Increasing control over biomineralization in conodont evolution

Vertebrates use the phosphate mineral apatite in their skeletons, which allowed them to develop tissues such as enamel, characterized by an outstanding combination of hardness and elasticity. It has been hypothesized that the evolution of the earliest vertebrate skeletal tissues, found in the teeth of the extinct group of conodonts, was driven by adaptation to dental function. We test this hypothesis quantitatively and demonstrate that the crystallographic order increased throughout the early evolution of conodont teeth in parallel with morphological adaptation to food processing. With the c-axes of apatite crystals oriented perpendicular to the functional feeding surfaces, the strongest resistance to uniaxial compressional stress is conferred along the long axes of denticles. Our results support increasing control over biomineralization in the first skeletonized vertebrates and allow us to test models of functional morphology and material properties across conodont dental diversity

Biostratigraphy and Paleoecology of Lower Paleozoic, upper Cretaceous, and Lower Tertiary Rocks in U.S. Geological Survey New Madrid Test Wells, Southeastern Missouri

The paleontology and biostratigraphy of Tertiary, Cretaceous, and Paleozoic rocks in the upper Mississippi embayment are incompletely known because marine fossils are only locally present in these rocks. This study concerns material from two U.S. Geological Survey test wells drilled in New Madrid County, southeastern Missouri, as part of earthquake hazard studies in the northern Mississippi Embayment. Test well1 sampled lower Tertiary strata to a depth of 146ft; these strata were found to be late Eocene in age on the basis of sporomorphs. Test well1-X, 29ft northwest of well1, provided cuttings and cores from lower Tertiary, Upper Cretaceous, and lower Paleozoic rocks to a total depth of 2,316 ft below the Kelly bushing (at an altitude of 288 ft). Lithologic evidence suggests that the base of the Jackson Formation may be at 270 ft, but sporomorphs indicate that the base of the Jacksonian Stage (upper Eocene) is possibly at a depth of about 350ft in test well1-X. Lithologic units of the Claibornian Stage (middle Eocene) here consist of the Cockfield(?) and Cook Mountain(?) Formations and Memphis Sand, in descending order. The Claibornian could not be subdivided using sporomorphs from cuttings, but sporomorphs show that the top of the Sabinian Stage (top of the lower Eocene) is at about 1,055 ft; lithologically, the top of the Wilcox Group (top of the Flour Island Formation) is at 1,048 ft. Sporomorphs suggest that the top of the lower Sabinian (top of the Paleocene) is within the Flour Island Formation at about 1,105 ft. The next major lithologic break, the top of the Fort Pillow Sand at 1,186 ft, does not coincide with any detectable biostratigraphic boundary. Lithologically, the interval from 1,339 to 1,377 ft may belong to the Old Breastworks Formation; dinoflagellates from cuttings indicate that this interval is likely to be late Midwayan in age and, therefore, correlative with the Naheola Formation of the eastern Gulf Coast. The Porters Creek Clay extends from a probable top at 1,377 ft to 1,691 ft, and the base of the underlying Clayton Formation (Tertiary-Cretaceous contact) is at 1, 703 ft. Calcareous nannofossils, dinoflagellates, foraminifers, mollusks, ostracodes, and sporomorphs from continuous cores of the Porters Creek Clay and Clayton Formation indicate that the upper half of the Porters Creek correlates with the upper part of the same formation or perhaps partly with slightly younger rocks in the eastern Gulf Coast, the lowermost Porters Creek appears to correlate with the upper part of the Clayton Formation of the eastern Gulf Coast and with the Kincaid Formation of Texas, and the thin Clayton of the test well probably correlates with only the lower part of the thicker Clayton of the eastern Gulf Coast. Sporomorphs from McNairy Sand (Upper Cretaceous) cores indicate Maestrichtian and perhaps latest Campanian Age. Lower Paleozoic dolostone from 2,023 ft to total depth at 2,316 ft is barren of identifiable fossils except for a probable fragment of the fish Anatolepis; the dolostone is probably Late Cambrian in age. The McNairy Sand of New Madrid well 1-X seems to have been deposited in nonmarine to nearshore marine environments; the Clayton Formation and lower part of the Porters Creek Clay were deposited under marine conditions during an early Paleocene marine transgression. A major regression followed that continued through the end of Porters Creek time and perhaps into Naheola (late Midwayan) time; the Sabinian, Claibornian, and Jacksonian strata in southeastern Missouri were deposited primarily or entirely in nonmarine environments.

Conodonts from El Paso Group (Lower Ordovician) of westernmost Texas and southern New Mexico

Samples were taken at 20-ft intervals through a 410 m (1,345-ft) section of the El Paso Group in the southern Franklin Mountains of westernmost Texas and southern New Mexico as part of a detailed study of the conodonts of that unit. The succession is predominantly dolostone in the lower part and limestone in the upper part of the section. All of the 1-kg samples processed yielded conodonts. More than 16,500 conodont elements were recovered, and preservation of the conodonts ranges from poor to good. Altogether, 145 species are described and/or discussed taxonomically. The conodonts are distributed among 30 genera, of which Cristodus is new. Of the 145 species, 41 are discussed in terms of multielement taxonomy, and 104 are discussed in form taxonomy. New species named herein include: Clavohamulus lemonei, Cristodus loxoides, Drepanodus pseudoconcavus s. f., Histiodella donnae, Juanognathus hayesi, Oistodu? lecheguillensis, Protopanderodus leei, Reutterodus borealis, Scolopodus abruptus s. f., Scolopodus acontiodiformis, S. acontiodiformis angularis, S. bolites, S. filosus xyron, S. kelpi, S. parabruptus, S. carlae, and S. floweri. The conodonts range in age from early (but not the earliest) to late Canadian. North American conodont Fauna C is represented in the lowermost beds of the El Paso Group in west Texas. Diagnostic conodonts of Fauna D appear between 56-91m (180-300 ft) above the base of the section. Elements of Fauna E appear abruptly at 256 m (839 ft). Fauna 1 is represented in the uppermost El Paso; diagnostic elements of this fauna appear between 347 m (1,140 ft) and 396 m (1,300 ft) above the section base. Correlation by means of conodonts is discussed, and the El Paso faunal sequence is compared with faunas from Lower Ordovician conodont-bearing successions in the United States and abroad.Conodonts of Early Ordovician age are as yet quite poorly known, especially considering the relatively widespread occurrence of rocks of that age. Contributing to the paucity of definitive studies of Early Ordovician conodonts, at least in North America, is the widespread occurrence through this interval of dolostones that are difficult to disaggregate. Sections that yield to acidizing techniques and that are partly to nearly continuous through the Lower Ordovician do exist, and several of these sections are presently the objects of intense conodont study. In North America, important sequences of this age being studied are those of the Great Basin; the Arbuckle Group of southern Oklahoma; the Canadian Rocky Mountains; the northern and central Appalachians; the Canadian Arctic; and the El Paso Group of western Texas, New Mexico, and Arizona.The primary purpose of this paper is to report, describe, and illustrate the conodont fauna recovered from the type section of the El Paso Group. Secondly, the conodonts of the El Paso Group are compared and correlated, where possible, with conodont faunas recovered from Lower Ordovician sequences elsewhere. Most of this comparison is done within taxonomies. Biostratigraphic and taxonomic work is continuing on the conodonts from the El Paso Group and related strata from sections in the Florida and Caballo Mountains, New Mexico, and from other sections in New Mexico and southeastern Arizona, as well as on the fauna recovered for this study.</jats:p

Basal Tissue Structure In The Earliest Euconodonts: Testing Hypotheses Of Developmental Plasticity In Euconodont Phylogeny

The hypothesis that conodonts are vertebrates rests solely on evidence of soft tissue anatomy. This has been corroborated by microstructural, topological and developmental evidence of homology between conodont and vertebrate hard tissues. However, these conclusions have been reached on the basis of evidence from highly derived euconodont taxa and the degree to which they are representative of plesiomorphic euconodonts remains an open question. Furthermore, the range of variation in tissue types comprising the euconodont basal body has been used to establish a hypothesis of developmental plasticity early in the phylogeny of the clade, and a model of diminishing potentiality in the evolution of development systems. The microstructural fabrics of the basal tissues of the earliest euconodonts (presumed to be the most plesiomorphic) are examined to test these two hypotheses. It is found that the range of microstructural variation observed hitherto was already apparent among plesiomorphic euconodonts. Thus, established histological data are representative of the most plesiomorphic euconodonts. However, although there is evidence of a range in microstructural fabrics, these are compatible with the dentine tissue system alone, and the degree of variation is compatible with that seen in clades of comparable diversity