MIDDLE ORDOVICIAN (CHAZYAN) SEA-LEVEL CHANGES AND THE EVOLUTION OF THE ORDOVICIAN CONODONT GENUS CAHABAGNATHUS BERGSTRÖM, 1983

ABSTRACT-

Two lineages exist for Cahabagnathus. Lineage 1 is C friendsvillensis-C. chazyensis-C. sweeti-C. carnesi. Three fresh species are proposed in lineage 2 which consists of C baueri n sp-C directus-C. cooperi n sp and C craigi n sp The distribution pattern of Cahabagnathus was powerfully influenced by Chazyan sea-level changes. The wide distributions of C friendsvillensis and C sweeti correspond to sum of two units large transgressions and the more narrow distribution of C chazyensis and C earnesi correspond to regression incidents A similar relationship exists between sea-level change and the distribution of species in lineage 2 Evolution of the cahabagnathids was apparently influenced by means of the rise of sea horizontal which widely distributed taxa, and the fall of sea horizontal which isolated taxa. We suggest that it was from peripheral isolates of widely distributed taxa that endemic Cahabagnathus taxa (C baueri, C cooperi, C craigi, C chazyensis, and C carnesi) evolved

INTRODUCTION

ACOMMON pattern in the evolution of Cambrian-Ordovician conodont lineages, especially those adapted to shallow marine faci??s, is the appearance of novel species at sea-level rises and disappearances at sea-level globules This simple picture can be complicated by dint of intervals that consistently have no examples of a particular lineage, smooth after extensive sampling of the greatest in quantity complete sections. Presumably the lineages survived in refugia, on the other hand commonly these endemic taxa remain unknown. Collecting unusual facies or stones that are difficult to proces may be wanted to recover such taxa and to re-establish evolutionary lineages (Lehnert et al., 2005) In contrast to shallow shelf areas, we can mark apparently complete evolutionary lineages of taxa in open-marine and deeper water environments (eg the Pygodus Lamont and Lindstrom, 1957 lineage; diocese Bergstr?¶m, 1983 and Zhang, 1998a). The distribution pattern of the endemic and more cosmopolitan species of Cahabagnathus Bergstr?¶m, 1983 (Table 1) demonstrates in what manner collecting material in less favorable lithologies for conodont sample processing can add to our understanding of the interplay between sea-level changes and evolutionary lineages. There are eight species of Cahabagnathus in the published literature at the near time that range from the early Pygodus serra band through the Baltoniodus gerdae Subzone of the Amorphognathus tvaerensis cincture (Bergstr?¶m, 1983; Bauer, 1990; Sweet, 1991; Leslie and Lehnert 1999) Five of the species are named. In this paper we formally document three fresh species and discuss how sea-level changes may have influenced the evolution of the species of Cahabagnathus.

EVOLUTIONARY LINEAGES OF CAHABAGNATHUS

The biostratigraphic value of the Cahabagnathus lineage was first discussed in detail by means of Bergstrom (1971, 1973). Bergstrom (1983) when he formally propos the name Cahabagnathus, used systematic and biostratigraphic data to suggest evolutionary relationships within the cahabagnathids. He documented what we deliver over to as evolutionary lineage 1 (Bergstr?¶m, 1983) This lineage, C friendsvillensis-C. chazyensis-C. sweeti-C. earnest (Figs. 1 2) is characterized by dint of the gradual disappearance of the anterior lobe of the anterolateral proces in the stelliplanate uncompounded body widening of the posterior proces in the pastiniplanate ultimate part and a progressive change in the main denticle line at the junction of the posterior and anterior processe in the pastiniplanate simple body (Bergstrom, 1983). This progression of characters is well documented in Whiterockian strata from the southern Appalachian Mountains (Bergstr?¶m, 1971 1973 1983; Bergstr?¶m and Carnes, 1976) and in part from the Arbuckle Mountains (Bauer, 1990) In the pastiniplanate uncompounded body there is also a decrease in the angle made by dint of the denticle row on the posterior proces with the denticle line on the lateral process from one side time from approximately 120?° in C.friendsvillensis (Bergstr?¶m, 1971) to approximately 90?° in C earnest Bergstr?¶m, 1983 (Fig. 1) In addition, a short anterolateral proces unfolded in the pastiniplanate elements of the sum of two units youngest taxa of lineage 1 [C sweeti (Bergstr?¶m, 1971) and C earnest]. Bauer (1987a) allude toed that there may be more than single lineage of Cahabagnathus based upon the recognition of two fresh species of Cahabagnathus. He illustrated Cahabagnathus baueri n sp (=Bauer, 1987a, Cahabagnathus n sp) and C directus Bauer, 1987b as members of a separate lineage of Cahabagnathus, with the former as the ancestor and the latter the descendant (Bauer, 1987a, fig. 41) the one and the other of these species possess an anterior lobe of the anterolateral proces in the stelliplanate ultimate part as do C. friendsvillensis and C chazyensis Bergstr?¶m, 1983 in lineage 1

Leslie and Lehnert (1999) further documented the idea that there were distinct Cahabagnathus lineages. Here, we expand upon this idea and formally define the novel taxa that illustrate the sum of two units lineage concept of the evolution of Cahabagnathus. Cahabagnathus craigi n sp and Cahabagnathus cooperi n sp are interpreted as the youngest representatives of the propos Cahabagnathus lineage 2 This lineage is the Cahabagnathus baueri-C. directus relationship propos by the agency of Bauer (1987a) with the recognition of Cahabagnathus cooperi from southwest Nevada and Cahabagnathus craigi from north-central Arkansas as the newly discovered youngest species in this phylogeny The sum of two units new species of Cahabagnathus recognized in this paper posses an anterior lobe upon the anterolateral process in the stelliplanate uncompounded body (Fig. 2). In C. cooperi and C craigi, the angle made between the denticle rank on the posterior process with the denticle file on the lateral process upon the pastiniplanate element does not fit well within progression observ in lineage 1 (Fig. 2) This is especially actual for C. cooperi, where the angle is nearly 180?° Lineage 2 is characterized by the agency of a large angle in all species. However, the large variance observ in this angle for C craigi implies that a link between this fresh species and lineage 1 cannot be rul on the outside If the proposed two lineage archetype is correct, Cahabagnathus lineage 1 and Cahabagnathus lineage 2 evolv from a for the use of all ancestor in southern Laurentia during the middle Whiterockian.