CONIASAURUS OWEN, 1850 (REPTILIA: SQUAMATA), FROM THE UPPER CRETACEOUS NIOBRARA CHALK OF WESTERN KANSAS

Journal of Paleontology, May 2006 by Shimada, Kenshu, Bell, Gorden L Jr

INTRODUCTION

CONIASAURUS OWEN, 1850 was a small (50-100 cm in length) marine lizard that has been sporadically reported from Late Cretaceous marine deposits of Germany, England, and the United States. These deposits were formed throughout the basins of the Tethys and Western Interior Seas (Diedrich, 1997: Caldwell, 1999; Caldwell and Cooper. 1999; Liggett, 2004). Coniasaurus is thus far known only from incomplete specimens. However, sufficient material exists to show that it possessed four short limbs and an elongated skull, neck, trunk, and tail, and was equipped with a specialized dentition (Caldwell and Cooper, 1999). Caldwell and Cooper (1999) inferred that Coniasaurus likely occupied a niche similar to small sauropterygians.

The age of previously reported Coniasaurus material ranges from the Early Cenomanian to at least the end of the Cenomanian in England, and Middle/Late Cenomanian to Middle Turonian in the United States (Caldwell and Cooper. 1999; Liggett et al., 2005). In this paper, we describe a previously unreported, isolated Coniasaurus vertebra (Fig. 1 ) that is housed in the Sternberg Museum of Natural History at Fort Hays State University (FHSM), Hays, Kansas. The vertebra was recovered from the Santonian portion (ca. 85 Ma) of the Niobrara Chalk of Kansas, and the specimen is noteworthy because it extends the biostratigraphic range of Coniasaurus and adds a new paleoecological component to the Niobrara fauna.

SYSTEMATIC PALEONTOLOGY

Class REPTILIA

Subclass DIAPSIDA

Infraclass LEPIDOSAURIA

Order SQUAMATA Oppel, 1811

Family DOLICHOSAURIDAE Gervais, 1852

Genus CONIASAURUS Owen, 1850

CONIASAURUS sp.

Figure 1

Description.-The vertebra has a procoelous. dorsoventrally compressed centrum that measures approximately 3.5 mm in height, 5.5 mm in width, and 12.5 mm in anteroposterior length. The articular surfaces of the centrum are slightly inclined from the vertical, with the cotyle oriented slightly downward and the condyle slightly upward. There is no significant constriction of the body of the centrum anterior to the condyle. The distal height and the robustness of the preserved left synapophysis indicate that the vertebra is from the middle thoracic region. The plane of zygapophyseal articulation is approximately horizontal. Although the right prezygapophysis and left postzygapophysis are damaged, the estimated maximum width across the prezygapophyses is 10.5 mm, and that across the postzygapophyses is 8 mm. The zygosphene is broken from the anterior end of the neural arch but their location is indicated by a well-formed zygantrum. The neural spine is not well preserved, but the thickest part was apparently the posterior portion. The dorsal portion of the neural spine is broken off and the anterior margin of the base of the spine is missing. However, the anteriormost two-thirds of the damaged edge (indicated by an asterisk in Fig. 1.3) is very thin and inclined posteriorly whereas the posterior third continues to rise and thickens posteriorly. This anteriorly thinned condition is suggestive of the original presence of a broad dorsal notch near the middle of the anteroposterior length of the neural spine. These vertebral features compare favorably to the diagnosis of Coniasaurus given by Caldwell and Cooper (1999. p. 430): "well-developed zygosphenes and zygantra: centrum not constricted anterior to condyle; neural arch of dorsal vertebrae notched in lateral profile."

Material examined.-FHSM VP-2219, an isolated dorsal (=trunk) vertebra from near "Castle Rock," southeastern Gove County, Kansas (Fig. 2: detailed locality information is on file at FHSM).

Occurrence.-The Smoky Hill Chalk Member of the Niobrara Chalk (Fig. 3). At the locality, the exposed rocks range from slightly below Hattin's (1982) lithostratigraphic Marker Unit (MU) 8 to slightly above MU 12. The specimen occurred somewhere between MU 8 and MU K). This stratigraphie range corresponds to Stewart's (1990) biostratigraphic zone of Clioscaphites vennifonnis (Meek and Hayden, 1862) and C. choteauensis Cobban, 1951, and is Middle Santonian in age (86.0-84.8 Ma: Kauffman et al., 1993).

Discussion.-Although monophyly of the family has not been demonstrated at this point. Dolichosauridae includes two genera in addition to Coniasaurus (cf. Haber and Polcyn. 2005): /\phanizocnennts DaI Sasso and Pinna. 1997, from the Cenomanian of Lebanon (DaI Sasso and Pinna. 1997). and Dolichosaiirus Owen, 1850, from the Cenomanian of Germany and England (Diedrich, 1999; Caldwell. 2000). Compared to Dolichosaurus and Coniasaurus, Aphanizocnemiis was a much smaller lizard (ca. 30 cm in length) that had laterally wider neural arches in the dorsal vertebrae. A full comparison between Dolichosaurus and Coniasaunis is not possible because of the limited number of anatomical elements available for each taxon. However, morphological features that are known, including those of the vertebrae, are particularly similar between these two genera (e.g.. see Lee and Caldwell, 2000). In fact. Dolichosaurus may be a junior synonym of Coniasauriis (Caldwell. 2000). Having considered these possibilities, we assign FHSM VP-2219 to Coniasaurus.