''Acanthomeridion'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

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 ...

found in the

Cambrian Stage 3 Cambrian Stage 3 is the still unnamed third stage of the Cambrian. It succeeds Cambrian Stage 2 and precedes Cambrian Stage 4, although neither its base nor top have been formally defined. The plan is for its lower boundary to correspond approxima ...

aged

Chengjiang biota The Maotianshan Shales () are a series of Early Cambrian sedimentary deposits in the Chiungchussu Formation or Heilinpu Formation, famous for their '' Konservat Lagerstätten'', deposits known for the exceptional preservation of fossilized orga ...

deposits of Yunnan,

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 ...

. It is a member of

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 ...

, placing it as a close relative of trilobites.

Morphology

Specimens of ''Acanthomeridion'' range in length from measured along the midline (

sagittal plane The sagittal plane (; also known as the longitudinal plane) is an anatomical plane that divides the body into right and left sections. It is perpendicular to the transverse and coronal planes. The plane may be in the center of the body and divi ...

). The exoskeleton was unmineralised. The head shield was roughly triangular in shape, rounded towards the front. A pair of compound eyes were accommodated by notches at the posterior edge of the head shield. Over the course of growth, the head became proportionally wider relative to body length. On the underside of the head, a pair of teardrop-shaped plates attached to the head shield, with their outer posterior edges forming a pair of spines projecting backwards from the head. The mouth region had an axe-shaped hypostome. The head bore four pairs of appendages, including a pair of antennae with at least 43 segments, with these segments becoming narrower and shorter towards the tip of the antennae. The three pairs of post antennae head limbs are poorly preserved in known specimens, but may be

biramous The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments (called podomeres) are of Latin origin, and may be confused with terms for bones: ''coxa'' (meaning hip, ...

(two-branched). The trunk is made of eleven segments/ tergites, which curve into a spine shape towards their outer edges. During growth, the trunk transitioned from being proportionally narrow as well as being noticeably convex (curved upwards) in young individuals, towards being more broad and flatter in mature individuals. One specimen indicates specimens of ''Acanthomeridion'' had paired gut diverticula within the body cavity. Each tergite/segment is associated with a pair of biramous appendages, which are all similar in size and morphology. The basal segment of the limbs (protopodite) had well developed spines facing towards the midline of the animal, forming a gnathobasic apparatus for processing food, with the underside of the protopodite also bearing longer endite spines. The

exopod The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments (called podomeres) are of Latin origin, and may be confused with terms for bones: ''coxa'' (meaning hip (a ...

s (upper limb branch) are slender, rod-like and elongate, though not exceeding the breadth of the trunk carapace. These exopods are covered in two rows of short lamellae. The

endopod The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments (called podomeres) are of Latin origin, and may be confused with terms for bones: ''coxa'' (meaning hip, : ...

s (lower, leg-like branches), were made of up seven segments/ podomeres each of trapezoidal shape. The first six of these segments bore endite spines arranged in rows on their undersides, with the number of rows gradually decreasing towards the tip of the limbs. The ends of the endopods on the final seventh podomere/segment bore a claw composed of three spines. The final body segment terminated with a slender spine.

Taxonomy

''Acanthomeridion'' and the species ''Acanthomeridion serratum'' was first described by Hou, Chen and Liu in 1989. In 1997, it was placed in its own,

monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unisp ...

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

, Acanthomeridiidae. In 2016, a second species ''Acanthomeridion anacanthus'', was described by Hou et al. Until 2024 the morphology of the limbs of ''Acanthomeridion'' was unknown. In that year, a comprehensive redescription of the genus was published by Du et al. This study considered ''A. anacanthus'' to be a junior synonym of ''Acanthomeridion serratum'', making the genus again monotypic. Early studies suggested an affinity with

Xandarellida Xandarellida is an extinct order of artiopod arthropods known exclusively from the Middle Cambrian of China and Siberia. Morphology In terms of anatomy, xandarellids share numerous characteristics such as a natant hypostome, alongside a uniq ...

(also known as Petalopleura) for ''Acanthomeridion''. However, Hou et al. 2016, found ''Acanthomeridion'' to be a basal (early branching) member of

, the clade containing

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

and their close relatives, including Xandarellida (which was found to not be particularly closely related to ''Acanthomeridion''). A 2019 study placed ''Acanthomeridion'' in the newly named clade Protosutura alongside ''

Zhiwenia ''Zhiwenia'' is a genus of artiopods that existed during Stage 3 of the Cambrian period and was uncovered in the Xiaoshiba Lagerstätte in China. Only one species is known, ''Zhiwenia coronata''. It is the sister taxon of '' Australimicola'' in ...

,'' also known from the Cambrian of China, and '' Australimicola'' from the Cambrian of Australia'','' based on the shared presence of dorsal ecdysial sutures (areas of the upper surface of the head shield where the exoskeleton splits during moulting/

ecdysis Ecdysis is the moulting of the cuticle in many invertebrates of the clade Ecdysozoa. Since the cuticle of these animals typically forms a largely inelastic exoskeleton, it is shed during growth and a new, larger covering is formed. The remnant ...

). This group was again placed as basal (early branching) members of Artiopoda. The 2024 redescription noted striking similarities of the head organisation of ''Acanthomeridion'' in particular to trilobites, and proposed that this may suggest an especially close relationship between the two groups within the Artiopoda, though the authors alternatively suggested that these similarities may be due to convergence, and that its placement within Artiopoda could not be precisely determined due to this uncertainty.

References

{{Taxonbar, from1=Q18643337, from2=Q149460 Maotianshan shales fossils Prehistoric arthropod genera Monotypic arthropod genera Enigmatic arthropod taxa Taxa described in 1989