Hurdiidae

Anatomy of the frontal appendage of a hurdiid.

Frontal appendages of various species of hurdiid.

Dorsal carapaces of various species of hurdiid.

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).[1]

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.[2] 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.[2] In most species, the endites were curved medially, so that the appendages formed a basket-like structure.[1] Some hurdiids had greater numbers of endites, with Cordaticaris bearing seven endites of equal length.[3] 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.[2]

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 6–15 millimetres (0.24–0.59 in), but it is not known whether this specimen was juvenile or adult.[4] Aegirocassis, the largest known hurdiid, was over 2 metres (6.6 ft) long, comparable in size to the largest known arthropods.[5]

The majority of hurdiids appear to have been predators that fed by sifting sediment with their frontal appendages, but Aegirocassis was a suspension feeder.[1]

Hurdiids had a global distribution.[3] 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.[6] The group increased in diversity during the Miaolingian epoch.[3] 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.[6][4]

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.[7]

The phylogeny of hurdiids, accompanying the description of the hurdiids Aegirocassis benmoulae, Titanokorys gainesii, and the analyzation of Stanleycaris hirpex as follows:[5]

Species include

• Buccaspinea cooperi[9]
• Cambroraster falcatus[10]
• Cordaticaris striatus[10]
• Hurdia triangulata[10]
• Hurdia victoria[10]
• Pahvantia hastata[10]
• Peytoia nathorsti[10]
• Peytoia infercambriensis[2]
• Schinderhannes bartelsi[10]
• Stanleycaris hirpex[10]
• Titanokorys gainesii[10]
• Ursulinacaris grallae[2]
• Zhenghecaris shankouensis[10]
• Aegirocassisinae
  • Aegirocassis benmoulai[10]
  • Pseudoangustidontus duplospineus[11]
  • Pseudoangustidontus izdigua