The sand dollar Periarchus lyelli (Echinoidea: Clypeasteroida: Scutelliformes) in the Caddell Formation (Upper Eocene) of Texas.
The sand dollar echinoid Periarchus lyelli (Conrad) is a distinctive and well-known element of the fauna of Upper Eocene (Jackson) marls and limestones across the eastern Gulf of Mexico and lower Atlantic coastal plain. Hurricane Rita struck the Texas coast in September, 2005, and caused significant erosion on the northern shore of the Sam Rayburn Reservoir and created fresh exposure of sandstones and shales of the Upper Eocene Caddell Formation in the immediate vicinity of the type area. A number of specimens of Periarchus lyelli were collected from these exposures following the storm. This report documents a geographic range extension of the species west from Mississippi to Texas.
STUDY AREA AND METHODS
The Caddell Formation was named by Dumble (1915) after the town of Caddell in southern San Augustine County, Texas (Figure 1). The town site was inundated by the Sam Rayburn Reservoir in the 1960s, although the cemetery remains. The type section was determined by Eargle (1959) and the locality was redescribed by the Gulf Coast Section of Economic Paleontologists and Mineralogists (1966) before inundation by the reservoir. The old town site is near the Harvey Creek Recreation Area at the end of FM2390 about 10 miles (by road) from Broaddus. There are poorly preserved fragments of sand dollar echinoids collected from this area in the Rio Bravo Collection (Molineux 2008) at the Texas Natural Science Center. The original collecting locality for these specimens, as recorded by specimen labels more than century old, was in a southward-facing slope of the valley of the Angelina River at Caddell. Loose cobbles and boulders of indurated sandstone contained fragments of sand dollars. In September, 2005, the eye of Hurricane Rita crossed the area and caused significant erosion on the northern shore of Sam Rayburn Reservoir. This erosion exposed fresh sections of sandstone and shale of the Caddell Formation. Outcrops along the irregular shoreline are now best accessed by boat, and the exposure of the formation extends from the boat ramp at the Harvey Creek Recreation Area to approximately 3 km southeastward, exposing a stratigraphic interval about 35 m thick. The lowest interval of the section is composed of interbedded argillaceous, iron oxide impregnated sandstone and sandy clay, with trace fossils and poorly preserved (moldic) megafauna. This is overlain by a fine-grained unit with calcareous lenses several meters in diameter containing the large oyster Crassostrea gigantissima (Finch). A thin layer of calcareous, very shelly sand less than a meter thick overlies the shale, grading upwards into indurated sandstone which weathers into large concretionary masses. Well-preserved fragments (Fig. 2a) of Periarchus lyelli are found in the thin calcareous unit and relatively whole specimens (Fig. 2b) occur on the weathered surfaces of the upper sandstone. Both occurrences are associated with unidentified nummulitid foraminifera.
[FIGURE 1 OMITTED]
All fossil material collected has been cataloged in the collections of the Nonvertebrate Paleontology Laboratory (NPL) of the Texas Natural Science Center (TNSC), on the Pickle Research Campus of the University of Texas at Austin.
Sand dollar echinoids, members of the order Clypeasteroida, first appeared in the fossil record of the Gulf of Mexico coastal plain in the early part of the Middle Eocene. The oldest known sand dollars from this region are the putative species Protoscutella mississippiensis found near the top of the Tallahatta Formation in western Alabama. Various nominal species of Protoscutella are found in abundance in slightly younger deposits from Texas to North Carolina. Periarchus lyelli first appeared in the Middle Eocene and by the Late Eocene it became the predominant species of sand dollar. Oddly, although the oldest occurrence of P. lyelli is in the Cook Mountain Formation in east Texas, the species was not definitely known from the Upper Eocene Jackson Group in Texas. Fragmentary evidence in the TNSC Rio Bravo Collection was suggestive of the occurrence of the species, but is not definitive.
Periarchus lyelli was first described by Conrad (1834) from (probably) the Moodys Branch Formation below Claiborne Bluff on the Alabama River (Monroe County, Alabama). The species is characterized by significant morphologic variability and widespread geographic distribution. This has resulted in several closely related species being described then variously reduced to synonyms or subspecies of P. lyelli by different authors. The species is especially abundant in the Moodys Branch Formation and equivalents from Mississippi east and north to North Carolina and as far south as central Florida. It has not been found in the Moodys Branch Formation exposures along the Quachita River in central Louisiana (Huner 1939), but Cooke (1942; 1959) reported that fragments of sand dollars attributable to this species were found by T. W. Vaughan in the Moodys Branch Formation exposed at Montgomery Landing (Creole Bluff) on the Red River in Louisiana (now inundated by water dammed behind navigational locks on the river). The species is not listed in the fauna from Montgomery Landing in Schiebout & van den Bold (1986), but Vaughan's specimens (27 small fragments) are in the Smithsonian paleobiology collections, catalog number USNM 164680, and do appear to be attributable to P. lyelli. Clark & Twitchell (1915) reported some occurrences of P. lyelli in Texas, but all of these were misidentifications of Protoscutella mississippiensis. The occurrence of P. lyelli noted by Cooke (1959) is from a Cook Mountain Formation locality in Sabine County, Texas (see Zachos & Molineux 2003). A probable identification of P. lyelli for the material in the Rio Bravo collection was reported by Zachos & Molineux (2003), but the latest collections reported here were taken in situ from the Caddell Formation and confirm the identification.
Better preserved specimens collected from the Caddell Formation show the conical "Chinese hat" profile characteristic of the putative subspecies P. lyelli pileussinensis (Figure 2c). This variety is generally found in Jackson (Bartonian) deposits and is evidence that the Texas occurrence is of equivalent age to the Moodys Branch Formation or younger deposits in the eastern Gulf of Mexico coastal plain.
[FIGURE 2 OMITTED]
Carter et al. (1989) showed that Periarchus lyelli in all its variants preferred a sand substrate. Carter & McKinney (1992) presented evidence that distribution patterns in Upper Eocene echinoid faunas were related to facies patterns, particularly in relation to sand/mud ratios. The extension of the geographic range westward into Texas demonstrates that P. lyelli had a preference for sandy substrate but was tolerant of composition (quartz/glauconite vs. calcite/aragonite) and minor variation in water temperature represented by differences in environmental regimes that included the Atlantic coast of the Carolinas and the carbonate platform of Florida.
Appreciation is extended to A. Molineux, Texas Natural Science Center, Austin for assistance with the collections of the Non-Vertebrate Paleontology Laboratory, and to C. Ciampaglio and T. Yancey for their critical review of the manuscript. This work was supported by the Geology Foundation of the Jackson School of Geosciences at the University of Texas at Austin., and by the Smithsonian Institution Fellowship Program.
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Molineux, A. 2008. The Rio Bravo Collection: Preserving a unique collection for future research in the Gulf Coast section. Gulf Coast Assoc. Geol. Soc. Trans. 58:699-700.
Schiebout, J. A. & W. van den Bold. 1986. Montgomery Landing Site, Marine Eocene (Jackson) of Central Louisiana, Thirty Sixth Annual Meeting of the Gulf Coast Association of Geological Societies, Baton Rouge, vi + 238pp.
Zachos, L. G. & A. Molineux. 2003. Eocene echinoids of Texas. Jour. Paleo. 77:491-508.
Louis G. Zachos
Department of Paleobiology MRC-121 National Museum of Natural History, Smithsonian Institution P.O. Box 37012, Washington, DC 20013-7012
LZ at: firstname.lastname@example.org
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|Author:||Zachos, Louis G.|
|Publication:||The Texas Journal of Science|
|Date:||Aug 1, 2009|
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