SAKMARIAN (EARLY PERMIAN) SULCUS-BEARING SWEETOGNATHID CONODONTS FROM NEVADA: EARLY EXPERIMENTATION AND NOVEL CARINAL CONFIGURATIONS

The middle part of the Lower Permian Riepe Spring Limestone at North Spruce Mountain Ridge, Elko Co., Nevada preserves a sparse yet intriguing Sakmarian (Cisuralian) age conodont fauna dominated by the Early Permian genus Sweetognathus. P1 elements of the genus are characterized by subtle, pustulose micro-ornamentation along the oral surface of the platform and may display an adenticulate, nodose, transversely ridged, or sulcus-bearing (troughed) carinal configuration. The high degree of morphological plasticity among the sweetognathids (e.g., Sweetognathus and Neostreptognathodus) is widely documented and has led to a number of conflicting taxonomic assignments and phylogenetic interpretations. Reevaluations of near-homeomorphic, apparently asynchronous forms have demonstrated the potential for reiterations of analogous carinal configurations within the sweetognathids. The North Spruce Mountain Ridge (NSMR) fauna provides examples of the recurrent evolution of sulci by illustrating additional instances of near-homeomorphy and two novel carinal configurations. The recently identified Type V and Type VI carinal configurations exhibit the characteristic pustules of members of Sweetognathus, but pair the ornamentation with either primitive pre-sulcus traits (“dimples”) or composite features of types previously described by Ritter (1986). Such features likely represent early morphological experimentation with troughed denticulation in a region of the Ely Shelf that may have been partitioned by remnants of the Antler orogenic belt. Other sulcus-bearing sweetognathid conodonts recovered include forms similar to those assigned to Sw. clarki and Sw. obliquidentatus, species that are generally known to occur later in time than the presented specimens. The Sakmarian age assignment is based on the co-occurrence of cosmopolitan conodonts including Sw. expansus, Sw. aff. Sw. merrilli, and Sw. binodosus, along with the fusulinaceans Schwagerina aculeata, Schwagerina aculeata plena, Schwagerina vervillei, and various forms of Paraschwagerina. Additionally, all illustrated specimens precede the first occurrence of Sw. aff. Sw. whitei in the NSMR section.