A NEW OCCURRENCE OF THE "HYDROZOAN" RADIOTRABECULOPORA RETICULATA FAN, RIGBY, AND ZHANG, 1991 IN THE PERMIAN OF CALIFORNIA

Journal of Paleontology, Mar 2004 by Rigby, J Keith, Linder, Gloria A, Stevens, Calvin H

INTRODUCTION

EXAMPLES OF the "hydrozoan" Radiotrabeculopora reticulata Fan, Rigby, and Zhang, 1991 have been recently recovered from Lower Permian rocks in the Inyo Mountains, Owens Valley area, of east-central California by G. Linder and C. Stevens. That genus and species were originally described from the Middle Permian (Leonardian-Lower Guadalupian) Maokou Formation from western Hubei, China, by Fan et al. (1991). This the first report of the genus and species from North America.

The fossils were collected from Unit 7 of the Owens Valley Group, U.S. Geological Survey Map I-1932 (Stone et al, 1989). This unit represents a carbonate buildup composed of wackestones, packstones, and grainstones that formed on an uplift composed of an Upper Carboniferous-lowermost Permian deepwater limestone sequence. Basinward, this unit lenses into rocks that accumulated in deeper water. Fossils from Unit 7, in order of abundance, include pelmatozoan parts, epimastoporid and phylloid algae, and fusulinids. Tubiphytes Maslov, 1956, bryozoans, and the "hydrozoans" are much less common, and brachiopods and corals are rare. Algae are abundant throughout the section, becoming dominant in the upper part, where they are associated with the "hydrozoans."

The thickness of the buildup from which the "hydrozoans" were collected ranges up to at least 180 m, the maximum thickness observed in the general area. Specifically the "hydrozoans" were collected from the central part of the westernmost outcrop of Unit 7, near its contact with alluvium, at an elevation of about 2,160 m (7,100 feet), north of Conglomerate Mesa, in the Ubehebe Peak 15' Quadrangle, Inyo County, California (Fig. 1). Fusulinids collected from the same locality include only Chalaroschwagerina Skinner and Wilde, 1965 (Fig. 2.5-2.8), indicating a late Wolfcampian age for the occurrence.

SYSTEMATIC PALEONTOLOGY Class? HYDROZOA Owen, 1843

An extensive fauna of Middle and Upper Permian "hydrozoans" from South China was described by Fan, Rigby, and Zhang (1991). They noted that classification of fossil hydrozoans remains controversial, and observed that at present it is nearly impossible to document whether the fossils are truly hydrozoans, calcareous sponges, sclerosponges, or algae, particularly if the fossils have experienced much diagenesis. In response to that uncertainty they chose to term the fossils they described as "hydrozoans" or, in formal terminology, as included in the Class? Hydrozoa Owen, 1843. We have chosen to follow the same terminology here.

Order UNCERTAIN

Family DISJECTOPORIDAE Tornquist, 1901

Genus RADIOTRABECULOPORA Fan, Rigby, and Zhang, 1991

Type species.-Radiotrabeculopora xiangboensis Fan, Rigby, and Zhang, 1991.

Discussion.-The genera Tubulispongia Wu (1991, p. 35), Flabellisclera Wu (1991, p. 36), Gigantosclem Wu (1991, p. 38), and Fungispongia Wu (1991, p. 39), described from Permian reefs at Xiangbo, in northwestern Guangxi, China, are here considered as synonyms of Radiotrabeculopora. They all have the same general skeletal structure as the earlier published Radiotrabeculopora, with coenostea composed of many trabeculae, of various widths, which are separated by skeletal openings that have a moderate range of diameters.

RADIOTRABECULOPORA RETICULATA Fan, Rigby, and Zhang, 1991

Fig. 2.1-2.4

Radiotrabeculopora reticulata FAN, RIGBY, AND ZHANG, 1991, p. 60-62, fig. 15.1-15.5.

Description.-Two large fragments are in the collection, both embedded in matrix, but on Sample 11, the weathered surface of a diagonal section shows the structure and form of the species moderately well. It is a cylindrical form with an originally open central cavity and moderately thick walls. It is at least 11 cm tall and approximately 5 cm in diameter, with an original somewhat irregular central, subcylindrical, cavity that is now matrix-filled and approximately 2 cm across in the lower part, but which decreases and becomes circular and approximately 8 mm across in cross section at the preserved upper end of the fragment. The outer surface is slightly irregular, with nodes or ridges up to 1 cm high or wide.

The other specimen, Sample 16, is also irregularly cylindrical and approximately 12 cm tall and 7 cm in diameter. The original subcylindrical central opening, which is now matrix-filled, is approximately 3 cm in diameter. That opening is surrounded by moderately uniform walls that appear to be approximately 13-15 mm thick, where a near vertical section is exposed.

Two thin sections of that specimen, both of which are at angles to principal axes of the fossil, show the internal skeletal and canal structure of the fossil moderately well. Thin section SJS-1307 #16H is at approximately 30 degrees from horizontal through the specimen. It shows prominent radial and concentric trabeculae that unite in three dimensions to produce a rectangular or quadrate skeletal framework. Radial trabeculae are spaced approximately 0.5 mm apart and new ones are inserted to keep the spacing uniform. Concentric "horizontal" trabecular layers are spaced approximately 1 mm apart. Individual trabecular segments are thin between junctions, 0.2-0.3 mm in diameter, but swell dramatically to 0.4-0.5 mm across at junctions. Trabeculae surround elliptical openings 0.2-0.4 mm wide, concentrically, and 0.6-0.8 mm high, radially. These openings interconnect in three dimensions so the skeletal structure is very porous.