SHORT PYGIDIAL SPINE IN AN INJURED(?) TRICREPICEPHALUS TEXANUS (TRILOBITA) FROM THE WEEKS FORMATION (CAMBRIAN, GUZHANGIAN), HOUSE RANGE, UTAH

An otherwise essentially complete Tricrepicephalus texanus found in the Weeks Formation (Cambrian, global Series 3, Guzhangian) at North Canyon, Utah, is missing the distal half of the left pygidial spine, with the broken surface slightly rounded off and more rugose than the smoother, intact part of the spine. The right spine is intact and complete. The full specimen (SUSA 3163) is 53 mm long, and most other elements of the skeleton appear intact (except the lateral part of the left librigena, including the genal spine, which is missing). The right pygidial spine is 11 mm long, but the broken left spine preserves only slightly over 5 mm of the basal portion. There is no extra mineralization or deformation indicative of a pathology or teratological origin for the shortened spine, and the completeness of the rest of the skeleton suggests that the short spine is not a taphonomic modification. The blunt, rounded nature of the shortened spine, and its slightly rugose texture, suggest that it was injured and healed, perhaps as a result of failed predation by an unknown predator. The broken spine differs significantly from that in a previously described Tricrepicephalus (also a left) from another formation in that the previous specimen lost the entire spine at the base (up against the pygidium), and it was the spine-pygidium junction that healed in that specimen. In being short, blunt, and possibly regenerated, the pygidial spine of SUSA 3163 is similar to a broken and healed right genal spine in a specimen of Cedaria minor from the Weeks Formation (KUMIP 259299).

Predation injuries are commonly recognized on pleural lobes (particularly posteriorly), pygidia, and cephala of trilobites; such injuries are expected on presumably defensive spines of the mineralized exoskeleton, but distinguishing these injured spines from those resulting from other causes can be difficult. Interestingly, injuries and other damage to the spines of olenelloid trilobites seem to be comparatively rare. The documentation of broken and healed, and otherwise pathological, pygidial spines in several specimens of Tricrepicephalus indicates both the defensive nature of the spines and the ability of the animals to survive what appears to be relatively frequent attempted predation, likely by other arthropods.