Hurdiidae

Hurdiidae (synonymous with the previously named Peytoiidae ) is an extinct cosmopolitan family of radiodonts, a group of stem-group arthropods, which lived during the Paleozoic Era. It is the most long-lived radiodont clade, lasting from the Cambrian period to the Devonian period.

Description
Hurdiidae is characterized by frontal appendages with distal region composed of 5 subequal blade-like endites, alongside the enlarged head carapaces and tetraradial mouthpart (oral cone).

The frontal appendages of hurdiids have a distinctive morphology, with the appendage of most species bearing five equally-sized elongate blade-like ventral spines known as endites. Subsequent podomeres were reduced in size and with only small endites or none. Each podomere bore only a single endite, unlike other radiodonts, in which the endites were paired. In most species, the endites were curved medially, so that the appendages formed a basket-like structure. Some hurdiids had greater numbers of endites, with Cordaticaris bearing seven endites of equal length. Ursulinacaris is unique among hurdiids in bearing paired endites, which is likely a transitional form between the appendage of other radiodonts and that of hurdiids.

Hurdiids exhibited a wide range of body size. The smallest known hurdiid specimen, of an unnamed species, is estimated to have had a body length of 6 –, but it is not known whether this specimen was juvenile or adult. Aegirocassis, the largest known hurdiid, was over 2 m long, comparable in size to the largest known arthropods.

Paleobiology
The majority of hurdiids appear to have been predators that fed by sifting sediment with their frontal appendages, but some members, like Aegirocassis, Pseudoangustidontus, and possibly Cambroraster were suspension feeders.

Distribution
Hurdiids had a global distribution. The earliest known hurdiid in the fossil record is Peytoia infercambriensis, which lived during the third age of the Cambrian in what is now the country of Poland. The group increased in diversity during the Miaolingian epoch. Post-Cambrian records of the group are rare, but the group lasted into the Devonian period, with the last known taxon being the Emsian Schinderhannes bartelsi from what is now Germany.

Classification
Hurdiidae is classified within Radiodonta, a clade of stem-group arthropods. Hurdiidae is defined phylogenetically as the most inclusive clade containing Hurdia victoria but not Amplectobelua symbrachiata, Anomalocaris canadensis, or Tamisiocaris borealis. Some authors have argued that Peytoiidae, which was named by Conway Morris and Robison, 1982, has priority over Hurdiidae, and that Hurdiidae has "yet to be properly established following ICZN standards".

The phylogeny of hurdiids, accompanying the description of the hurdiids Aegirocassis benmoulae, Titanokorys gainesii, and the analyzation of Stanleycaris hirpex as follows:

Species include

Tauricornicaris was previously considered as a member of hurdiid, but later reinterpreted as euarthropod tergites.
 * Buccaspinea cooperi
 * Cambroraster falcatus
 * Cordaticaris striatus
 * Hurdia triangulata
 * Hurdia victoria
 * Pahvantia hastata
 * Peytoia nathorsti
 * Peytoia infercambriensis
 * Schinderhannes bartelsi
 * Stanleycaris hirpex
 * Titanokorys gainesii
 * Ursulinacaris grallae
 * Zhenghecaris shankouensis?
 * Aegirocassisinae
 * Aegirocassis benmoulai
 * Pseudoangustidontus duplospineus
 * ''Pseudoangustidontus izdigua

Zhenghecaris is originally described as a thylacocephalan, but it is later considered as hurdiid dorsal carapace. However, placement is questioned by some researchers, because classification as radiodont is majorly based on characters of Tauricornicaris.

Schinderhannes was originally described as stem-arthropod which have both characters of euarthropods and radiodonts, this interpretation was denied and most researchers agree that is hurdiid radiodont. There are some researchers who questioned its classification as hurdiid.

Huangshandongia yichangensis, Liantuoia inflata and Proboscicaris hospes  may represent species of Hurdia.