# Acanthomeridion

Acanthomeridion is an extinct arthropod found in the Cambrian Stage 3 aged Chengjiang biota deposits of Yunnan, China. It is a member of Artiopoda, placing it as a close relative of trilobites.

## Morphology

Specimens of Acanthomeridion range in length from 2 to 7.5 centimetres (0.79 to 2.95 in) measured along the midline (sagittal plane). The exoskeleton was unmineralised. The head shield was roughly triangular in shape, rounded towards the front. A pair of compound eyes were accomodated 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 (two-branched).
The trunk is made of 11 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 exopods (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 endopods (lower, leg-like branches), were made of up 7 segments/podomeres each of trapezoidal shape. The first 6 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 7th 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 family, 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 (also known as Petalopleura) for Acanthomeridion. However, Hou et al. 2016, found Acanthomeridion to be a basal (early branching) member of Artiopoda, the clade containing trilobites 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, 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). 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.