amber | Department of Organismic and Evolutionary Biology.

Another favourite Creationist article of faith was shattered again today with the publication of an open access article in Science Advances by a Chinese team of researchers, describing a Cretaceous crab that shows incomplete transition between an aquatic and a terrestrial existence.

The crab is beautifully preserved in amber and shows an amazing level of detail of soft tissues that are rarely fossilised, even down to the fine hairs on its mouth-parts. It has the gills of an aquatic crab, and lacks the lung-like adaptations of gills characteristic of terrestrial crabs, yet it is clearly terrestrial hence becoming trapped in amber. It thus represents a transition between marine and terrestrial crabs.

The team used micro-CT technology to examine the crab, Cretapsara athanata. At 100 million years old it is the oldest modern-looking crab ever described and is from the Cretaceous era when crabs were diversifying - the so-called Cretaceous Crab Revolution - when the modern crabs originated. According to the Harvard University Department of Organismic & Evolutionary Biology (OEB) press release:
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.

The more we studied the fossil, the more we realized that this animal was very special in many ways.

Now we were dealing with an animal that is likely not marine, but also not fully terrestrial. In the fossil record, nonmarine crabs evolved 50 million years ago, but this animal is twice that age.

Dr Javier Luque, Lead author
Postdoctoral researcher
Department of Organismic and Evolutionary Biology,
Harvard University.
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 animal's carapace – claws and legs found in sedimentary rocks. That is until now with the discovery of Cretapsara athanata

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…

3D mesh of Cretapsara athanata Luque gen. et sp. nov. holotype LYAM-9.(A to E) 3D mesh extracted from reconstructed micro-CT data in VGSTUDIO MAX, remeshed in MeshLab, and visualized using Autodesk Maya: (A) dorsal, (B) ventral, (C) right lateral, (D) oblique postero-dorsal, (E) oblique antero-ventral views, showing the claws of equal size and four pairs of slender legs similar in shape and size, with P5 slightly smaller than the other 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, thin arrows), a posterolateral margin bearing at least four small and equidistant tubercles (F, small arrows), straight posterior margin, slender coxae of the pereopods, a typical heterotreme eubrachyuran sternum (G), and a reduced and folded pleon with the first pleonites dorsally exposed. Left fifth pereopod digitally reattached. bcg, branchiocardiac groove; ca, carpus; cg, cervical groove; cx, coxa; da, dactylus; ib, ischiobasis; ma, manus or palm of claw; P1, claws or chelipeds; P2 to P5, pereopods or walking legs 2 to 5; po, pollex or fixed finger cheliped propodus; pr, propodus.

Images by E.G.C. Figure by J.L.
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.

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.
In their open access paper published in Science Advances the authors say:

Amber fossils provide snapshots of the anatomy, biology, and ecology of extinct organisms that are otherwise inaccessible. The best-known fossils in amber are terrestrial arthropods—principally insects—whereas aquatic organisms are rarely represented. Here, we present the first record of true crabs (Brachyura) in amber—from the Cretaceous of Myanmar [~100 to 99 million years (Ma)]. The new fossil preserves large compound eyes, delicate mouthparts, and even gills. This modern-looking crab is nested within crown Eubrachyura, or “higher” true crabs, which includes the majority of brachyuran species living today. The fossil appears to have been trapped in a brackish or freshwater setting near a coastal to fluvio-estuarine environment, bridging the gap between the predicted molecular divergence of nonmarine crabs (~130 Ma) and their younger fossil record (latest Cretaceous and Paleogene, ~75 to 50 Ma) while providing a reliable calibration point for molecular divergence time estimates for higher crown eubrachyurans.

The great thing about these frequent transitional forms and so incidental refutations of a basic article of faith of the Creationist cult, is the way Creationists and the frauds who live off their gullibility, studiously ignore them and continue to insist they don't exist. It's as though they live in some sort of weird parallel Universe in which reality obediently conforms to their requirements.

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