Paleomerus is a genus of strabopid, a group of extinct arthropods. It has been found in deposits from the Cambrian period (Atdabanian epoch). It is classified in the family Strabopidae of the monotypic order Strabopida. It contains two species, P. hamiltoni from Sweden and P. makowskii from Poland. The generic name is composed by the Ancient Greek words παλαιός (palaiós), meaning "ancient", and μέρος (méros), meaning "part" (and therefore, "ancient part"). Paleomerus is one of the oldest arthropods, being sometimes interpreted as the model of the first arachnomorphs. It is part of the order Strabopida, a poorly known group closely related to the aglaspidids of uncertain affinities, often being ignored by researchers and authors due to the poor preservation and abundance of their fossils. It has been suggested that Paleomerus and the closely related Strabops could be synonymous with each other, since they differ only in the size of the telson (the posteriormost division of the body) and the position of the eyes. These two genera were originally deferred by a hypothetical twelfth segment in Paleomerus, but after the discovery and description of a fourth specimen of P. hamiltoni, it has been shown that this segment actually represents the tail of the animal.
Description
As the other strabopids, Paleomerus was a small-sized arthropod. The bigger species was P. hamiltoni at 9.3 centimetres (3.7 inches), while the smaller P. makowskii reached only 7.3 cm (2.9 in). Within Strabopida, the closely related Strabops thacheri exceeded the length of Paleomerus with 11 cm (4.3 in), while Parapaleomerus sinensis reached a total length of 9.2 cm (3.6 in). Like some other arthropod clades, the strabopids possessed segmented bodies and jointed appendages (limbs) covered in a cuticle composed of proteins and chitin. The arthropod body is divided into two tagmata (sections); the frontal prosoma (head) and posterior opisthosoma (abdomen). The appendages were attached to the prosoma, and although they are unknown in strabopids (except for one undescribed specimen of Parapaleomerus), it is most likely they owned several pairs of them. Although the chemical composition of the strabopid exoskeleton is unknown, it was probably mineralized (with inorganic substances), sturdy and calcareous (containing calcium). The head of the strabopids was very short, the back was rounded and lacked trilobation (being divided in three lobes), the abdomen was composed by eleven segments and succeeded by a thick tail-like spine, the telson. In Paleomerus, the prosoma was parabolic (approximately U-shaped), and the opisthosoma can increase or decrease in width slightly or strongly. It was very flexible, possibly being able to flex laterally and roll up itself. The telson was trapezoidal, broad and tapering. Paleomerus may be the best model of a primitive arachnomorph. Paleomerus differs from Strabops only in the position of the eyes, which are farther from each other and closer to the margin than in Strabops, and the size of the telson, being shorter and broader than in the latter.
History of research Paleomerus has two species described; all other described strabopid genera are monotypic. The Norwegian paleontologist and geologist Leif Størmer erected the genus based on two specimens (Ar. 47071, holotype, and Ar. 47073, the paratype) and tentatively classified it under the order Aglaspida in its own family, Paleomeriidae (also spelled Paleomeridae). Paleomerus is translated to "ancient part", with its name derived from the Ancient Greek words παλαιός (palaiós, ancient) and μέρος (méros, part). The designated type species, P. hamiltoni, is named after Count Hugo Hamilton, who sent both fossils to the Swedish Museum of Natural History, Stockholm. In this species, the prosoma was short and had a parabolic shape with a somewhat concave posterior border. The posterolateral borders were rounded and lacked spines. The compound eyes appear as anterolateral reniform (bean-shaped) elevations in the surface of the prosoma. The opisthosoma increased slightly its width in the third and fourth segments, then decreased slightly until the seventh segment. All the tergites (the dorsal part of the segments) were similar, superimposing strongly on each other, with the half of the first segment being hidden under the prosoma. The rest of tergites were more slightly superimposed, indicating a great flexibility and even the ability to roll up itself. The telson was trapezoidal, but Størmer mistook it as the twelfth segment of the body, suggesting a hypothetical lanceolate (lance-shaped) or fan-shaped telson. It is estimated that the size of this species was of 9.3 cm (3.7 in) judging by the length of the paratype. In 1971, the Swedish geologist and paleontologist Jan Bergström described a new specimen of P. hamiltoni, RM Ar. 47170, found in the same place as the previous two. This new poorly preserved fossil contains molds of the first eleven segments, with a total corporal length estimated at 6.4 cm (2.5 in, being slightly larger than the holotype). Bergström tentatively removed Strabopidae (at that time containing Strabops and Neostrabops) and Paleomeridae (only Paleomerus) from the order Aglaspidida based on the fact that the head tagma was too short to accommodate the six pairs of appendages then assumed to be present in aglaspidids. Instead, he classified them in an uncertain order in the Merostomoidea class together with the emeraldellids. Ironically, Bergström speculated that the number of pairs of appendages present in the three genera could be fewer than seven, as well as including a possible antennal segment. This is currently observed as an overestimate. A study published by Derek Ernest Gilmor Briggs et al. in 1979 has shown that the aglaspidid Aglaspis spinifer had between four and five pairs of appendages, but not six, weakening Bergström's argument.
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