NEW LINGULIFORM BRACHIOPODS FROM UPPER CAMBRIAN AND LOWEST ORDOVICIAN (MILLARDAN/IBEXIAN) STRATA IN CENTRAL TEXAS: CORRELATIONS WITH THE GREAT BASIN, WYOMING, AND BEYOND

Intensive bulk sampling of the upper three members of the Wilberns Formation (Moore Hollow Group) and lower member of the Tanyard Formation (Ellenburger Group) has yielded 3840 specimens of linguliform brachiopods from 66 samples. The fauna includes many taxa not previously documented in Texas. The samples are from the classic Threadgill Creek measured section, near Doss, Texas. Trilobite and conodont zonations for this section have been documented previously. Samples were taken across two stage boundaries defined by trilobite extinctions.

The lowest brachiopod occurrence, in upper Steptoean strata, is Linnarssonella girtyi Walcott, 1902, which is widespread in Laurentian North America. Brachiopods first appearing near the base of the Sunwaptan stage (just above a trilobite extinction) have short ranges and can be used to correlate to coeval strata in Wyoming where linguliform brachiopod zonation was established previously. Sunwaptan species which have their lowest occurrence in the mid-Idahoia trilobite Zone and above generally have longer ranges and are reported previously from the Great Basin instead. All Sunwaptan linguliform brachiopod species apparently are endemic to North America.

There is a complete turnover of brachiopod fauna at the top of the Sunwaptan Stage, apparently occurring in coincidence with a second trilobite extinction event. The globally distributed species Eurytreta sublata Popov, 1988 appears in strata above the extinction event. This species was reported previously from Kazakhstan, Iran, and Utah. A siphonotretid with hollow spines also appears in lower Skullrockian strata and appears to be coeval with a previous report from Iran of the oldest occurrence of siphonotretids of this type. Because Laurentia, Kazakhstan, and Iran were widely separated geographically as well as latitudinally, the occurrence of Eurytreta sublata and the siphonotretid suggests that linguliform brachiopods were able to disperse particularly widely in the wake of whatever event caused Sunwaptan/Skullrockian trilobite extinction in Laurentia.

New zonation based on occurrences of linguliform brachiopods is proposed for the Sunwaptan and lowest Skullrockian of North America, and some of these zones may prove useful for global correlation as well.