Freeman et al.—Upper Cambrian linguliform Brachiopods from Texas


valve from Utah appears to be less inflated and have more spines and fewer nodes than this Texas material.


Results


Sunwaptan ties to the Great Basin.—Quadrisonia lavada- mensis Popov, Holmer, and Miller, 2002 occurs in the Sunwaptan part of the section. This species was reported previously in Utah (Popov et al., 2002), and the genus has a wide distribution in strata equivalent to the Sunwaptan globally (Freeman and Miller, 2011b). Zhanatella utahensis Popov et al., 2002 also ranges through the Sunwaptan part of the section (Rasettia magna Subzone of the Saukia trilobite Zone, Fig. 2), which is a distribution similar to the other reported occurrence of the species in Utah (Popov et al., 2002; Freeman and Miller, 2011b). Although Z. utahensis is known only from these two occurrences in North America, the genus Zhanatella is globally distributed and is reported from Kazakhstan (Koneva, 1986; Popov and Holmer, 1994; Holmer et al., 2001; Koneva and Ushatinskaya, 2008), the Montagne Noire region of France (González-Gómez, 2005; Álvaro et al., 2007), and Antarctica (Henderson et al., 1992). Specimens of Wahwahlingula Popov, Holmer, and Miller, 2002 occur in middle Sunwaptan strata in Texas (Taenicephalus Zone to Rasettia magna Subzone). This cosmopolitan genus occurs in Russia (Popov et al., 2002), Iran (Ghobadi Pour et al., 2011), as well as possibly East Gondwana (Australia; Brock and Holmer, 2004) and the Western Gondwanan Montaigne Noire region of France (González-Gómez, 2005). Stittia ornata Freeman and Miller 2011b was previously


reported only from Utah, in equivalent Sunwaptan strata (Freeman and Miller 2011a, 2011b). This genus is endemic to North America.


Sunwaptan–Skullrockian turnover.—The turnover of brachio- pod taxa is complete across the Sunwaptan–Skullrockian boundary at both the generic and specific levels. However, a 23.5m interval encompassing most of the Prosaukia serotina and Eurekia apoposis trilobite zones and the Cambrooistodus minutus and Hirsutodontus hirsutus conodont subzones (Fig. 2) yielded no identifiable brachiopods. Sampling thoroughly cov- ered the interval, but the ~125 samples processed yielded either no brachiopods at all, or linguliform shell hash so thoroughly comminuted that identification was impossible.


Cosmopolitan Skullrockian fauna.—The lowest identifiable Skullrockian brachiopod is a single specimen of Eurytreta Rowell, 1966 in the highest part of the Eurekia apopsis trilobite Zone (Fig. 2). The specimen is not identifiable to species. Eurytreta is associated particularly with Skullrockian and higher strata in North America (Popov et al., 2002; Holmer et al., 2005; Miller et al., 2011; Miller et al., 2014), but has a cosmopolitan distribution (Ushatinskaya, 2010). Stitt (1971b, 1975, 1977) identified the Eurekia apopsis trilobite zone as the critical interval (stage four of Stitt’s four-stage biomere devel- opment). Taylor (2006) considered the overlying Apoplanias trilobite Zone to be the true critical interval, and samples from this interval did not yield recognizable brachiopods in Texas. The species Eurytreta sublata Popov in Koneva and Popov,


763


1988 tentatively is identified in lower Skullrockian strata (Fig. 2). This species has been reported previously from Utah (Popov et al., 2002), Kazakhstan (Holmer and Popov, 2001), and Iran (Popov et al., 2009a). Distinctive fragments of a hollow- spined siphonotretid, possibly Siphonobolus Havlíček, 1982, occur in the same sample, in the Symphysurina trilobite Zone (Fig. 2). A similar siphonotretid occurs in equivalent strata in Iran, based on conodont stratigraphy (Popov et al., 2009a) and is associated with Eurytreta sublata. Siphonotretids with hollow spines are also reported from slightly older strata (possible Proconodontus tenuiserratus conodont Zone) from a different location in Iran (Popov et al., 2009b), which is the oldest report of such a morphology. Two species of Schizambon Walcott, 1884 appear in lower


Skullrockian strata (Symphysurina trilobite Zone) at a similar stratigraphic level as the occurrence of the oldest known species reported from Laurentia, S. typicalis Walcott 1884 (Walcott, 1884, 1912; Popov et al., 2002). The genus Schizambon ranges upper middle Cambrian to Ordovician (Holmer and Popov, 2000) and has a global distribution (Popov et al., 2013).


Discussion


Brachiopods and sequence boundaries.—The Sunwaptan– Skullrockian boundary in the study area has been interpreted to coincide with a period of lowered sea level, the Lange Ranch Lowstand or Lange Ranch Eustatic Event (Miller, 1978, 1984, 1992; Miller et al., 2003). The absence of linguliform brachio- pods in any form other than comminuted shell hash could support the interpretation of a shallow, high-energy depositional environment, although the comminuted hash can alternatively be interpreted as a bioclastic lag associated with transgression. Detailed sampling across the same interval in Utah also failed to yield identifiable linguliform specimens (Popov et al., 2002). If this interval is interpreted as the beginning of a transgression, then the coincidence of the trilobite extinction/apparent brachiopod extinction with the underlying sequence boundary is consistent with the prediction that apparent extinction events may be related to stratigraphic processes (e.g., Holland and Patzkowsky, 2015). The appearance of brachiopods with global affinities as sea level rose (Osleger and Read, 1993; Miller et al., 2003) is consistent with cratonward-shifting biofacies during the subsequent transgression. The presence of unidentifiable comminuted linguliform brachiopod hash in some samples above the ‘extinction,’ and before the ‘recovery’ indicates that our perception of the timing of these events is affected by taphonomic factors related to depositional conditions.


Potential Kazakhstan/Gondwana to Laurentia Skullrockian post-extinction migration.—The distribution of Eurytreta sublata in Iran (Popov et al., 2009a) and Kazakhstan (Homer et al., 2001) suggests migration between Laurentia, Kazakhstan, and Gondwana after the Sunwaptan–Skullrockian extinction. The lowest appearance of Eurytreta cf. E. sublata in lower Skullrockian strata in both the Great Basin and Texas suggests migration of that genus from outside of Laurentia. One possi- bility is Kazakhstan, where another Eurytreta species occurs in Sunwaptan-equivalent strata in the Malyi Karatau Range (Holmer et al., 2001). Another older occurrence of species of


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