Appendages and habits of the Upper Ordovician trilobiteTriarthrus eatoni

Abstract

Sixteen specimens from the classic locality quarried by C. E. Beecher, in the Frankfort Formation near Rome, New York State, U.S.A., have been examined. Pyrite has replaced the exoskeleton and lines or infills confined spaces such as within the doublure and appendages. Appendages are compressed beneath the flattened exoskeleton and retain their original relationship to one another; biramous appendages have in most cases been rotated, commonly showing the posterior face beneath the cephalon, the anterior face beneath thorax and pygidium. The specimens were collected and prepared by C. D. Walcott, most having been exposed from the ventral side and lie approximately parallel to the bedding, three are oblique-lateral in relation to bedding. Seven have been additionally prepared using a gas pressure abrasion machine, photographs taken in reflected light, the specimens submerged in alcohol, and explanatory camera-lucida drawings made. The antennae may not have had an originally lyriform configuration. Additional evidence supports the view that the cephalon bore three pairs of biramous appendages as J. L. Cisne claimed (Science, Wash. 186, 13-18 (1974);Fossils Strata4, 45-63 (1975);Palaeontogr. Am. 9, 99-142 (1981)), but there is no indication that a metastome was present. Coxae are poorly preserved, the last-formed posterior coxa small, triangular, coxae becoming progressively elongate forward along the series, and deeper beneath the cephalon. Neither the ventral membrane nor the coxa-body junction has been observed. The interpretation of the form of the coxa, and attachment of leg branches, by Cisne (1975, 1981), is considered to have been based on misinterpretation of an X-ray stereograph; the leg branch is inserted into the full depth of the abaxial coxal margin. Endites of the podomeres of the leg branch are deep, acutely triangular in shape, on podomeres 1-4 of the posterior branches; podomeres become progressively more elongate forward along the series, endites less acutely triangular, and are present only on the proximal podomere anteriorly, as C. E. Beecher (Am. J. Sci. 1, 251-256 (1896)) showed. The tips of the endites were spinose. We consider that there was no `post-pygidial abdomen', as claimed by Cisne (1975, 1981), the structure so interpreted being the most posterior, six or seven tiny coxae and leg branches, preserved crowded together and overlapping, backwardly directed across and behind the pygidial doublure. The shaft of the exite was rigidly attached to the upper, posterior side of the coxa, was broad proximally, tapering, and obliquely subdivided. It bore about 50 filaments and a small, setose terminal lobe. The filaments are preserved as imbricated, flattened strips, closely spaced and parallel, always dorsal to the leg branches. The most posterior limb pair was the last-formed, differing growth rates between portions of the earlier-formed limbs led to the graded differentiation shown by the biramous limb series. The differences between coxae and leg branches in particular regions of the body were concerned with capture and ingestion of food. A new reconstruction is made, the exoskeletal convexity based on uncrushed, enrolled specimens of the similar speciesTriarthrus beckii. Exoskeletal structures show thatT. eatonicould have enrolled to form an enclosed capsule, within which all the appendages must have been accommodated. The biramous limbs are restored in a hanging stance, the inner ends of the coxae close together. Both branches of the limbs projected well below the margins of the exoskeleton, but only the tips were visible in dorsal view.T. eatoniwas a benthic animal that walked on the substrate, and could have launched itself off the bottom and drifted above it. It was probably a generalized deposit feeder, scavenger and predator, exploring the surface of the mud and digging into it for small organic fragments, and capable of catching and squeezing small prey in the formidable array of posterior endites and spines. Food was passed forward to the mouth by the interaction of the spinose mesial edges of pairs of coxae, larger cephalic coxae helped push food toward the mouth.T. eatoniis preserved predominantly in dark shales, that were formed in muddy environments of the outer continental shelf and upper slope.