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).[2]
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.[3] 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.[3] In most species, the endites were curved medially, so that the appendages formed a basket-like structure.[2] Some hurdiids had greater numbers of endites, with Cordaticaris bearing seven endites of equal length.[4]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.[3]
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 (0.24–0.59in), but it is not known whether this specimen was a juvenile or an adult.[5]Aegirocassis, the largest known hurdiid, was over 2 metres (6.6ft) long, comparable in size to the largest known arthropods.[6]
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.[2][7][8]
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.[10] 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", due to its first definition not having a character-based diagnosis, and the second being published in an online-only journal without being registered in the ZooBank database.[1][11]
Cladogram of Hurdiidae after Moysiuk & Caron, 2025:[12]
Tauricornicaris was previously considered as a member of Hurdiidae, but fossils of it were later reinterpreted to be euarthropod tergites.[15][16]
Zhenghecaris was originally described as a thylacocephalan, but it was later considered a hurdiid dorsal carapace.[17][18] However, its placement is questioned by some researchers, since its classification as a radiodont is largely based on the characters of Tauricornicaris.[14]
While Schinderhannes was originally described as a stem-arthropod with characters of both euarthropods and radiodonts, this interpretation was denied and most researchers now agree that it is a hurdiid radiodont.[19][20][21][22][18][23][24][25][26] There are some researchers who have questioned its classification as a hurdiid.[27][28]
1 2 Pates S, Lerosey-Aubril R, Daley AC, Kier C, Bonino E, Ortega-Hernández J. 2021. The diverse radiodont fauna from the Marjum Formation of Utah, USA (Cambrian: Drumian) PeerJ 9:e10509 https://doi.org/10.7717/peerj.10509
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