Kochiella Poulsen, 1927, and Hadrocephalites new genus (Trilobita: Ptychopariida) from the early middle Cambrian of western North America

Journal of Paleontology, Jan 2002 by Sundberg, Frederick A, McCollum, Linda B

ABSTRACT-Kochaspids are an informal group of ptychopariid trilobites that were both abundant and widespread in the early Middle Cambrian of North America. Based on the reassociation of pygidia and cranidia of some kochaspids, Kochiella Poulsen, 1927, is redefined and Hadrocephalites n. gen. is proposed. Hadrocephalites includes taxa previously assigned by Rasetti and Palmer to Schistometopus Resser, 1938a. Schi.stometopus is considered nomen dubium. Representatives of Kochiella and Hadrocephalites from the Pioche Shale and Carrara Formation of Nevada are described, including the new species Kochiella rasettii, K. brevaspis, Hadrocephalites Ivndonensis, and H. rhVidodes. Other kochaspids previously assigned to Kochaspis Resser, 1935; Eiffelaspis Chang, 1963; Schistometopus: and Kochiella are discussed and some are reassigned. The type specimens of Kochiella augusta (Walcott, 1886); K. crito (Walcott, 1917b); K. chares (Walcott, 1917a); K. mansfieldi Resser, 1939; K. arenosa Resser, 1939; K. arenosa Resser, 1939; Hadrocephalites carina (Walcott. 1917b), and H. cecinna (Walcott. 1917b) are re-illustrated.

INTRODUCTION

DURING A study of trilobites from the Plagiura-Poliella Biozone of the Pioche Shale and Carrara Formation of southeastern Nevada, it became apparent that the cranidia and pygidia of some kochaspid trilobites have been incorrectly associated (e.g., Resser, 1935, 1938a, 1938b, 1939; Rasetti, 1951, 1957; Palmer and Halley, 1979). These incorrect associations probably resulted from the misassociation of the cranidium and spinose pygidium of Kochaspis liliana (Walcott, 1886, type species of Kochaspis Resser, 1935) and the placement of the spinose pygidium of Kochaspis augusta (Walcott, 1886) to the same genus. Walcott (1886, 1890, 1916) originally assigned these species to Crepicephalus Owen, 1852, and then Resser (1935) assigned them to Kochaspis. Walcott's and Resser's incorrect assignments probably resulted from several kochaspids occurring at the type locality (personal observation of type lot USNM locality 31 a and nearby localities) and the lack of articulated material.

This paper proposes new pygidial associations for some kochaspids; differentiates the distinctive genera Kochiela Poulsen, 1927, and Hadrocephalites n. gen. based on these reassociations; and describes their representatives in the Pioche Shale and Carrara Formation of southeastern Nevada. Hadrocephalites includes species previously assigned to Schistometopus Resser, 1938a, by Rasetti (1951, 1957) and Palmer (in Palmer and Halley, 1979). Schistometopus is considered as nomen dubium because of the poor and partial preservation of its type species S. typicalis Resser, 1938a, as discussed under Hadrocephalites.

This study is part of a series of papers that includes the reanalysis of the kochaspids and the description of trilobites from the Lower-Middle Cambrian boundary interval of the southern Great Basin (Sundberg and McCollum, 1997, 2000; Eddy and McCollum, 1998). Kochaspids are an informal group of ptychopariid trilobites from the early Middle Cambrian of North America (Palmer and Halley, 1979, p. 99). These trilobites are characterized by granular ornamentation and similar cranidia, including a prominent tapered glabella with several pairs of moderately to strongly developed lateral glabellar furrows, in which the glabella tends to be slightly concave opposite of the second pair (S2) of lateral glabellar furrows. Genera were based primarily on pygidial characteristics.

Palmer (in Palmer and Halley, 1979) included Caborcella Lochman, 1948; Kochaspis Resser, 1935; Kochiellina Palmer (in Palmer and Halley, 1979); Nyella Palmer (in Palmer and Halley, 1979); and Schistometopus; and possibly Kochiella and Kochina Resser, 1935, within his concept of kochaspids. Eokochaspis Sundberg and McCollum, 2000, should also be included in the kochaspids. These trilobites are common in strata ranging from the Eokochaspis nodosa to Albertella biozones of western North America, including the Arrojos Formation of Mexico (Lochman, 1948); the Carrara Formation (Palmer and Halley, 1979), Pioche Shale (Walcott, 1886; Palmer in Merriam, 1964; Sundberg and McCollum, 1997, 2000; Eddy and McCollum, 1998; this report), Brigham Quartzite (Resser, 1939), and Langston Formation (Resser, 1939) of the United States; and the Mount Whyte Formation (Walcott, 1917b; Rasetti, 1951, 1957), Cathedral Formation (Walcott, 1917a; Rasetti, 1951), and an unnamed formation (Norford, 1968) of Canada.

STRATIGRAPHY

The kochaspid trilobites described in this report are from the lower Middle Cambrian portion of the Lower and Middle Cambrian Pioche Shale and Carrara Formation in southeastern Nevada. This study is confined to the mountain ranges near the type area of the Pioche Shale in eastern Lincoln County, Nevada, and to the Carrara Formation exposed in the northern Groom Range in the western part of Lincoln County (Fig. 1).

The Pioche Shale and Carrara Formation consist of a mixed siliciclastic and carbonate facies assemblage. The bases of both formations appear to be time-equivalent, i.e., Arcuolenellus arcuatus assemblage, upper Olenellus Biozone. The top of the Pioche Shale coincides with the top of the Albertella Biozone, whereas the upper Carrara Formation includes the lower portion of the overlying Glossopleura Biozone (Palmer and Halley, 1979; Eddy and McCollum, 1998). Both formations have been divided into a series of members (Fig. 2). Merriam (1964) proposed two formal and four informal members for the Pioche Shale, and subsequently Eddy and McCollum (1998) and Sundberg and McCollum (2000) have replaced the informal A- through D-shale members with formal member names. Palmer and Halley (1979) proposed nine formal members for the Carrara Formation.