''Callichimaera perplexa'' is a

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

of small

crab Crabs are decapod crustaceans of the infraorder Brachyura (meaning "short tailed" in Greek language, Greek), which typically have a very short projecting tail-like abdomen#Arthropoda, abdomen, usually hidden entirely under the Thorax (arthropo ...

s known from the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

Churuvita Group of

Colombia Colombia, officially the Republic of Colombia, is a country primarily located in South America with Insular region of Colombia, insular regions in North America. The Colombian mainland is bordered by the Caribbean Sea to the north, Venezuel ...

and the Frontier Formation of the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

. 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 monotypi ...

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.

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 (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...

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 Eye, visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidium, ommatidia, which are tiny independent photoreception units that consist of a cornea, lens (anatomy), lens, and p ...

s, a small

fusiform Fusiform (from Latin ''fusus'' ‘spindle’) means having a spindle (textiles), spindle-like shape that is wide in the middle and tapers at both ends. It is similar to the lemon (geometry), lemon-shape, but often implies a focal broadening of a ...

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 biology, developmental process in an organism compared to its ancestors or other organisms. This lea ...

. These crabs were small, with carapace length up to , and width up to . ''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, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...

environment.

Fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

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 in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation ...

living high in the

water column The (oceanic) water column is a concept used in oceanography to describe the physical (temperature, salinity, light penetration) and chemical ( pH, dissolved oxygen, nutrient salts) characteristics of seawater at different depths for a defined ...

. ''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