The distribution and inferred larval dispersion of Rhondellina dorei: a new Cambrian brachiopod (Acrotretida)

• Published in: Journal of Paleontology
• Main Author: Rowell, A. J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cambridge University Press (CUP) 1986
• Subjects: Paleontology; Greenland; Peary; Peary Land; North Greenland
• Online Access: http://dx.doi.org/10.1017/s0022336000022587; https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022336000022587

Rhondellina dorei n. gen., n. sp. is morphologically a very unusual acrotretid in possessing a marginal ventral beak and vestigial ventral pseudointerarea. In common with all other acrotretaceans it has a pitted first-formed shell that is an evolutionary novelty for the clade. By analogy with developmental stages of living lingulids and discinids it is concluded that these pitted first-formed valves are not protegula, but are larval valves developed during a protracted pelagic phase. The distinction is important in considering dispersal of the organisms. Although some acrotretids are endemic and limited to rocks deposited around a single Cambrian continental landmass, others are cosmopolitan. The wide dispersal of such forms supports earlier conclusions that the functional significance of the pits in the larval valves was an adaptation to increase buoyancy and suggests that the larvae were planktotrophic. Because of energy problems inherent with planktotrophic larvae and small parents, it is conceivable that acrotretacean development was mixed and that a relatively small number of lecithotrophic eggs were brooded prior to release of the free-swimming planktotrophs. Rhondellina dorei was seemingly endemic to Cambrian North America. Although its larvae apparently were unable to cross oceanic barriers, the species dispersed around the periphery of the craton. It is known from western Utah and eastern Nevada and also occurs in Peary Land, North Greenland, on the opposing side of the continent. In the western United States it occurs only in the Crepicephalus Zone (lower Dresbachian, Upper Cambrian), but in Greenland it appears earlier in the upper Lejopyge laevigata Zone, in beds that are the temporal equivalents of the North American Cedaria Zone.