Alkenopterus
Alkenopterus is a genus of prehistoric eurypterid classified as part of the family Onychopterellidae. The genus contains two species, A. brevitelson and A. burglahrensis, both from the Devonian of Germany.
Like the other onychopterellids, Alkenopterus was a small eurypterid. The largest species was A. brevitelson, being 7.5 centimetres (3.0 inches) long.[1] The other species, A. burglahrensis, represents in fact the smallest species of eurypterid known as far, only measuring 2.03 cm (0.80 in).[2]
The prosoma (head) was large, with a subquadrate (almost square) to semielliptic (nearly elliptic), horseshoe-like outline. It was anteriorly surrounded by a broad and flat marginal rim that reached its posterior corners. The carapace (the exoskeleton part covering the prosoma) was rounded in the front. Its surface was somewhat inflated, being distinguished several narrow grooves and ridges, most of them wrinkle-like. The prominent lateral eyes were placed in the center of the carapace. They were reniform (bean-shaped), with a strongly arcuate visual surface (a "half moon" in the eye). Between the eyes was located a tubercle or node carrying the ocelli (simple eye-like sensory organs).[1][2] A. brevitelson also had between the eyes, behind the tubercle, certain grooves resembling the reversed V-shaped structure found in some stylonurines.[1]
The appendages (limbs) of Alkenopterus are not known in much detail. A pair of poorly preserved appendages of A. brevitelson representing the sixth (and last) pair of them is known.[2] The podomeres (leg segments) were more or less rectangular and constant in width. In the end there was a spine about as long as the podomeres which was slightly curved, with a long longitudinal groove.[1] The third to fifth appendages are also known, but they are not exceptionally preserved. Nevertheless, in all of them a distal spine can be identified. Regarding A. burglahrensis, only an appendage belonging to the sixth pair with five distal podomeres (podomeres that were not underneath the prosoma) is known. The seventh and eighth podomeres (and perhaps more of them) were somewhat prolonged outwards[2] and flattened. They featured spine-like immovable spurs on their anterior margins. On the posterior margin of the seventh podomere was the movable spine-like podomere 7a, characteristic of the eurypterines.[3] All the podomeres had similar proportions, except the distal spine which was slightly curved and probably had a pointed tip.[2]
The opisthosoma (abdomen) suffered a strong to moderate first order differentiation, that is, it was divided into a preabdomen (body segments 1 to 7) and a postabdomen (segments 8 to 12). The preabdomen had lateral convex margins and was quite short and broad, with the first tergite (dorsal half of the segment) being less wide than the subsequent ones. The postabdomen was narrow, had a constant width and did not have epimera (lateral "extensions" of the segment), like the preabdomen. The segments of the whole body were hardly distinguishable from each other. The integument of the body lacked ornamentation[2] and was very smooth.[3] The main difference between A. brevitelson and A. burglahrensis was the length of the telson (the posteriormost division of the body).[2] The one of A. brevitelson was short, measuring only 0.55 cm (0.22 in) in a 7.5 cm (3.0 in) long specimen. The ratio between the total body length and telson of this specimen is around 13.6. It was slightly subtriangular (almost triangular) and had a median "keel" (ridge), with an expanded anterior base articulated to the pretelson (segment that preceded the telson).[1] In the other hand, the telson of A. burglahrensis was long, measuring 0.35 cm (0.14 in) in a specimen with 2.03 cm (0.80 in) in total, with a ratio significantly small of 5.8. It had an expanded anterior portion as well. The telson of both species had, however, the same styliform shape.[2]
