Abstracts of Recent Articles


Yin, Jiarun and C. A. McRoberts. 2006. Latest Triassic-Earliest Jurassic bivalves of the germig Formation from Lanongla (Tibet, China). Journal of Paleontology, vol. 80, no. 1, pp.104-120.

ABSTRACT: The Germig Formation of the Tethyan Himalaya of southern Tibet contains an exceptionally abundant bivalve fauna which has been found in association with choristoceratid and psiloceratid ammonoids and spans the Triassic/Jurassic boundary. The bivalve fauna consists of 25 species, including four new species: Newaagia lanonglaensis, Persia hallami, Liostrea tibetica, and Ctenostreon newelli. The fauna comprises three biostratigraphically controlled bivalve assemblages: 1) an upper Rhaetian Palaeocardita–Krumbeckiella Assemblage including seven species; 2) a high diversity transitional Rhaetian-Hettangian Persia–Plagiostoma Assemblage with many as 19 species; and 3) a low diversity lower Hettangian Liostrea–Chlamys Assemblage containing three species. The transitional Rhaetian-Hettangian Assemblage is dominated by cementing species and exhibits a high degree of endemism. A large proportion of lower latitude and cementing taxa from the lower two levels may indicate that they inhabited shallow subtidal tropic or subtropic paleoenvironments.

McRoberts, C. A., and N. D. Newell. 2005. Marine Myalinidae (Bivalvia: Pterioida) from the Permian of West-Texas. American Museum Novitates No. 3460, pp. 1-15.

Marine bivalves of the family Myalinidae are an important benthic constituent of the Permian Reef Complex of West Texas and New Mexico. We describe and summarize the myalinids from Lower and Middle Permian reef and near-reef settings and infer living habits as either epifaunal or semi-infaunal byssally attached suspension feeders. The six myalinid species described are exceptionally preserved as silica pseudomporphs. Included in the fauna are two new taxa: Myalina lamellosa, a species with distinctive commarginal lamellae, and Myalina plicata, the only known myalinid with prominent radial plicae. The family Myalinidae is placed in the Ambonychioidea (Order Pterioida) and an emended diagnosis incorporates ligament characters and details of shell ultrastructure.

Hopkin, E. K. and C. A. McRoberts. 2005. A New Middle Triassic Flat Clam (Pterioida: Halobiidae) from the Middle Anisian of North-Central Nevada, USA. Journal of Paleontology, vol. 79, pp. 796-800.

McRoberts C. and Blodgett, R. 2002. Late Triassic (Norian) Molluscs from the Taylor Mountains, Alaska. U.S. Geological Survey Professional Paper 1662, pp. 55-75

ABSTRACT: We describe a diverse molluscan fauna of silicified fossils from two localities of the Taylor Mountains D-3 quadrangle of southwestern Alaska. The molluscan fauna consists of at least 8 species of bivalves, including 1 new species. Cassianella cordillerana McRoberts n.sp., and at least 11 species of gastropods, including 2 new species, Neritaria nuetzeli Blodgett n.sp. and Andangularia wilsoni Blodgett n.sp. Bivalve and gastropod affinities suggest an early Norian age with taxonomic similarities to several southern Alaskan tectonostratigraphic terranes (for example, Alexander and Chulitna), as well as to the South American Cordillera of Peru. The mollusks are associated with numerous brachiopods that also support a Norian age and similar biogeographic affinities.

McRoberts, C. A. 2001.Triassic bivalves and the initial marine Mesozoic revolution: A role for predators?. GEOLOGY, vol 29, pp. 359-362.

ABSTRACT: Marine bivalves document the long-term increase in generic richness through the early Mesozoic. Following the end-Permian crisis, the Early Triassic was marked by a gradual recovery in generic richness (57 Induan and 66 Olenekian genera). Diversity slowly increased in the Middle Triassic (98 Anisian and 121 Ladinian genera) and peaked in the Late Triassic (171 Carnian, 165 Norian, and 143 Rhaetian genera). These data support earlier hypotheses that the recovery following the end-Permian extinction was very gradual and was not completed (in terms of both richness and ecologic complexity) until the Ladinian. Although a Carnian-Norian extinction is not evident in the data and may be a regional event limited to the Tethyan Realm, the end-Triassic extinction is profound--less than 30 genera (<35%) survived into the Jurassic. Diversity metrics are not equally distributed among bivalve living habits. The generally epifaunal Pteriomorphia and Isofilibranchia exhibit higher extinction rates compared to the ordinarily infaunal Heteroconchia (especially the Veneroida and Trigonoida). This pattern of selective extinction led to a gradual increase in generic richness of infaunal suspension feeders through most of the Triassic. Contrary to previous hypotheses, this increase in infaunalization may not have been related to the evolutionary expansion of major predatory groups (e.g., shell-crushing cephalopods, crustaceans, sharks, fish, and reptiles), which had typically low abundances and limited distribution during the Triassic. Drilling predators, although present during the Triassic, are not considered to be prominent causes of mortality among bivalves. Instead, the infaunalization of bivalves during the Triassic may have been due to several interconnected abiotic and biotic causes associated with the recovery after the end-Permian mass extinction.

McRoberts, C. A., and N. D. Newell. 2001. A new Permian myalinid genus, Elversella, of west Texas. American Museum Novitates, 3311, pp. 1-5.

ABSTRACT: A distinctive myalinid bivalve Elversella rugosa new genus and species from the Middle Permian of west Texas is described. Elversella rugosa is characterized as being inequivalved, with a larger left valve covered rhythmically by coarse rugae and a smooth right valve that is somewhat smaller and less convex.

Orchard, M. J., Zonneveld, M. J., Johns, M. J., McRoberts, C. A., Tozer, E. T., and Sandy, M. R., 2001, Fossil succession and sequence stratigraphy of the Upper Triassic of Black Bear Ridge, northeast British Columbia, a GSSP prospect for the Carnian-Norian boundary : Albertiana, vol. 25, p. 10-22.

ABSTRACT: Black Bear Ridge on Williston Lake is one of several important North American reference sections for intercalibrating the ranges of Upper Triassic ammonoids, conodonts, bivalves, ichthyoliths, and brachiopods. The section, largely comprising the Ludington and Pardonet formations, consists of four sequences (BB-I through IV) spanning the uppermost Carnian through Hettangian of the Lower Jurassic. These sequences record deposition on the distal slope and adjacent abyssal plain, west (seaward) of the Pangean continental shelf. Fossil fauna and sequence stratigraphy serve to identify significant biological and sedimentary events in the history of the Late Triassic. Each of the four sequences consist of a comparably coarse- grained lowstand succession which grades up through a thick transgressive succession marked by peak abundances of the conodont (Norigondolella) and ichthyolith (Birgeria). Highstand systems tracts within the study interval are thin, consisting of condensed intervals with few fossiliferous limestone beds. The Carnian-Norian boundary interval lies within sequence BB-1 and the transitional beds between the Ludington and Pardonet Formations, wherein a lineage of several Metapolygnthaus conodont species provide two potential datums for boundary definition: the base of the Communisti and Primitius zones. Within the same interval, several ammonoids and bivalves occur, and there are several changes in the ichthyolith fauna.

Orchard, M. J., C. A. McRoberts, E. T. Tozer, M. J. Johns, M. R. Sandy, and J. S. Shaner. 2001. The Upper Triassic of Black Bear Ridge, Williston Lake, northeast British Columbia: an integrated biochronology. Geological Survey of Canada Current Research. vol. 2001-A6, p. 1-21.

ABSTRACT: Black Bear Ridge on Williston Lake is one of several important North American reference sections for intercalibrating the ranges of Upper Triassic ammonoids, conodonts, bivalves, ichthyoliths, and brachiopods. The section (Ludington and Pardonet formations) spans the uppermost Carnian through Hettangian of the Lower Jurassic. The co-occurrence of several fossil groups facilitates an integrated biozonation of the interval. Fossil fauna and biofacies serve to identify significant biological and sedimentary events in the history of the Late Triassic. Conodont and ichthyolith faunal change is gradual through the Carnian-Norian boundary, whereas conodont and ammonoid changes are dramatic across the Lower-Middle Norian boundary. Major transgressions are recognized in both the Lower and Middle Norian, and are marked by a conodont (Norigondolella) and ichthyolith (Birgeria) biofacies.

McRoberts, C.A. 2000. A primitive Halobia (Bivalvia: Halobioidea) from the Triassic of northeast British Columbia. Journal of Paleontolgy, vol. 74, pp. 599-603.

ABSTRACT: Halobia daonellaformis new species is described from the lowermost Carnian of northeast British Columbia. Halobia daonellaformis is regarded as a primitive Halobia characterized by external ornamentation similar to Daonella lommeli, but with a poorly developed anterior auricle. Morphologic characters of H. daonellaformis suggest that Halobia may not be a natural taxon, but a polyphyletic group with one or more ancestors from Daonella and Aparimella and/or other posidoniid(s). The sudden appearance of Halobia throughout the marine Triassic suggests a rapid dispersal mechanism following a Ladinian origin. Larval shell morphology indicates a planktotrophic developmental strategy for H. daonellaformis, and by extension to other halobiids, may explain the widespread distribution of many halobiid species.

Pandey, D.K., C.A. McRoberts, and M.K. Pandit. 1999. Dimorpharaea (Scleractinia, Anthozoa) from the Middle Jurassic Of Kachchh, India, Journal of Paleontology, vol. 73, pp. 1015-1028.

ABSTRACT: Classification of scleractinian corals based upon gross morphological features has been found unsatisfactory in the light of additional information from skeletal microarchitecture and microstructure. It is necessary to investigate microstructural details and limits in morphologic variation within and between different coral clades before a revised classification is constructed.
Variations in morphologic characters and microstructural details from a population of Dimorpharaea (Family Microsolenidae) from Upper Bathonian (Jumara Dome) strata in Kachchh are described. The data used include the diameter (D) and height (H) of the corallum, number of corallites in the colony (NC), number of septa in the mother corallite at the center of the colony (NS), minimum distance between centers of central corallite and corallite of the inner ring (C1), minimum distance between corallite centers of the outer ring (C2), septal density (DS) and trabecular density (DT). The principal components analysis reveals that most of the variation is explained by "size" related characters (D and H) while corallite density (NC and C1) and septal structures (DS and DT) contribute to the second and third principal component axes, respectively. The microarchitecture and distribution of characters observed in the Kachchh Dimorpharaea require a re-evaluation of familial-specific concepts and suggest that the population belongs to a single species, Dimorpharaea stellans, rather than four morphospecies (D. stellans, D. distincta, D. continua and D. orbica) as has been assumed by previous authors.

McRoberts, C.A, 1998. Late Triassic (Norian-Rhaetian) bivalves from the Antimonio Formation, northwest Sonora, Mexico.Revista Mexicana de Ciencias Geológicas, vol. 14, no. 2 (1997), pp. 167-177

ABSTRACT: The Norian-Rhaetian bivalve fauna from the Upper part of Antimonio Formation is taxonomically and ecologically more diverse than previously reported. Of the ten bivalve taxa described herein, eight have never been previously described from Sonora and at least four represent previously unknown forms. Two new species are formally recognized: (i) ?Lopha cordillerana n. sp., a large ostreid from a Norian bioherm facies, and (ii) Xiaoschuiculana tozeri n. sp., a nuculanid from fine-grained sand and siltstones of probable Rhaetian age. Other taxa, while ecologically important, are too few in number or too poorly preserved to be adequately assigned to species. Although the Antimonio bivalve fauna shows strong taxonomic affinities with South American faunas, similarity to other North American localities, such as from the Luning-Gabbs sequence of Nevada or co-eval rocks in the Cordilleran terranes of California and Oregon, cannot be assessed due to inadequacy in species-level taxonomy.

McRoberts, C.A. , H. Furrer, and D. Jones. 1997. Palaeoenvironmental interpretation of a Jurassic-Jurassic boundary section from western Austria based on palaeoecologic and geochemical data. Palaeogeography, Palaeoclimatology, Palaeoecology, vol. 136, pp. 79-95.

ABSTRACT: A sequence spanning the Triassic-Jurassic boundary is described from near the village of Lorüns in the Vorarlberg region of western Austria. At Lorüns, the latest Triassic is characterized by bedded carbonates of the Kössen Formation supporting a stenotopic fauna indicative of a shallow sub-tidal environment of normal marine salinity. The Triassic-Jurassic boundary may be represented as a sequence boundary developed on top of a 1.1 m thick boundary interval of red mudstones of the lower Schattwald Shale which is interpreted to have been deposited in a marginal marine environment and quite possibly a mud flat. Above the boundary interval, the upper Schattwald Shale is characterized by thin-bedded marl and dark limestone beds with a Hettangian macrofauna dominated by epifaunal filter-feeding bivalves including ostreids and mytilids, which suggest a shallow, subtidal, salinity-controlled environment typical of an interplatform lagoon. Carbonate production rejuvenated in the later Early Hettangian with development of the Lorüns Oolite, a shallow subtidal oolitic and oncolitic unit bearing echinoderms indicative of normal marine conditions.

Lithologic and geochemical evidence for the spread of anoxic waters during marine transgression is absent from Lorüns where Th/U values, determined by gamma-ray spectrometry, are above 5 in the boundary interval. A negative excursion in d13C in the boundary interval can be interpreted in two different ways: (i) a short-term decrease in primary productivity or (ii) precipitation of caliche during paleosol development. The first alternative is favored due to the positive excursion in d18O within the same interval, even though such a change may have been short-lived. Although short-term changes in paleotemperature may have occurred within the boundary interval, comparison of d18O values from the Kössen Formation and Lorüns Oolite indicate no significant long-term changes in paleotemperature.

McRoberts, C.A. and Newell, N.D. 1997. Novaculapermia, gen. nov., a transitional myalinid bivalve from the Lower Permain of west Texas. Palaeontology. , v. 40, pp.487-495.

ABSTRACT. Novaculapermia boydi gen. et sp. nov. is a remarkable Lower Permian vertically elongate bivalve that superficially resembles 'razor' clams of the superfamily Solenoidea. Our 'razor' clam possessed a duplivincular ligament and early ontogeny of the Myalinidae. The flattened, equiconvex form suggests that Novaculapermia was a shallow vertical burrower in soft sediments possibly anchored by byssal attachment; evidently it was not a reef dweller, but lived in near-reef environments.

McRoberts, C.A. and Aberhan, M. 1997. Marine diversity and sea-level changes: numerical tests for association using Early Jurassic bivalves. Geologische Rundschau, v. 86, pp. 160-167.

ABSTRACT: Past attempts to substantiate the species-area effect by correlating changes in sea-level and marine diversity have met with limited success. Partial rank correlation and concordance analyses are used as two complementary numerical methods to examine the association between sea-level changes and diversity as predicted by the species-area effect. When applied to Early Jurassic bivalve species from northwestern Europe, the numerical analyses failed to discriminate an association (r = -0.298 for the partial rank correlation and p = 0.45 for the concordance probability). Additional analysis using subsets of the data or recoding periods of anoxic water as periods of reduced habitable area (in addition to marine regression) also failed to show a significant association. The absence of significant correlation is likely to be due to numerous biotic and abiotic factors that cannot directly be measured from sediments or fossil assemblages. A multitude of interrelated cause and effect relationships renders the species-area effect a poor predictor of the influence of sea-level changes on marine diversity.

Return to McRoberts' home page