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Late Triassic North American continental mollusks--thoughts on distinctive assemblages seemingly worlds apart.

INTRODUCTION-PREMISE

Important questions concern the composition, chronostratigraphy, and origin of continental molluscan assemblages of the Late Triassic, Late Jurassic and mid-to-Late Cretaceous. These assemblages are separated by many millions of years and sometimes great distances. A related question concerns the first appearance of a taxon into these freshwater or terrestrial habitats and a means of showing evolutionary continuity. This preliminary assessment on the subject focuses on the Triassic record of North America. Reported freshwater mussels and gill- and lung-bearing snails of Triassic age have global distribution, but occurrences are widely separated geographically resulting in limited bio- and chronostratigraphic continuity. The North American record consists of mollusks from the American southwest and rift valleys of northeast states.

UNIONOIDS--COLORADO PLATEAU

Good provided a thorough examination of freshwater mussels from the Upper Triassic Chinle, Dockum, and Delores Formations of the southwest United States and noted the great evolutionary significance of documenting the first appearance of undisputed members of the superfamily Unionoidea (1). Good reported the shells of the mussel fauna to be small- to medium-sized, very thick, and lanceolate-shaped indicating high-energy fluvial systems. Good (1) recognized mussel families Hyriidae (e.g., Antediplodon) and Unionidae (e.g., Pleisielliptio?, Uniomerus), and snail families of Cerithioidea--Pleuroceriidae (Lioplacodes) and Ampullaridae (Ampullaria). These families were best documented from late Carnian and Norian (mid Late Triassic). Since Good's study, Rinehart and Lucas (2) identified an early mid-Norian the hyriid Antediplodon from the Chinle Group (3) of Texas. Good recognized ecophyenotypic changes in mussel parameters upsection with shells becoming slightly larger. Unionoid diversity also increased slightly, with less thick shells in the Sonsela Member of the Petrified Forest Formation. These characters indicate low current velocity (1). Only hyriids are present in the Owl Rock Formation, which Good suggested indicated a decline in mussel fluvial habitat quality. Sedimentological evidence for a large lacustrine system (1) would not necessarily be detrimental to mussel survival or reduction in diversity. Declining water quality may have been a factor in mussel survival with an increase in aridity.

UNIONOIDS--EAST COAST RIFT BASINS

Freshwater mussels from Upper Triassic Newark Supergroup of Atlantic coast rift-basin lakes were interpreted to belong to Mulleriidae (was Mycetopodidae; Mycetopoda), Hyriidae (Diplodon), and Unionidae (Anoplophora, Unio) by Pilsbry (4,5). Pilsbry's taxa occur in the Upper Triassic New Oxford Formation of York County, Pennsylvania. Earlier described taxa by Emerson and Troxell are from the Lower Jurassic Portland Formation (Hettangian-Sinemurian; 6) somewhere near Wilbraham, Hampden County, Massachusetts. Bogan et al. (5) described the Norian-age Triaslacus from the Sanford Formation of the Chatham Group, Durham County, North Carolina, and from the Newark Supergroup (undivided), Fauquier County, Virginia. Tentatively assigned to Unionidae, Bogan et al. hypothesized that this taxon and others from the rift lakes were from an extinct radiation and, as such, were not related to the currently recognized six families of the Unionida. Thus they did not represent the origin of any of the modern Unionida families.

FIRST APPEARANCE VERSUS COMMON ANCESTOR

Pilsbry (4,7) referred Triassic freshwater mussels from the Newark Supergroup to modern South American families Mycetopodidae based on similarity of shell shape and Hyriidae based on radial umbo sculpture (5). Remaining taxa were described or moved to the genus Unio and assumed to belong in the family Unionidae. Good (1) suggested the small or "dwarfed" size of basal southwestern unionoids was due to "environmental conditions not optimum for this group." Small shell size is common in modern and fossil unionoids and not here considered dwarfed. The continuity of globally widely spaced Late Triassic freshwater mussels (in particular) requires a global, phylogenetic approach to understanding its early ancestry.

ACKOWLEDGEMENTS

We wish to thank Art Bogan for his review of an earlier version of this manuscript.

1) Good, SC (1998) in Bivalves: An Eon of Evolution, p. 223-249. 2) Rinehart, LF and Lucas, SG (2013) New Mexico Museum of Natural History and Science Bulletin 61, p. 500-523. 3) Lucas, SG, Heckert, AB, and Estep, JW, and Anderson, OR (1997) New Mex. Geolog. Soc. Guidebook, 48th Field Conferences, p.

Connor Lindenberg * and Joseph H. Hartman

Harold Hamm School of Geology and Geological Engineering, University of North Dakota, Grand Forks, ND 58202
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Title Annotation:UNDERGRADUATE COMMUNICATIONS IN THE A. ROGER DENISON COMPETITION
Author:Lindenberg, Connor; Hartman, Joseph H.
Publication:Proceedings of the North Dakota Academy of Science
Geographic Code:1U4ND
Date:Jan 1, 2014
Words:687
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