For the reptiles, see Anapsida; for the crustaceans, see Anaspididae

Anaspididae

Anaspididae is a family of freshwater crustacean that is endemic to Tasmania, Australia. The family contains 3 genera and 5 species. This group of crustaceans are considered living fossils. They are commonly and collectively known as the Tasmanian anaspid crustaceans. Anaspidids have stalked eyes,...

The Anaspida ("without shield") are stem gnathostomes, and are classically regarded as the ancestors of lampreys. Anaspids were small marine agnatha

Agnatha

Agnatha is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. The group excludes all vertebrates with jaws, known as gnathostomes....

ns that lacked scales and paired fins, but have a striking highly hypocercal tail. They first appeared in the early Silurian

Silurian

The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

, and flourished until the Late Devonian extinction

Late Devonian extinction

The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota. A major extinction, the Kellwasser Event, occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, , about 374 million years ago...

, during the late Devonian

Devonian

The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

, where most species, save for lamprey

Lamprey

Lampreys are a family of jawless fish, whose adults are characterized by a toothed, funnel-like sucking mouth. Translated from an admixture of Latin and Greek, lamprey means stone lickers...

s, went extinct due to the environmental upheaval during that time.

Anatomy

Unusually for an agnathan, anaspids did not possess a bony shield or armor. The head is instead covered in an array of smaller, weakly mineralized scales. They have large laterally placed eyes with no sclerotic ring

Sclerotic ring

Sclerotic rings are rings of bone found in the eyes of several groups of vertebrate animals, except for mammals and crocodilians. They can be made up of single bones or small bones together. They are believed to have a role in supporting the eye, especially in animals whose eyes are not spherical,...

, and the gill

Gill

A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

s opened as a row of holes along either side of the animal, typically numbering anything from 6-15 pairs. The major synapomorphy

Synapomorphy

In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...

 for the anaspids is the large, tri-radiate spine behind the series of the gill openings.

Phylogeny and Taxonomy