Aglaspidida is an extinct order of marine

arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

s known from fossils spanning the Middle

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

to the Upper

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

. Initially considered

chelicerates The subphylum Chelicerata (from Neo-Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. Chelicerates include the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, tic ...

, modern anatomical comparisons demonstrate that the aglaspidids cannot be accommodated within this group, and that they lie instead within the

Artiopoda Artiopoda is a clade of extinct arthropods that includes trilobites and their close relatives. It was erected by Hou and Bergström in 1997 to encompass a wide diversity of arthropods that would traditionally have been assigned to the Trilobitomor ...

, thus placing them closer to the

trilobite Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...

s, being placed in the artiopod subgroup

Vicissicaudata Vicissicaudata is an unranked group of artiopodan arthropods, containing Cheloniellida, Aglaspidida and several other genera outside these groups like ''Sidneyia'' and ''Emeraldella.'' Description Vicissicaudatans are similar to most other ...

. With 38 known valid species as of 2017, they represent one of the most diverse groups of early Paleozoic arthropods, after trilobites.Lerosey-Aubril, R., Ortega-Hernández, J., Van Roy, P., Zhu, X., (2017)
The Aglaspidida: a poorly-known, yet important order of early Palaeozoic arthropods
''International Workshop on Evolution of Cambrian Arthropods - Taxonomy, Ontogeny and Phylogeny''. 1-6 Sep. 2017, Xian, China. Aglaspidid fossils are found in

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

(United States and Canada),

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

, and

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

Description

The exoskeletons of aglaspidids have frequently been suggested to have been

phosphatic Phosphates are the naturally occurring form of the element phosphorus. In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphor ...

, though this may actually represent post-mortem

taphonomic Taphonomy is the study of how organisms decay and become fossilized or preserved in the paleontological record. The term ''taphonomy'' (from Greek , 'burial' and , 'law') was introduced to paleontology in 1940 by Soviet scientist Ivan Efremov ...

mineral replacement, and at least some members of the group definitively had

calcified Calcification is the accumulation of calcium salts in a body tissue. It normally occurs in the formation of bone, but calcium can be deposited abnormally in soft tissue,Miller, J. D. Cardiovascular calcification: Orbicular origins. ''Nature Mat ...

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 (anteriorly) and towards the midline (medially), though some species appear to have lost their eyes entirely. The underside of the headshield (

cephalon Cephalon, Inc. was an American biopharmaceutical company co-founded in 1987 by pharmacologist Frank Baldino Jr., Frank Baldino, Jr., neuroscientist Michael Lewis, and organic chemist James C. Kauer—all three former scientists with the DuPont ...

) had either four or possibly five pairs of attached limbs (including a pair of antennae). The trunk segments (

tergites A ''tergum'' (Latin for "the back"; : ''terga'', associated adjective tergal) is the dorsal ('upper') portion of an arthropod segment other than the head. The anterior edge is called the 'base' and posterior edge is called the 'apex' or 'margin'. ...

) were freely articulating (with the exception of the posteriormost one), 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 Sclerosis (also sclerosus in the Latin names of a few disorders) is a hardening of tissue and other anatomical features. It may refer to: * Sclerosis (medicine), a hardening of tissue * in zoology, a process which forms sclerites, a hardened exo ...

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 (which constitute the majority of the group) 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 (less than half the length of the trunk), reduction or complete loss of eyes, the headshield having rounded angles, the trunk having a vaulted shape and being composed of 11 or less (minimum 6) 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 (probably posteriorly directed) mouth. They are thought to have mostly been

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

animals that inhabited the seafloor (with trace fossils probably made by aglaspidids on the seafloor having been reported from the Upper Cambrian of Wisconsin), though ''

Cyclopites ''Cyclopites'' is a genus of aglaspidid arthropods that lived in shallow seas in what is now Wisconsin during Late Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian laste ...

'' may have been a swimming,

nekton Nekton or necton (from the ) is any aquatic organism that can actively and persistently propel itself through a water column (i.e. swimming) without touching the bottom. Nektons generally have powerful tails and appendages (e.g. fins, pleopods, ...

ic animal.

Evolution

Aglaspidida first appeared in

Laurentia Laurentia or the North American craton is a large continental craton that forms the Geology of North America, ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of ...

(what is now North America) during the

Guzhangian The Guzhangian is an uppermost stage of the Miaolingian Series of the Cambrian. It follows the Drumian Stage and precedes the Paibian Stage of the Furongian Series. The base is defined as the first appearance of the trilobite '' Lejopyge laevi ...

stage of the upper

Miaolingian The Miaolingian is the third Series of the Cambrian Period, and was formally named in 2018. It lasted from about to million years ago and is divided in ascending order into 3 stages: the Wuliuan, Drumian, and Guzhangian. The Miaolingian is prec ...

(Cambrian Series 3), before undergoing a major radiation during the

Furongian The Furongian is the fourth and final epoch and series of the Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding ...

(Upper Cambrian), reaching a worldwide distribution during this period. During the Ordovician, remains are known from

Avalonia Avalonia was a microcontinent in the Paleozoic era. Crustal fragments of this former microcontinent are terranes in parts of the eastern coast of North America: Atlantic Canada, and parts of the East Coast of the United States, East Coast of the ...

(modern Wales),

Gondwana Gondwana ( ; ) was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia (continent), Australia, Zea ...

(Morocco) and South China, with the youngest representative being known from the

Katian The Katian is the second stage of the Upper Ordovician. It is preceded by the Sandbian and succeeded by the Hirnantian Stage. The Katian began million years ago and lasted for about 7.6 million years until the beginning of the Hirnantian mill ...

stage of the Upper Ordovician in Morocco.

Taxonomy

When aglaspidids were first described in the 1860s, they were initially considered to be

crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

s. 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

. 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

with taxa like ''

Sidneyia ''Sidneyia'' is an extinct marine arthropod known from fossils found from the Early to the Mid Cambrian of China and the Mid Cambrian Burgess Shale of British Columbia, Canada. Description Sidneyia size comparison.svg, Size comparison of ''Si ...

'', ''

Emeraldella ''Emeraldella'' is a genus of arthropod known from the Middle Cambrian of North America. The type species ''E. brocki'' was described in 1912 from the Burgess Shale. 21 specimens of ''Emeraldella'' are known from the Greater Phyllopod bed, where th ...

'' (both formerly classified as "xenopods")

Cheloniellida Cheloniellida is a taxon (usually referred to as an order) of extinct Paleozoic arthropods. As of 2018,Wendruff, Andrew James, et al. "New cheloniellid arthropod with large raptorial appendages from the Silurian of Wisconsin, USA." BioRxiv (2018) ...

, and ''

Kodymirus ''Kodymirus'' is a genus of Early Cambrian arthropod, known from the Czech Republic. Although it possessed great appendage-like raptorial arms, it was not homologous with those of megacheirans. ''Kodymirus'' is currently considered a member of V ...

'', based on shared traits of the organisation of the final segments of the trunk. Vicissicaudata is generally placed within the

, the broader group which contains

s and their close relatives.

List of genera

* ''

Australaglaspis ''Australaglaspis stoneyensis'' is an aglaspid that superficially resembles a horseshoe crab, or trilobite. It is known from Idamean-aged strata (Late Cambrian) at Stoney Point in north-west Tasmania Tasmania (; palawa kani: ''Lutruwi ...

'' (Late Cambrian, Australia) * '' Beckwithia'' (Middle-Late Cambrian, United States) * Tremaglaspididae ** '' Brachyaglaspis'' (Early Ordovician, Morocco) ** '' Chlupacaris'' (Late Ordovician, Morocco) ** ''

'' (Late Cambrian, United States) ** '' Flobertia'' (Late Cambrian, United States) ** '' Quasimodaspis'' (Middle-Late Cambrian, United States) ** '' Tremaglaspis'' (Middle Cambrian, United States, Early Ordovician, Wales, unnamed possible species also known from the Early Ordovician of Morocco) * Aglaspididae ** '' Aglaspella'' (Late Cambrian, United States, South China) ** ''

Aglaspis ''Aglaspis'' is the type genus of the family Aglaspididae within the arthropod order Aglaspidida. It lived on the seafloor in what is now Wisconsin during late Cambrian times. References External links ''Aglaspis''at the Paleobiology Databas ...

'' (Late Cambrian, United States) ** ''

Aglaspoides ''Aglaspoides'' is an extinct genus of aglaspid arthropod. External links ''Aglaspoides''at the Paleobiology Database The Paleobiology Database (PBDB) is an online resource for information on the distribution and classification of fossil anima ...

'' (Late Cambrian, United States, possibly a synonym of ''Glyptarthus'') ** '' Chraspedops'' (Late Cambrian, United States) ** '' Glypharthrus'' (Late Cambrian, United States, Canada, South China) ** '' Gogglops'' (Upper Ordovician, China) ** '' Hesselbonia'' (Late Cambrian, United States) ** '' Setaspis'' (Late Cambrian, United States) ** '' Tuboculops'' (Late Cambrian, United States) ** '' Uarthrus'' (Late Cambrian, United States) Additionally, '' Obrutschewia, Angarocaris'' and '' Intejocaris'', known from the Ordovician of the Siberian Platform, may also be members of the group. Certain other similar extinct arthropods are suggested to possibly be closely related to the aglaspidids, including members of the poorly known order

Strabopida Strabopidae is the only Family (biology), family of the order Strabopida, an extinct group of arthropods known from the Cambrian period. The order Strabopida contains three valid genera and one doubtful, ''Khankaspis'', ''Paleomerus'', ''Parapal ...

. Cladogram after Jiao et al. 2021.

References

*Hesselbo, SP. 1992. Aglaspidida (Arthropoda) from the Upper Cambrian of Wisconsin. Journal of Paleontology 66(6)885-923. *Raasch, GO. 1939. Cambrian Merostomata. Geological Society of America Special Paper 19, 146p.

External links

Virtual fossli museum on AglaspidaMerostomata genera
{{Taxonbar, from=Q394207 Prehistoric arthropod orders Miaolingian first appearances Late Ordovician extinctions