Paleoecologic implications of epibionts on the Silurian lichid trilobite Arctinurus

Abstract

The Wenlockian lichid trilobite ArctinUDls boltoni has been regarded as a classic example of a "snow-shoe" type adaptation. That is, the broad flat doublure has been interpreted as a resting surface, with well-developed telTBCe ridges serving as either sensory or frictional devices. New data from the Rochester Shale of western New York State indicate that this interpretation is incorrect The discovery of the small, calcareous wonn tube Comulites on complete, articulated specimens of Arctinuros indicates that these trilobites seldom rested their doublure on the sediment surface. Apertures of all specimens of Cornulites are oriented posteriorly, suggesting a rheophilic response to persistent water circulation patterns beneath the trilobite. The presence of suspension feeding organisms on the doublure also indicates that the water currents beneath the trilobite were free of suspended mud, so that Arctinuros may not have processed sediment in search of food.Large specimens of Arctinums also have numerous epibionts on the dorsal shield, including cyclostome bryozoans, brachiopods (Ste&erhyncbus) and Cornulites. The presence of these suspension feeders indicates that Arctinuros did not burrow or even partially cover itself with mud The large number of epibionts on adult specimens of Arctinurus, including several age classes for SteGrbynchus, indicates that molting stopped or at least slowed greatly once a certain size was attained, and that these trilobites may have had fairly long life spans.

A Trilobite Cluster from the Silurian Rochester Shale of New York: Predation Patterns and Possible Defensive Behavior

“…boltoni have been previously documented (table 1), as well as specimens bearing epibionts (e.g., brachiopods, bryozoans, worm tubes) that are attached to the dorsal and ventral sides of the exoskeleton (Tetreault, 1992;Taylor and Brett, 1996;Brett, 2015). Arctinurus boltoni was there¬ fore subject to both commensalism and potentially predation.…”

Supplemental Material for 'A trilobite cluster from the Silurian Rochester Shale of New York : predation patterns and possible defensive behavior. (American Museum novitates, no. 3937)'

“…Arctinurus boltoni (Bigsby, 1825) is a large (150+ mm long) lichid trilobite from the mid-Silurian (Wenlock) Rochester Shale of New York: a deposit known for several trilobite-rich horizons, with articulated specimens of many species (including A. boltoni) highly sought after by fossil collectors (Whiteley et al, 2002;Chinnici and Smith, 2015). Injured specimens of A. boltoni have been previously documented (table 1), as well as specimens bearing epibionts (e.g., brachiopods, bryozoans, worm tubes) that are attached to the dorsal and ventral sides of the exoskeleton (Tetreault, 1992;Taylor and Brett, 1996;Brett, 2015). Arctinurus boltoni was therefore subject to both commensalism and potentially predation.…”

Paleoecology of commensal epizoans foulingFlexicalymene(Trilobita) from the Upper Ordovician, Cincinnati Arch region, USA

“…However, studies describing episkeletozoans on trilobites are relatively few in number (e.g., Brandt, 1996;Kloc, 1997). Moreover, most studies involve few encrusted trilobites (e.g., Prokop, 1965;Kesling and Chilman, 1975;Clarkson and Tripp, 1982;Snajdr, 1983;Mikulic, 1990;Tetreault, 1992Tetreault, , 1997Budil and Saric, 1995;Kâcha and Saric, 1995;Taylor and Brett, 1996;Brett and Taylor, 1997;Pratt, 2001). Exceptions are Kloc (1992Kloc ( , 1993Kloc ( , 1997 and Brandt (1996) who documented the frequency, stratigraphie occurrence, and paleoecology of trilobite episkeletozoans from sizeable collections of articulated specimens.…”

“…D. hamatus elegantus provided the necessary hard substrate as well as increased feeding opportunities and mobility for the sessile episkeletozoans, whereas the episkeletozoans produced a living camouflage that functioned to make the outline of the host less conspicuous to predators. Tetreault (1992Tetreault ( , 1997 examined the autecology of several trilobite species from the Rochester Shale (Silurian) of Ontario and New York. Numerous specimens of the large lichid Arctinurus boltoni (Bigsby, 1825) supported Cornulites sp.…”

“…on the doublure were oriented posteriorly, suggesting a rheophilic response to persistent water circulation patterns beneath the trilobite. Tetreault (1992Tetreault ( , 1997 concluded that the upper part of the exoskeleton of Arctinurus sp. remained mud free because of the large number of epizoozoans fouling that area.…”