''Thylacares'' is a genus of

thylacocephala The Thylacocephala (from the Greek or ', meaning " pouch", and or ' meaning "head") are group of extinct probable mandibulate arthropods, that have been considered by some researchers as having possible crustacean affinities. As a class they ...

n containing only the single species ''Thylacares brandonensis''. Found in

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

period strata from the

Brandon Bridge Formation The Brandon Bridge Formation is a geologic formation in Wisconsin. It preserves fossils dating back to the Silurian period, including those of graptolites, conodonts and '' Parioscorpio.'' See also * List of fossiliferous stratigraphic units ...

Waukesha, Wisconsin Waukesha ( ) is a city in and the county seat of Waukesha County, Wisconsin, United States. The population was 71,158 at the 2020 United States census, 2020 census. Located along the Fox River (Illinois River tributary), Fox River adjacent to th ...

U.S. 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 states and a federal capital district, Washington, D.C. The 48 contiguous ...

, the species is distinguishable from other thylacocephalans by its smaller

raptorial In biology (specifically the anatomy of arthropods), the term ''raptorial'' implies much the same as ''predatory'' but most often refers to modifications of an arthropod leg, arthropod's foreleg that make it function for the grasping of prey whi ...

appendages and

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. The body is fully encased in a

bivalve shell A bivalve shell is the enveloping exoskeleton or mollusc shell, shell of a bivalve mollusc, composed of two hinged halves or ''valve (mollusc), valves''. The two half-shells, called the "right valve" and "left valve", are joined by a ligament and ...

, with only the eyes protruding on stalks. The species' trunk is composed of about 22 segments. It was once considered the oldest know thylacocephalan, before older specimens were found in

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

rocks.

References

Prehistoric crustaceans Silurian arthropods Silurian arthropods of North America Fossil taxa described in 2014 Paleontology in Wisconsin {{paleo-crustacean-stub Thylacocephala