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Abnormal Pygidial Spine in an Injured(?) Trilobite (Tricrepicephalus Texanus) from the Weeks Formation (Cambrian, Guzhangian), House Range, Utah

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Abnormal Pygidial Spine in an Injured(?) Trilobite (Tricrepicephalus Texanus) from the Weeks Formation (Cambrian, Guzhangian), House Range, Utah Lucas, S. G., Hunt, A. P. & Lichtig, A. J., 2021, Fossil Record 7. New Mexico Museum of Natural History and Science Bulletin 82. 71 ABNORMAL PYGIDIAL SPINE IN AN INJURED(?) TRILOBITE (TRICREPICEPHALUS TEXANUS) FROM THE WEEKS FORMATION (CAMBRIAN, GUZHANGIAN), HOUSE RANGE, UTAH JOHN R. FOSTER Utah Field House of Natural History State Park Museum, 496 East Main St., Vernal, Utah 84078, [email protected] Abstract—An essentially complete Tricrepicephalus texanus found in the Weeks Formation (Cambrian, Miaolingian, Guzhangian) at North Canyon, Utah, is missing the distal half of the left pygidial spine, which appears to have healed. This represents a rare instance of the loss of only a portion of a spine for a trilobite. The spine was likely broken during a failed predatory attack on the individual. In addition, a shortened T2 pleural spine and damaged librigena suggest the individual may have had multiple injuries. INTRODUCTION complete. The right pygidial spine is 11 mm long, but the broken The Weeks Formation is a middle-late Cambrian left spine preserves slightly over 5 mm of the originally proximal (Miaolingian, Guzhangian) stratigraphic unit restricted to half. The lateral edge of the distal tip of the shortened left spine the southern House Range of Millard County, Utah (Walcott, curves gently toward the medial edge, which, in contrast, is 1908; Robison, 1964; Hintze and Davis, 2002, 2003). It has nearly straight, suggesting that the original break may have been produced a wide variety of trilobites, unmineralized arthropods, diagonal across the spine from proximolateral to distomedial. sponges, worms, and other fossil taxa (Adrain et al., 2009; There is no extra mineralization or deformation indicative of a Lerosey-Aubril et al., 2012, 2018; Robison et al., 2015; Foster pathology or teratological origin for the shortened spine, and and Gaines, 2016). One of the less common trilobites is the the completeness of the rest of the exoskeleton suggests that the ptychopariid Tricrepicephalus texanus of the Cedaria Biozone. short spine is not a taphonomic modification. Tricrepicephalus was defined by Kobayashi (1935) based on the The specimen also has an abnormally short, apparently species Arionellus (Bathyurus) texanus Shumard, 1861 from broken right pleural spine on thoracic segment T2 (Fig. 1C). The Texas. A number of other species of Tricrepicephalus have been right T2 pleural spine has a blunt, shorter tip compared to the left described from elsewhere in North America (e.g., Resser, 1938; T2 pleural spine, which is intact. It is unclear if the T2 pleural Lochman and Duncan, 1944; Stitt and Perfetta, 2000), although spine break is a healed injury or, perhaps less likely, postmortem Pratt (1992) recognized only T. texanus and T. tripunctatus. The damage. Tips of the right pleural spines of T10 and T11 are also specimen described here, with an abnormal left pygidial spine broken off but are clearly unhealed and were probably a result and right T2 pleural spine, was found at the North Canyon of postmortem damage; the otherwise intact condition of the locality in the Weeks Formation of Utah in June 2016 (Quarry 1 carcass suggests this damage was probably not transport-related. in Lerosey-Aubril et al., 2018). The left librigena is damaged and only the proximal portion Institutional Abbreviations—FHPR, Utah Field House is preserved; the lateral portion of the plate, lateral border, and of Natural History State Park Museum, Vernal, Utah; KUIMP, genal spine are gone (Fig. 1). The remaining proximal part University of Kansas Invertebrate Paleontology Collection, of the librigenal is broken into several polygonal fragments Lawrence, Kansas; SUSA, Southeastern Utah Society of Arts but is otherwise articulated in life-position. This suggests the and Sciences, Moab Museum, Moab, Utah; USNM, National possibility of a lethal injury to the left librigena that removed the Museum of Natural History, Washington, D.C. border and spine and broke up the more proximal portion. Fieldwork sampling that produced this specimen (FHPR DESCRIPTION 16570) also recorded two isolated pygidia of Tricrepicephalus, The full trilobite specimen (FHPR 16570; formerly SUSA both with complete spines on each side. 3163, Foster and Hunt-Foster, 2016) is 53 mm long, with most INTERPRETATION elements of the skeleton intact except for the lateral part of the left librigenal and genal spine (Fig. 1). The specimen is a ventral Trilobite exoskeletal abnormalities may result from injuries view of the interior of the dorsal exoskeleton; the hypostome (mechanical damage from predation attempts or accidental self- appears to be approximately in place as well, suggesting it is a inflicted damage), teratological issues (developmental problems), carcass. When found, the specimen was covered entirely with or pathologies (disease or parasitic infections) (Bicknell et al., matrix, and it was exposed through air-abrasion preparation, so 2018; Bicknell and Pates, 2019). Non-lethal injuries can be it is known that missing elements had not split off at discovery. identified by healing and scar formation (especially of U-, V-, Shafts of each pygidial spine are crushed, and the broken and W-shaped indentations of elements) and other deformities pieces at the thick spine bases reveal the original hollowness of normal elemental shape. Teratological abnormalities may be of the spines (Fig. 1B), a characteristic clear in specimens identified by additional or unusual spines or segments (or other from other formations that are preserved three dimensionally. elements), and pathologies are usually identified by atrophy or Three dimensionally preserved Tricrepicephalus specimens infected growth of elements (Bicknell et al., 2018; Bicknell and from limestones in other parts of North America indicate that Pates, 2019). the pygidial spines were hollow, round in cross-section, and The fact that the broken edge of the pygidial spine of FHPR projected posterodorsally from the pygidium (Lochman, 1936, 16570 is neither jagged nor flat suggests that the tip was partly 1941; Pratt, 1992). Those from the Weeks Formation are regenerated on the now-shorter spine during subsequent molts; flattened due to compaction. The now-shortened left pygidial taphonomic or other postmortem breakages would be unlikely to spine of FHPR 16570 has a tip surface that is rounded off produce the blunt, conical tip. Similarly, the tip does not appear asymmetrically and is slightly less smooth than the intact part indicative of a disease pathology. That the skeleton includes of the spine along the shaft. The right spine is fully intact and the hypostome and articulated right librigena suggests that the 72 FIGURE 1. Trilobite Tricrepicephalus texanus from the Weeks Formation, North Canyon, House Range, Millard County, Utah (FHPR 16570). A, As preserved, after air-abrasion preparation, ventral view of dorsal exoskeleton (left side of animal on right in photo), lighting from left in photo. Scale bar = 1 cm. B, Close up of pygidial spines showing complete right spine (on left in photo) with compacted hollow shaft base and shortened left pygidial spine with oblique healed repair surface (arrow), lighting from left. Scale bar = 1 cm. C, Close up of thoracic segments T1 (displaced slightly anteriorly under cranidium) through T8, showing broken right pleural spine of T2 (arrow). Scale bar = 1 cm. specimen is a carcass, and thus the broken spine is unlikely to injured and healed. Lochman (1941) illustrated a pygidium of be damage from the last molt (Owen, 1985). Other than a healed Tricrepicephalus from the Bonneterre Dolomite of Missouri break to the spine, the only other plausible cause of the shortened with an entirely missing left spine that broke and healed at the spine and blunt tip might be a developmental (teratalogical) base of the spine up against the body of the pygidium. In being anomaly resulting in a less sharp tip and a spine about half the short, blunt, and possibly partly regenerated, the short pygidial normal length. Such abnormalities are known from trilobites, spine of FHPR 16570 is similar to a broken and healed right though often in the thoracic segments (Pocock, 1974). genal spine in a specimen of the trilobite Cedaria minor from FHPR 16570’s shortened left pygidial spine shows no signs the Weeks Formation (KUIMP 259299) illustrated by Babcock of infected growth or clear teratology, but its apparently rounded (1993). The Weeks Tricrepicephalus specimen also differs from off new spine tip suggests repair of mechanical damage from an a pathological or teratological left pygidial spine illustrated by injury, most likely resulting from predation or damage during Lochman (1936), also from the Bonneterre Dolomite, with a molting. These two modes of injury can be difficult to distinguish curled distal tip (USNM 91866). (Brandt, 2002). Predation may be a preferred interpretation of Predation appears to have arisen very early in the history of injury in cases in which there is healing and multiple segments animals (Bengtson, 2002), and predation injuries are commonly are affected or there are “substantial single spine injuries” recognized on pleural lobes (particularly posteriorly), pygidia, (Bicknell et al., 2019). These two criteria appear to be met with and cephala of trilobites of various geologic ages (Ludvigsen, FHPR 16570, and in addition, injuries are the most common 1977; Conway Morris and Jenkins, 1985; Babcock, 1993, 2003, abnormalities for extant xiphosurids, which demonstrate some 2007; Owen and Tilsley, 1996); such injuries are expected on of the same types of injuries as trilobites (Bicknell and Pates, presumably defensive spines, but distinguishing these injured 2019). The abnormal pygidial spine of FHPR 16570 is therefore spines from those resulting from other causes can be difficult. interpreted here as resulting from the healing of a predation Though spines may have served several functions (Brandt, injury. 2002), the documentation of broken and healed pygidial spines DISCUSSION in several specimens of Tricrepicephalus indicates both the potentially defensive nature of the spines and the ability of the The blunt, rounded nature of the shortened spine in FHPR animals to survive attempted predation.
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