860


Journal of Paleontology 91(5):859–870


their diagnoses in an effort to better understand their global representation. This study focuses on Gigantopteridium through examination of specimens attributed to Gigantopteridium amer- icanum (White) Koidzumi, Cathaysiopteris yochelsonii Mamay, and a previously unrecognized species similar to Cathaysiopteris whitei (Halle) Koidzumi, and on published descriptions of Gigantopteridium huapingense and Cathaysiopteris whitei.The result is a reevaluation of taxonomic relationships among these species,which leads further to a reevaluation of the biogeographic distribution of gigantopterid genera. After compilation of foliar morphologic features, including second and third order venation structure and margin characters, we suggest that G. americanum, G. yochelsonii,and G. utebaturianum n. sp. comprise a Gigantopteridium morphogroup of one genus with three species, which, to the extent presently understood, is unique to the Euramerican portion of the Pangean equatorial regions.


Geologic setting


All field localities are in North Central Texas, USA (Fig. 1). This area is on the eastern shelf of the Midland basin (Tabor and Montanez, 2004), which was a coastal zone in western equa- torial Pangea during the Cisuralian (early Permian). The ter- restrial sediments derived from the Ouachita foldbelt and Muenster highlands to the present day north of the study area. Lithology for the study area transitions from coal-bearing strata, including abundant mudstones, sandstones, and limestones, deposited during the Virgilian (Gzhelian), Late Pennsylvanian into primarily mudstone with deposits of fluvial sandstone bodies and marine limestones deposited during the early Permian. Terrestrial depositional environments found through- out the study area vary among field sites and range from sand-/ gravel-rich channel bars to pointbar and floodplain deposits. Most of the plant-bearing beds were deposited in floodplain lakes, representing abandoned segments of channels, in wetlands developed within such settings, or in waning-flow portions of bars in active-channel settings. The earliest confirmed gigantopterid fossils in the North


Central Texas stratigraphic section are Gigantopteridium and occur immediately above the Elm Creek Limestone, at the base of the Petrolia Formation (Hentz and Brown, 1987). Species of Gigantopteridium, the focus of this paper, have not been found, to date, above the Clear Fork Group. This stratigraphic interval encompasses the late Wolfcampian through the early Leonardian, in American regional terminology, of early Permian age. In international stratigraphic terms, this is the late Artinskian through the early part of the Kungurian, based on marine microfossils extracted from limestones that are part of the regional strata (Wardlaw, 2005). According to the International Union of Geological Sciences 2016 Chronos- tratigraphic Chart, the Artinskian-Kungurian boundary is at ~283.5±0.6 Ma.


Nomenclature


The species Gigantopteris americanum was initially described from North America by White (1912) on the basis of specimens collected from the Wichita Formation of North Central Texas in


1909 and 1910. This species was later transferred by Koidzumi (1934) to a new genus, Gigantopteridium. Gigantopteridium has been reported from the early Permian of the southwestern United States as well as the Guadalupian (cf. middle Permian) of southern China, each composed of a single species, G. americanum andG. huapingense, respectively (White, 1912; Shen, 1995). The type species of Cathaysiopteris, C. whitei, was chosen


by Koidzumi (1934) when he erected the genus. It originally had been described by Halle (1927) from the middle Permian of the Shihhotse Series of central China as a species of Gigantopteris.Aswith Gigantopteridium, Cathaysiopteris has been recognized in both the southwestern United States and Chinese floras of Permian age, by a single, morphologically distinct, species in each geographic area. The sole North American species, C. yochelsonii, was first noted by Read and Mamay (1964) as a distinctive form and later described by Mamay (1986). The extent to which the North American species of the


genera Gigantopteridium and Cathaysiopteris overlap with each other and possibly with other, related genera has been con- sidered in several publications (Mamay, 1986, 1988; DiMichele et al., 2011), but a formal reclassification of this group of mor- phologically similar species has not been undertaken until now. For this article, we transfer the North American Cathaysiopteris species,C. yochelsonii,intoGigantopteridiumand recognize three species of Gigantopteridium:G. americanum,G. yochelsonii,and G. utebaturianum n. sp.


Materials and methods


Morphologic descriptions are based on total of 188 specimens, both partial (>50% of entire specimen; ~76 specimens) and fragmentary (<50% of entire specimen; ~112 specimens). Of these specimens, 146 were determined to be Gigantopteridium americanum; 16 were determined to be Gigantopteridium yochelsonii n. comb.; and 26 were assigned to Gigantopter- idium utebaturianum n. sp. As material allowed, each specimen was characterized for gross architecture, leaf venation pattern, and vein density using macromorphology via the naked eye and low-power lenses, and micromorphology via light microscopy. All material used for data collection was examined from original specimens through primary observation.


Repositories and institutional abbreviations.—Paleobotanical specimens analyzed as part of this study were collected during the years 1909–2013 by geologists of the US Geological Survey, USA (USGS) and US National Museum of Natural History, Washington, DC, USA (USNM) and are housed at the collections of the Paleobiology Department of the USNM. All type and illustrated specimens are housed in the USNM fossil plant type and illustrated collection. Additional institutional abbreviations used in this paper include Florida Museum of Natural History, Florida, USA (FLMNH) and Swedish Museum of Natural History, Stockholm, Sweden (NRM). Note: collec- tions described here as formerly held by the USGS have been transferred to the USNM.


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