Aligned trace fossils from the Utica Shale: implications for mode of life and feeding in the trilobiteTriarthrus beckii

Abstract

The olenid trilobite Triarthrus commonly occurs in nearly monospecific assemblages within otherwise relatively barren black shales. As such, it has been proposed that these trilobites preferred dysoxic or even anoxic habitats and suggested feeding habits range from predation and particle feeding to chemoautotrophism. A unique bedding-plane assemblage of aligned traces that grade from Rusophycus to Cruziana, with associated Triarthrus beckii carcasses, are described from three localities in the Indian Castle Formation (upper Utica Shale) just below a K-bentonite bed. Although few body fossils are preserved, it is clear that the 15-cm-thick, laterally extensive Thruway K-bentonite created a unique taphonomic window that preserved the activities of numerous olenid trilobites. Rusophycus and Cruziana, consistently observed in densities above 100 trackways/m 2 , were excavated by the trilobites into the upper surface of a micrograded bed, likely a distal turbidite deposit. Sedimentological and trace metal data (Mo, Mn, V and U) support dysoxic but not persistently anoxic conditions through this interval. Measurements from over 500 individual trackways have mean orientations of 259.1°(WSW) at the Myers Rd. locality and 224.59°(SW) at the Dolgeville Dam locality. These orientations are closely similar to independent sedimentological indicators of current transport direction at these sites, which indicates that the trilobites preferentially faced into the prevailing current. The trackways provide compelling evidence that Triarthrus beckii individuals were engaged in feeding, probably assisted by current transport of particles. These results do not support previous suggestions that Triarthrus may have relied upon a chemoautotrophic life habit, but are consistent with suggestions of predation or scavenging from the sediment. □ Black

Digestive and appendicular soft-parts, with behavioural implications, in a large Ordovician trilobite from the Fezouata Lagerstätte, Morocco

“…The stout spines of this trilobite species are more likely associated with a digging activity than the sharp, fine spines of trilobites such as Olenoides , which probably had a grasping and ripping function for predation or scavenging22. Although those fine spines are present in some deep-water Ordovician olenid trilobites such as Triarthrus 1223, in the asaphid trilobite studied here, the direct evidence does not suggest this as the principal feeding habit. Furthermore, Cruziana rugosa is regarded as a combined feeding and locomotion trace (‘pascichnion’) most likely produced by the food searching strategy of asaphoid trilobites1824.…”

“…( b ) Resting burrow, Rusophycus morgati Baldwin, Armorican Sandstone (Floian) from the Armorican Massif, western France, interpreted as made by the trilobite Ogyginus 14, IGR-114954. ( c,d ) Resting burrows , Rusophycus carleyi (James), upper Fezouata Shale (early Floian beds), from SW Ouzina, southern Tafilalt (eastern Anti-Atlas, Morocco), locally associated with the trilobite Asaphellus 23. ( c ) MGM-6759X.…”

Gut evolution in early Cambrian trilobites and the origin of predation on infaunal macroinvertebrates: evidence from muscle scars inMesolenellus

“…Cisne ; Lerosey‐Aubril et al . ) and yet, analyses of its appendicular morphology and organization (Whittington & Almond ) and the fossilized traces of its activity (Boyer & Mitchell ) suggest that it was a surface deposit feeder able to employ some level of sorting, which might have ingested organic particles and microscopic organisms contained in organic‐rich, uppermost substrate layers (Watling ). This example reminds us, however, that the presence of a crop should not be used alone to infer predatory habits in fossil arthropods ( contra Fortey & Owens ).…”