Extremes of Pit Infestation and Growth Deformity in a Crinoid Column, Permian of Timor

ABSTRACT

Crinoids are diverse and well-known from the Permian of Timor, but the literature has failed to document the numerous specimens of crinoid pluricolumnals from the fauna, many showing unusual morphology or yielding palaeoecological information. A curious and instructive specimen demonstrates the relationship between a living Permian crinoid and coeval invasive, pit-forming, invertebrates in detail. The pit-former is not preserved; most likely it was unmineralized or, if mineralized, then the shell simply dropped out. The infesting organism made pits assigned to the ichnospecies Oichnus paraboloides Bromley. The pit-former was unusually site selective. Either (1) one spatfall attached to just one side of the elevated (either up-current or down-current) or recumbent column and each individual centered their pits on the sutures between adjacent columnals; or (2) a single individual migrated along the column. The living crinoid showed an extreme reaction to this infestation. Excess stereom growth on the side of the pits transformed what was a circular column by addition of a thick, triangular ridge on the pitted side.

KEYWORDS:

• Palaeoecology
• Oichnus
• Site selectivity: Artinskian
• Paleopathology

Acknowledgments

Debbie Schoor thanks Naturalis Biodiversity Center, Leiden, for giving her the opportunity to work on this project as a part of her M.Sc. training. A thought-provoking review by Professor Chris Paul (University of Bristol) of an early incarnation of this paper is gratefully acknowledged. We thank our two external peer reviewers, Drs Olev Vinn (University of Tartu, Estonia) and James R. Thomka (University of Akron, Ohio), for their supportive and incisive comments.