''Callichimaera perplexa'' is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

of small crabs known from the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

Churuvita Group of Colombia and the

Frontier Formation The Frontier Formation is a sedimentary geological formation whose strata date back to the Late Cretaceous. The formation's extents are: northwest Colorado, southeast Idaho, southern Montana, northern Utah, and western Wyoming. It occurs in many ...

of the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...

. Because it possesses a strange combination of anatomical features, it is said to be the "

platypus The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal endemic to eastern Australia, including Tasmania. The platypus is the sole living representative or mono ...

of crabs". The presence of certain features in this species, such as its large claws and swimming limbs, confirm that those features were present in the crab lineage up to 95 million years ago. It evolved during the

Cretaceous crab revolution The Cretaceous crab revolution refers to a major diversification event of Brachyura crabs (also known as true crabs) that took place during the Cretaceous Period, from 145 to 66 million years ago. Nearly 80% of modern groups of crabs originated d ...

Etymology

The generic name ''Callichimaera'' is derived from the Ancient Greek prefix "calli-" (kalós), meaning beautiful, and "chimaera" (chímaira), meaning a composite mythological animal, a reference to the bizarre forms and characteristics of this species. The specific name is derived from the Latin "perplexus", meaning intricate (or perplexing in that manner), a reference to the complex anatomical affinities this species displays.

Description

''Callichimaera'' possesses features that are commonly associated with the

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. ...

l crab stage called the

megalopa Crustaceans may pass through a number of larval and immature stages between hatching from their eggs and reaching their adult form. Each of the stages is separated by a moult, in which the hard exoskeleton is shed to allow the animal to grow. The ...

. This includes features like large

compound eye A compound eye is a visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidia, which are tiny independent photoreception units that consist of a cornea, lens, and photoreceptor cells which disti ...

s, a small fusiform body, and thin mouth parts. However, the fossil crabs also show evidence of sexual maturity, suggesting they are adult forms. This species likely evolved those larval features via a process called

heterochrony In evolutionary developmental biology, heterochrony is any genetically controlled difference in the timing, rate, or duration of a developmental process in an organism compared to its ancestors or other organisms. This leads to changes in the ...

. These crabs were small, with many that are nearly the size of a U.S.

quarter A quarter is one-fourth, , 25% or 0.25. Quarter or quarters may refer to: Places * Quarter (urban subdivision), a section or area, usually of a town Placenames * Quarter, South Lanarkshire, a settlement in Scotland * Le Quartier, a settlement ...

. The crabs range in size from a few millimeters up to two centimeters, also suggesting that both juveniles and adults are present. ''Callichimaera'' does not have the typical crab-like shape, something that has occurred several times among crabs since the Cretaceous. Because ''Callichimaera'' has large paddle-like legs, it was probably an active swimmer, opposed to predominantly living on the seafloor.

Paleoecology

''Callichimaera'' lived in a

tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...

environment. Fossils are rarely preserved in tropical settings in comparison to other parts of the world, making this fossil an even more unusual discovery. An in-depth study of the crab's eyes indicated that ''Callichimaera'' was an active

predator Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...

living high in the

water column A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...

. ''Callichimaera'' was discovered in deposits also containing comma shrimp and caridean shrimp.

References

{{Taxonbar, from=Q105722227 Crabs Late Cretaceous crustaceans Late Cretaceous animals of South America Late Cretaceous animals of North America Cretaceous arthropods of North America Fossil taxa described in 2019 Fossils of Colombia Cretaceous Colombia Cretaceous United States Fossils of the United States Cretaceous arthropods of South America