Olenelloid tribolites: type species, functional morphology and higher classification

Abstract

Type or topotype specimens of the type species of the generaOlenellus,Nevadia,WanneriaandElliptocephalaare redescribed, together with similar specimens of0. vermontanusand0 . robsonensis. Entire, or almost entire, exoskeletons of all these species are known. In the light of this re-investigation, and of recent escnp ions o Canadian, Scandinavian and Russian material, the exoskeletal morphology of olenelloids is reviewed. The exoskeleton was convex, sloping gently downwar s rom the axial furrow, more steeply distally. The perrostral suture and probably a circumocular suture were present, dorsal sutures are unknown. A macropleural third segment, a large axial spine on the fifteenth segment, and a subdivision o t e t orax at this segment, appear to characterize species ofOlenellusand its close relatives. In species of other genera there may be a transition between the subdivisions of the thorax, either the macropleura or the axial spine may be absent, or none ol these features may be developed. Articulating flanges in the thoracic exoskeleton ol0. thompsoniare described, and may occur in other genera ; it appears probable that olenelloids could partly enrol. The small pygidium consisted of few segments, the convex axis being ill-defined, the posterior margin of the pleural region entire, bilobed, or spinose; the doublure extended under much of the posterior pleural region. The olenelloid pygidium included the telson and a few segments, and was a terminal tagma of the body, as in all trilobites. On the external dorsal surface of the exoskeleton a reticulate pattern of raised ridges is present in some species, in others the ridges anastomose, or granulation may occur. The doublure bore terrace ridges. From the earliest olenelloid developmental stages the perrostral suture is present, the dorsal branches of the sutures absent, and the macropleura may disappear, or be retained, during ontogeny. The configurations of holaspid moults suggest that partial enrolment was essential to the process of exuviation. The perrostral and circumocular sutures facilitated such exuviation, this sutural arrangement being different from, but as effective as, the arrangements in contemporaneous and younger trilobites. Exoskeletal characters considered distinctive of olenelloids are all known in other trilobites; none is unique. The developmental stages of olenelloids, redlichioids and paradoxidoids show similarities, and holaspides have many characters in common. The order Redlichiida, embracing these groups and possibly also ellisocephaloids, is advocated, heavy weight being placed on similarities in growth stages as evidence of relationship. There is no basis for K.-E. Lauterbach’s concept of the ‘telosom a’, or for the pygidium being an intercalary, rather than a terminal, tagma of the body in trilobites. Characters of the olenelloid exoskeleton are not like those of any Cambrian or younger chelicerate.