Orygmaspis is a genus of asaphid trilobite with an inverted egg-shaped outline, a wide headshield, small eyes, long genal spines, 12 spined thorax segments and a small, short tail shield, with four pairs of spines.
Asaphida is comprised of six superfamilies found as marine fossils that date from the Middle Cambrian through to the Ordovician — Anomocaroidea, Asaphoidea, Cyclopygoidea, Dikelocephaloidea, Remopleuridoidea and Trinucleioidea. It was here, in the Ordovician, that five of the six lineages met their end along with 60% of all marine life at the time. They did leave us with some wonderful examples of their form and adaptations.
The stubby eyed Asaphids evolved to give us Asaphus kowalewskii with delightfully long eyestalks. These specialized protrusions would have given that lovely species a much better field of view in which to hunt Ordovician seas — and avoid becoming the hunted.
The outline of the exoskeleton Orygmaspis is inverted egg-shaped, with a parabolic headshield — or cephalon less than twice as wide as long. Picture a 2-D egg where the head is wider than the tail.
The glabella, the well-defined central raised area excluding the backward occipital ring, is ¾× as wide as long, moderately convex, truncate-tapering, with 3 pairs of shallow to obsolete lateral furrows.
The occipital ring is well defined. The distance between the glabella and the border (or preglabellar field) is ±¼× as long as the glabella. This fellow had small to medium-sized eyes, 12-20% of the length of the cephalon. These were positioned between the front and the middle of the glabella and about ⅓ as far out as the glabella is wide.
The remaining parts of the cephalon, the fixed and free cheeks — or fixigenae and librigenae — are relatively flat. The fracture lines or sutures — that separate the librigenae from the fixigenae in moulting — are divergent just in front of the eyes. These become parallel near the border furrow and strongly convergent at the margin.
From the back of the eyes, the sutures bend out, then in, diverging outward and backward at approximately 45°, cutting the posterior margin well within the inner bend of the spine — or opisthoparian sutures.
The thorax or articulating middle part of the body has 12 segments. The anteriormost segment gradually narrows into a sideward directed point, while further to the back the spines are directed outward and the spine is of increasing length up until the ninth spine, while the spine on the tenth segment is abruptly smaller, and 11 and 12 even more so.
This fellow has a wee pygidium or tail shield that is only about ⅓× as wide as the cephalon. It is narrowly transverse about 2× wider than long. Its axis is slightly wider than the pleural fields to each side, and has up to 4 axial rings and a terminal and almost reaches the margin. Up to 4 pleural segments with obsolete interpleural grooves and shallow pleural furrows. The posterior margin has 3 or 4 pairs of spines, getting smaller further to the back.
References:
Chatterton, Brian D. E.; Gibb, Stacey (2016). Furongian (Upper Cambrian) Trilobites from the McKay Group, Bull River Valley, Near Cranbrook, Southeastern British Columbia, Canada; Issue 35 of Palaeontographica Canadiana; ISBN: 978-1-897095-79-9
Moore, R.C. (1959). Arthropoda I - Arthropoda General Features, Proarthropoda, Euarthropoda General Features, Trilobitomorpha. Treatise on Invertebrate Paleontology. Part O. Boulder, Colorado/Lawrence, Kansas: Geological Society of America/University of Kansas Press. pp. O272–O273. ISBN 0-8137-3015-5.
