Aglaspidida
Aglaspidida is an extinct order of marine arthropods known from fossils spanning the Middle Cambrian to the Upper Ordovician. Initially considered chelicerates, modern anatomical comparisons demonstrate that the aglaspidids cannot be accommodated within this group, and that they lie instead within the Artiopoda, thus placing them closer to the trilobites, being placed in the artiopod subgroup Vicissicaudata.
With 38 known valid species as of 2017, they represent one of the most diverse groups of early Paleozoic arthropods, after trilobites. Aglaspidid fossils are found in North America, Europe, Australia, and China.
Description
The exoskeletons of aglaspidids have frequently been suggested to have been phosphatic, though this may actually represent post-mortem taphonomic mineral replacement, and at least some members of the group definitively had calcified exoskeletons. The headshields ancestrally have a pair of unstalked eyes attached to the upper surface, which are attached to a raised region of the head that merges with the rest of the headshield frontwards and towards the midline, though some species appear to have lost their eyes entirely. The underside of the headshield had either four or possibly five pairs of attached limbs. The trunk segments were freely articulating, and bore outwardly projecting pleurae. The body ends with a tailspine. A distinctive feature of aglaspididans are "postventral plates", a pair of two flat sclerotized plates located on the underside of the posterior final few segments of the body, covering the base of the tailspine. Most members of the group are around in length, though their length varies from up to a maximum of including the tailspine.Members of the family Agaspididae are additionally defined as having a flat, wide body, with the posterior outer corners of the headshield having an acute to spinose shape, with the trunk ancestrally composed of 12 tergites, with the tailspine being elongate and fused to the posteriormost 12th trunk segment/tergite.
Members of Tremaglaspididae are characterised by having a short tailspine, reduction or complete loss of eyes, the headshield having rounded angles, the trunk having a vaulted shape and being composed of 11 or less tergites.
Ecology
Aglaspidids are thought to have primarily inhabited shallow marine environments. Aglaspidids are suggested to have been predatory or scavengers, using their legs to pass food towards the mouth. They are thought to have mostly been benthic animals that inhabited the seafloor, though Cyclopites may have been a swimming, nektonic animal.Evolution
Aglaspidida first appeared in Laurentia during the Guzhangian stage of the upper Miaolingian, before undergoing a major radiation during the Furongian, reaching a worldwide distribution during this period. During the Ordovician, remains are known from Avalonia, Gondwana and South China, with the youngest representative being known from the Katian stage of the Upper Ordovician in Morocco.Taxonomy
When aglaspidids were first described in the 1860s, they were initially considered to be crustaceans. Following the pioneering work on the group by Gilbert O. Raasch in his 1939 publication Cambrian Merostomata, who named the group and many genera of aglaspidids, they were considered to be chelicerates. However, research in the late 20th century challenged this hypothesis, and during the 21st century it became recognised that they belonged to a group called Vicissicaudata with taxa like Sidneyia, Emeraldella Cheloniellida, and Kodymirus, based on shared traits of the organisation of the final segments of the trunk. Vicissicaudata is generally placed within the Artiopoda, the broader group which contains trilobites and their close relatives.List of genera
Australaglaspis Beckwithia Tremaglaspididae- * Brachyaglaspis
- * Chlupacaris
- * Cyclopites
- * Flobertia
- * Quasimodaspis
- * Tremaglaspis Aglaspididae
- * Aglaspella
- * Aglaspis
- * Aglaspoides
- * Chraspedops
- * Glypharthrus
- * Gogglops
- * Hesselbonia
- * Setaspis
- * Tuboculops
- * Uarthrus
Cladogram of the position of Aglaspdida within Vicissicaudata, after McCoy et al. 2025:
Cladogram of Aglaspdida after Jiao et al. 2021.