NEWLY REPORTED TRIGONOTARBID SPECIMENS FROM OKLAHOMA AND INDIANA

Occurring from the late Silurian to the early Permian, the spider-like members of the ancient arachnid order Trigonotarbida represents some of the earliest fully terrestrial fauna and are the oldest known non-scorpion arachnids. Despite this significance, description of North American trigonotarbid specimens has remained largely incomplete with roughly a quarter of the known specimens only identified to order (even though many have ample diagnostic features to be further identified). This deficiency of North American trigonotarbid research presents great potential for expanding our understanding of trigonotarbid taxonomy, biogeography, and evolutionary history.

Examination of the known trigonotarbid specimens held in multiple museums across the U.S. and Canada has yielded 17 species of North American trigonotarbids (in 10 genera across 5 families) from 12 reported localities. Here, three undescribed trigonotarbid specimens (Field Museum of Natural History invertebrate paleontology collection) are reported for the first time from localities in Oklahoma and Indiana. All three specimens are preserved in iron carbonate concretions. The Indiana specimen (FMNH PE 9940) is from the middle Pennsylvanian Shelburn Formation (Westphalian D, 311–306 Ma), and both Oklahoma specimens (FMNH PE 56932 and 56955) are Pennsylvanian from the shale above the Croweburg Coal (Desmoinesian, 313–305 Ma).

All three specimens consist of a part and counterpart and are preserved as dorsal/ventral prosomal and opisthosomal external molds with only specimen 56932 having more than minimal appendage preservation. Both specimens 9940 and 56932 exhibit heavily ornamented dorsal surfaces (with the lobed, subtriangular carapace of 56932 clearly visible) indicating both belong to the monophyletic eophrynid assemblage (Eophrynidae, Kreischeriidae, and Aphantomartidae). Specimen 9940 possesses two pairs of terminal opisthosomal spines that are characteristic of both kreischeriids and eophrynids. These spines are absent in specimen 56932, suggesting its relation to aphantomartids.

These newly reported trigonotarbid occurrences increase the number known in North America and carries potential implications for the taxa’s taxonomy and biogeography through expanding the paleogeographic distributions of their families.