Non-trilobite arthropods from the Silver Peak Range, Nevada

Journal of Paleontology, Jul 2003 by Waggoner, Ben

USNM loc. 42036: SE 1/4 SE 1/4 sec. 30 (unsurveyed) T. 1 N., R. 37. E, Silver Peak Range, Esmeralda County, Nevada (Rhyolite Ridge NW 7.5'' quadrangle, 1987 edition). Outcrops exposed on low hill and small wash near the Emigrant Pass road, about 50 meters W of locality 42035. Collected by B. Waggoner and S. Richardson, 2000.

USNM loc. 42037: Same as USNM loc. 42036. Collected by M. Brents, 2002.

ARTHROPODA Siebold and Stannius, 1848

Incertae sedis

Genus ESMERALDACARIS new genus

Type species.-Esmeraldacaris richardsonae new species.

Diagnosis.-As for type species.

Etymology.-From Esmeralda, the county where the fossils were found; caris, Latin "shrimp." Gender feminine.

ESMERALDACARIS RICHARDSONAE new species

Figures 6.1-6.5, 7, 8.1-8.2

Diagnosis.-Arthropods with triangular head shield with doublure, trapezoidal tail shield of size and proportions equal to head shield, up to seven segments with posteriormost free segment shorter than others, ventral cuticle equally sclerotized as dorsal.

Description.-Medium-sized arthropods with cephalon, maximum of seven body segments, and tail shield. Overall body form ovoid, roughly three times longer than wide; greatest width near center. Maximum observed length 33.0 mm; maximum observed breadth 9.5 mm. Exoskeleton with sparsely punctate dorsal surface; irregular, taphonomically induced wrinkles imply weak mineralization in life. No defined axial region. Cephalon roughly triangular with short blunt anterior medial protrusion; pair of short spines extending forward from anterolateral margins of cephalon; prominent doublure along anterolateral margin of cephalon, with median suture along midline of doublure. Body segments inferred to be nearly tubular in life, with cuticle preserved on both dorsal and ventral surfaces and sediment commonly filling the space in between; no difference in composition, texture, or scleritization between dorsal and ventral cuticle. Segments all perpendicular to long axis of body, with no deflection of the distal ends of segments; no lateral spines or other extensions. Posteriormost segment about half as long in the sagittal direction as the rest. Tail shield rounded trapezoidal, equal to cephalon in size, with blunt, square posterior terminus. Appendages unknown.

Discussion.-Esmeraldacaris is difficult to interpret, in part because it is not obvious which end is anterior and which is posterior. Because the specimens are all composed of delicate, partially exfoliated cuticle in a hard matrix, preparation of the fossils was considered likely to damage them and was not done. Formal cladistic analysis of Esmeraldacaris was also not done, because the combination of missing characters and plesiomorphies leads to poor phylogenetic resolution.

The apparent doublure at one end of the body of the holotype specimen is taken here to indicate the head. This orientation implies that the body segment that is shorter in the sagittal direction is the posteriormost. The complete specimen with six segments (USNM 520699, 19.5 mm) is smaller than the holotype (USNM 520698, 33 mm) with seven, suggesting that the two represent younger and older developmental stages. This is consistent with segments successively being released from the tail shield at each molt, with the shortest segment the youngest, a pattern seen in nektaspids and some trilobites (Fortey and Theron, 1994; Budd, 1999). None of the specimens of Esmeraldacaris are larger than the holotype, and it is tentatively assumed that the holotype represents the morphology at maturity.