The crab family has just received a series of new cousins, including a 95-million-year-old species that will force scientists to rethink the definition of a crab – and perhaps the different evolution of animals over time.
An international team of researchers headed by Yale paleontologist Javier Luque announced the discovery of hundreds of exceptionally well-preserved specimens from rock formations in Colombia and the United States, dating back to the mid-Cretaceous period of 90 to 95 million years. The cache contains hundreds of small fossils with prawns, their telltale curve-like curve; several carideans, the widespread "real" shrimp; and a whole new branch of the evolution tree for crabs.
The most intriguing discovery, the researchers say, is Callichimaera perplexa, the earliest example of a floating arthropod with paddle-shaped legs since the extinction of scorpionfish more than 250 million years ago. The name comes from the Chimera, a mythological creature that has body characteristics of more than one animal. Callichimaera's full name means "amazingly beautiful chimera".
Videocredit: Images by Daniel Ocampo R., Vencejo Films & Javier Luque, Yale University / Animation and 3D Reconstruction by Alex Duque.
Callichimaera is about a quarter tall. Luque notes are "unusual and sweet" – large compound eyes – without eye sockets, curved claws, leg-like jaws, exposed tail and long body – are typical of cancer larvae from the open sea. This suggests that some ancient crabs have retained some of their larval characteristics into adulthood, reinforced and developed a new body architecture. This is an evolutionary process called "heterochrony."
"Callichimaera perplexa is so unique and alien that it can be considered a platypus of the crab world," said Luque. "It suggests how new forms evolve over time and become so different. Usually we think of crabs as big animals with wide carapaces, powerful claws, small eyes in long eye stems and a small tail stowed under the body. Well, Callichimaera resists all these "crabby" features and forces us to rethink our definition of what makes a crab a crab. "
Luque also pointed out the importance of discovery in a tropical region of the world. There are fewer researchers actively looking for fossils in the tropics, he said, and the amount of land cover and dense vegetation of tropical forests make access to well-exposed rocks more difficult.
"It is very exciting to find new branches in the tree of life from a distant past, especially from regions like the tropics, which, although today hotspots of diversity, are places where we have the least know about their past diversity, "Luque said.
Luque's team consisted of researchers from the University of Alberta, Kent State University, the University of Montreal, the Smithsonian Tropical Research Institute of Panama, the Canadian Parks and Wilderness Society, the National Autonomous University of Mexico, the University of Nevada and the College of Communication and Design in Boca Raton, Florida.
Publication: J. Luque, et al., "Extraordinary Conservation of Mid-Cretaceous Maritime Arthropods and Development of New Forms by Heterochrony", Science Advances, April 24, 2019: Vol. 5, no. 4, eaav3875; DOI: 10,1126 / sciadv.aav3875