Thursday, 10 January 2019

Sertulaster keslingi and Delicaster hotchkissi: Two new species of Starfish from the Ordovician and Carboniferous of eastern North America.

Starfish, Asteroidea, are important members of benthic invertebrate communities, often playing a major role in shaping marine ecosystems. They have a long fossil record, having first appeared in the Ordovician, but this record is extremely sparse due to the nature of their exoskeletons, which are made up of large numbers of tiny elements, which become disarticulated very easily upon the death of the animal, and when fossils are exposed at the surface. This makes it very hard to understand the evolutionary history of Starfish as a group, due to the limited number of preserved skeletons.

In a paper published in the Journal of Paleontology on 7 November  2018, Daniel Blake of the Department of Geology at the University of Illinois, and Joseph Koniecki of Ann Arbor in Michigan describe two new species of Starfish from the Ordovician and Carboniferous of eastern North America.

The first new species is named Sertulaster keslingi, where ‘Sertulaster’ means ‘garlanded star’ in reference to the primary circlet of skeletal plates, which resembles a garland, and ‘keslingi’ honours palaeontologist Robert Kesling for his work on ancient Starfish and other Echinoderms. The species is described from four partially complete specimens, one from the Verulam Formation at the LaFarge Belleville Quarry, near Belleville in Ontario, and three from the upper Bobcaygeon Formation of the Kirkfield Quarry at Kirkfield, Ontario; both formations are Late Ordovician in age. These specimens have an arm-tip radius of up to 15 mm, and a body radius of up to 5 mm.

(1)–(6) Sertulaster keslingi, family Palaeasteridae; specimens wetted to delineate ossicular shapes and arrangements. Surfaces are finely pustulate but enlarged spine bases are lacking. (1)–(3) Holotype UMMP 74694: (1) complete specimen, central disk ossicles lost or collapsed into disk interior; arrow at superomarginal (SM) series; (2) inclined view, inset primary circlet interradial (upper arrow) abuts two radials, primary circlet does not include supplemental ossicles; the interradial bears two ventrally directed flanges and is separated by two upright disk superomarginals from axillary (lower arrow), which lies between two arm marginal series; no intermarginal series is developed; (3) dorsal aspect, radial (arrow) at head of carinal series. (4) Paratype UMMP 74695, arrow at SM series; (5) paratype UMMP 74696, left arrow at SM series, right arrow at axillary, is beneath enlarged disk SM pair; (6) paratype UMMP 74697, arrow at SM series. (7) Eriaster ibexensis, is most similar to Sertulaster among known palaeasterid genera; overall view of holotype; primary circlet ossicle (left arrow) is subcircular and pustulate; enlarged carinal series ossicles extend to the arm tip and overlie marginal and intermarginal series (right arrow. Scales bars are 5 mm. Blake & Konieki (2018). 

The second species described is placed in the genus Delicaster, and given the specific name hotchkissi, in honour of palaeontologist Frederick Hotchkiss for his work on Starfish and other Echinoderms, including obtaining the specimen from which Delicaster hotchkissi is described. The species is described from a single specimen from the shale above the Willow Point Limestone Member of the Pennsylvanian (Carboniferous) Canyon Series at Bridgeport Clay Pit in Wise County, Texas. This specimen has a maximum arm radius of about 30 mm and an inner body radius of 12 mm, though this is likely to have been increased by flattening during burial.

Delicaster hotchkissi, holotype and only known specimen, YPM IP 238703. The two columns of figures illustrate the opposite surfaces of the single-known specimen, rotated 180° about the “vertical” axis. Disk ossicles are in disarray although arm intervals are largely intact. (1) The two more complete arms showing primarily marginal and ambulacral ossicular form in dorsal aspect, axillaries at arrows; (2) axillary at left arrow, madreporite immediately beyond axillary at right arrow; (3) disk region, ossicles largely disrupted; axillaries marked by two upper arrows, madreporite near upper left arrow; primary circlet ossicles and adradial face of ambulacral at two lower right arrows; (4) arm to left exposes the ventral surface; arm to right is exposed in dorsal view, folded across the ventral disk surface; adambulacral series at upper right arrow belongs to an arm obscured by the folded arm; lower arrows locate adambulacral ossicles; (5),upper left arm; adambulacral double series along midline is partially obscured and disrupted; adambulacral spines at arrow; (6) proximal interval of folded arm, partially displaced axillaries at arrows to left; small pustules along dorsal edge of marginals (middle arrow); adambulacral series in ventral view, ossicular outlines rectangular with transverse series of spine bases (right arrow). Scale bars are 10 mm. Blake & Konieki (2018).

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