An isolated squamate dorsal vertebra from the Late Cretaceous Greenhorn Formation of Mitchell County, KansasMichael J. everhart1 and Gail Pearson2

1. Sternberg Museum of Natural History, Fort Hays State University, Hays, Kansas meverhar@fhsu.edu 2. North Central Kansas Technical College, Beloit, Kansas

A procoelous dorsal vertebra collected from the Hartland Shale Member of the Late Cretaceous Greenhorn Formation represents the oldest specimen of a large squamate reptile, possibly a dolichosaur or early mosasauroid, from Kansas. The isolated vertebra was collected near the contact between the Hartland Shale and Jetmore Chalk (late Cenomanian - early Turonian) members of the Greenhorn Formation in Mitchell County, Kansas. A latest Cenomanian age was determined by analysis of the nannofossils contained in matrix recovered from the neural canal of the specimen.

Key words: Dolichosauridae, Mosasauridae, procoelous, Hartland Shale, Jetmore Chalk, Cenomanian, Turonian. marine, Western Interior Seaway

transactions of the Kansas acadeMy of science

Vol. 117, no. 3-4 p. 261-269 (2014)

introduction

Squamates, or scaled reptiles, compose the largest and most diverse order within Reptilia, including all lizards and snakes. Mosasaurs were marine squamates that evolved in the oceans during the last 30 million years of the Late Cretaceous Period of the Mesozoic. The ancestors of mosasaurs were most likely near-shore dwelling squamate lizards, similar to dolichosaurs (Jacobs et al. 2005).

Squamates are distinguished by their scale-covered skins, which are shed periodically, and by the unique joint (mobile quadrates) between their skulls and jaws which allow them to open their mouth wide and consume large prey (e.g. snakes). Their vertebrae are typically procoelous (anteriorly concave and posteriorly convex).

Although these ancestral squamates were relatively small, probably about the size of modern marine iguanas (1.2 – 1.5 m), the mosasaurs increased rapidly in size, becoming fully marine by the Middle Turonian, and 12-15 m leviathans by Early Campanian. Superbly adapted as apex predators in the

oceans of the Late Cretaceous (Massare 1987, 1988; Lindgren et al. 2010; Lindgren, et al. 2011), mosasaurs became extinct at the end of the Mesozoic along with many other groups of terrestrial and marine vertebrates. Lizards and snakes survived and flourished, however, and now comprise the largest order of modern terrestrial vertebrates.

The remains of the ancestral forms of the mosasaurs are extremely rare in marine deposits from the Late Cretaceous. New specimens are being described, but the early lineages of mosasaurs are still laregely unresolved. The smaller, early forms of these marine reptiles were less numerous and may have been preyed upon by other marine reptiles such as pliosaurs (Schumacher 2011; Schumacher et. al 2013), and by large lamniform sharks such as Cretoxyrhina, Cretodus and Cardabiodon (see Everhart 2004 for discussion). These Late Cretaceous shark genera reached sizes comparable to those of modern Great white (6 m) and would likely have been formidable predators on much smaller air-breathing marine vertebrates.

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In addition, the likelihood of finding the remains of these ancestral marine reptiles in the Greenhorn Formation in Kansas is certainly diminished by the lack of extensive exposures (See Schumacher and Everhart 2005 for a further discussion of the “Fort Benton Group”).

In that regard, relatively few remains of other vertebrates have previously been reported from the Greenhorn Formation, and most of those occurred in the basal (upper Cenomanian) Lincoln Member (Williston 1900; Stewart 1898; Liggett et al. 2005; Schumacher and Everhart 2005; Shimada and Martin, 2008; Everhart and Bell 2009; Shimada and Nagrodski 2010; Bell and Everhart 2011.

Nagrodski et. al (2012) reported on marine vertebrates from the Hartland Shale of southeastern Colorado, including two teeth of a squamate lizard, Coniasaurus crassidens (FHSM VP-17539; VP-15740). Most recently, Nelms, et. al (2014; this issue) described fish remains from the Jetmore Chalk Member (Lower Turonian) of the Greenhorn Formation in Washington County. The vertebra reported here is latest Cenomanian in age on the basis of the nannofossil analysis of matrix contained in the neural canal of the vertebra. Watkins (pers. comm., 2014) “identified 46 species including an assemblage of marker taxa that place this sample firmly at or just above the Cenomanian-Turonian boundary.”

Abbreviations: FHSM – Fort Hays State University, Sternberg Museum of Natural History, Hays, Kansas; KUVP – University of Kansas Museum of Natural History, Lawrence, Kansas.

Systematic Paleontology Class Reptilia Subclass Diapsida Infraclass Lepidosauria Order Squamata Oppel, 1811

locality / stratiGraPhy

FHSM VP-17246 was collected at the base of a vertical exposure of the Jetmore Chalk Member of the Greenhorn Formation near Salt Creek in southern Mitchell County, Kansas by the second author about 1975, and retained

Figure 1. Photograph taken in 2008 showing the site in Mitchell County where the FHSM VP-17246 squamate vertebra was collected. The photo includes almost all of the Jetmore Chalk Member (JT-1 to the JT-13 Shellrock limestone bed) and about 2 m of the lower part of the Pfeifer Shale. The watershed lake below the outcropping was constructed after the specimen was collected.

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Figure 2. A generalized stratigraphic column of Cretaceous rocks in Kansas showing the approximate stratigraphic occurrence of coniasaur, mosasauroid and mosasaur specimens prior to the deposition of the Smoky Hill Chalk and Pierre Shale. Mosasaurs are common occurrences in the Smoky Hill Chalk Member of the Niobrara Formation, and in the Pierre Shale. See text for details.

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in his personal collection until 2008 when it was donated. The locality exposes the entire Jetmore Chalk Member and also includes the contact with the overlying Pfeifer Chalk Member (Fig. 1). Detailed locality information is available in the records of the Sternberg Museum of Natural History.

Salt Creek was dammed to create a watershed lake in the years following the collection of the specimen and water currently covers the contact of the Jetmore with the Hartland Shale.. The locality is now at the edge of the lake and relatively inaccessible. At this site, the Jetmore is overlain by about 2 m of the lower Pfeifer Member. An abandoned

Fencepost Limestone quarry (top of the Pfeifer Shale Member of the Greenhorn Formation) is visible further up the slope. The Greenhorn Formation was deposited “on a broad, flat, gently subsiding cratonic shelf along the eastern side of the Western Interior Sea” (Hattin 1975, p. 106). The deposition of the Jetmore Chalk and Pfeifer Shale members occurred as the Greenhorn Transgression approached its maximum expanse during the Early Turonian. Hattin (1975) suggested that the maximum depth was no more that 90 m at the peak of the transgression and that sediment accumulation was slow, and diversity of macro-invertebrates was low, due in part to low oxygen levels on the sea floor.

Figure 3. Micro-photographs of calcareous nannofossils recovered from the matrix preserved within the neural canal of the FHSM VP-17246 specimen. A. Corollithion exiguum B. Percivilia fenestratus C. Microrhabdulus decoratus and D. Quadrum intermedium. Photos provided by D.K. Watkins.

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The Hartland Shale is an olive gray shaly chalk, ranging from 2.6 m in thickness to the north of Mitchell County to nearly 21 m in Ford and Hodgeman counties in western Kansas. Hattin (1975, p. 24) also suggests that it “does not consist of “shale” in the usual sense of the term,” and that the name should be changed to simply the “Hartland Member.”

The Jetmore Member of the Greenhorn Formation rests conformably on the Hartland Shale and is composed of alternating beds of tan chalky limestone and gray shaly chalk. Acccording to Hattin (1975), it has an average thickness of 5.7 m. The ledge-forming Shellrock limestone bed (JT-13; Hattin 1975, fig. 10C), about 0.3 m thick, is the uppermost subunit of the Jetmore (Fig. 1, arrow).

The overlying Pfeifer Member is characterized by layers of chalky limestone and numerous limestone nodules. It varies from 4.6 to 7.9 m in thickness where exposed in Kansas (Hattin 1975). The boundary between the Lower and Middle Turonian has been placed in the upper Pfeifer Shale (Fig.2).

Nannofossil analysis and age – Matrix from the neural canal of the FHSM VP-17246 vertebra was sent to David K. Watkins at the University of Nebraska. His 2014 report is included verbatim here:

“Calcareous nannofossils were examined from a sediment sample extracted from the neural canal of the vertebra. Calcareous nannofossils comprise approximately 80% of the sediment examined, with the remaining sediment composed of fragmented nannofossils and indeterminate carbonate. This abundant nannofossil assemblage is well-preserved, with few specimens so diagenetically altered that species affiliation could not be determined. Approximately 50 species were identified from this sample, including several taxa that are age-diagnostic. The presence of Microrhabdulus decoratus (Deflandre 1959; Fig. 3C) in the absence of Quadrum gartnerii (Prins &

Perch-Nielsen 1977) and Helenea chiastia (Worsley 1971) indicates Upper Cenomanian nannofossil Subzone CC10b of Perch-Nielsen (1985). The co-occurence of Corollithion exiguum (Stradner 1961), Percivilia fenestratus [(Worsley) Wise 1983] and Quadrum intermedium (Varol 1992) (Figs. 3A, 3B and 3D) indicate a placement in the uppermost part of CC10b (Corbett, et al., 2014), indicating that this sample was deposited in the latest Cenomanian, near the Cenomanian-Turonian boundary. This age is consistent with the uppermost Hartland and the lowermost Jetmore Members of the Greenhorn Formation.”

descriPtion

FHSM VP-17246 is a single, moderately crushed dorsal vertebra (Fig, 4), missing the neural spine and portions of both postzygapophyses. As preserved, the total length of the vertebra, including the prezygapophyses is 42 mm. The ventral centrum length is 38 mm; the cotyle is laterally oval in shape, measuring 20 mm in width by 17 mm in height, although some of the flattening may be the result of crushing during preservation. The cotyle measures 15 by 12 mm. Both the condyle and cotyle are slightly inclined anteriorly, again likely the result of crushing. The condyle is inclined at an angle of about 15 degrees, well within the range of basal mosasauroid specimens reported by Bell and VonLoh (1998). Both synapophyses are present, but severely damaged. They provide an approximate width of 36 mm for the vertebra at the anterior end. However, they do not preserve rib articulations.

The ventral surface of the centrum bears a low longitudinal ridge on either side of the midline and two small foramina adjacent to the ridges. Viewed ventrally, the centrum is not constricted immediately posterior to the cotyle, but is moderately constricted ahead of the condyle.

Dorsally, the neural arch is complete and moderately crushed. The base of the missing neural spine is about 2 mm wide anteriorly,

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widens to about 5 mm posteriorly. As in all squamates, the neural arch is solidly fused to the centrum. Matrix was removed from the moderately large, subtriangular neural canal for nannofossil analysis at the University of Nebraska.

In dorsal view, FHSM VP-17246 is triangular, lacking the condylar constriction of the “Y-shaped” Middle Turonian mid-dorsal Mosasaurinae vertebrae (FHSM VP-2218) reported by Polcyn et al. (2008, fig. 3), or the Middle Turonian Blue Hill Shale mosasaur vertebra reported by Shimada (2006, fig. 5; KUVP 6176). The vertebrae is also distinctly different in dorsal view from “T-shaped” trunk vertebrae of the Middle Turonian mosasauroid Dallasaurus turneri (Bell and Polcyn 2005, fig. 3). It is also more than twice the length of the comparable vertebrae reported from Dallasaurus turneri (Bell and Polcyn 2005). This suggests that the Kansas mosasauroid would have been approximately 2 m in overall length.

discussion

In Kansas, the vertebrae and teeth of squamate lizards called coniasaurs have been reported from the basal (Upper Cenomanian) Lincoln Member of the Greenhorn Formation (Liggett et al. 2005), the Middle Turonian Fairport Chalk (KUVP 32453, KUVP 97602 - Everhart and Darnell 2004; MJE pers. obs.), and as high as the Middle Santonian Smoky Hill Chalk (FHSM VP-2219; Shimada and Bell 2006).

A mid-dorsal and a caudal vertebrae from a mosasauroid (FHSM VP-2218 and VP-2217) were collected at different times from the same roadside exposure of the Pfeifer Shale / Fairport Chalk contact (Middle Turonian) in Russell County. Although reported as separate specimens by Polcyn et al. (2008), Schumacher (2011) considers them to have been originally from the same individual.

Figure 4. FHSM VP-17246 squamate dorsal vertebrae in A) dorsal, B) ventral, C) anterior, D) right lateral, E) left lateral, and F) posterior view. Scale bar = 2 cm

Transactions of the Kansas Academy of Science 117(3-4), 2014 267

The fragmentary remains of three relatively large mosasaurs were reported by Martin and Stewart (1977) from the Middle Turonian Fairport Chalk / Blue Hill Shale (jaw fragment - KUVP 25869; two caudal vertebrae (KUVP 27032), and four dorsal vertebrae (KUVP 6176). A mosasaur frontal (KUVP 97200) was figured by Everhart (2005b) from the Fairport Chalk and more fully described in Polcyn et al. (2008). Schumacher (2011) notes that the preservation of this specimen indicates that more likely originated in the Blue Hill Shale.

FHSM VP-17564 is an articulated series of 7 mosasaur caudal vertebrae reported from the middle of the Fairport Chalk by Schumacher (2011). Based on comparisons with other mosasaur remains, Schumacher estimated that the length of this mosasaur was about 3.2 m. The presence of haemal facets on the vertebrae suggest that the individual was either an ancestral Tylosaurus or a Platecarpus.

Everhart (2005a) described the partial vertebral column, a limb element (left coracoid), costal cartilage and ribs of an early Tylosaurus sp. (FHSM VP-2297) from the Early Coniacian Fort Hays Limestone. FHSM VP-2297 is the most complete of any pre-Smoky Hill Chalk mosasaur specimen yet collected in Kansas, and represents an individual that was about 6.7 m in length. Tylosaurus kansasensis Everhart 2005 is the earliest tylosaur species and is first documented from the basal (Late Coniacian) Smoky Hill Chalk. Based on the skull of FHSM VP-13742, Everhart (2005c) estimated that this species reached lengths of about 7 m.

The FHSM VP-17246 vertebrae represents a relatively small squamate lizard compared to the 8-12 m tylosaurs (Tylosaurus proriger) that inhabited the Western Interior Sea during the deposition of the Smoky Hill Chalk. While FHSM VP-17246 occurred in marine sediments, it does not provide data as to whether the animal was fully adapted to life at

sea with paddles instead of feet (hydropedal), or still able to move about on land (plesiopedal) like Dallasaurus (see Bell and Polcyn 2005). However, it does indicate that some squamate species were already becoming much larger than the contemporary coniasaurs.

The fact that the vertebra was collected from sediments laid down far from shore suggests that the squamate reptile was fully marine, although post-mortem transport from a distant coast can not be ruled out. Living in a marine environment with massive 7-8 m pliosaurs and Great white-sized sharks would have been extremely hazardous for such a small marine lizard. conclusion

FHSM VP-17246 represents the earliest known remains of a squamate reptile in Kansas, and raises further questions regarding the origin of later groups of marine reptiles. Although small by comparison to the later occurring mosasaurs, the specimen represents an individual that was already larger than its contemporary squamates, such as dolichosaurs, that have been suggested as possible mosasaur ancestors. It is also larger than a recently described early mosasauroid from Texas (Dallasaurus turneri) that retains plesiopedal limbs. Unfortunately, a single vertebra does not provide much information in terms of describing or identifying the complete animal. Additional specimens are required to better understand the early evolution of squamate lizards in the oceans of the Late Cretaceous.

acKnowledGMents

Comments by two anonymous reviewers improved the content of this paper. Richard Zakrzewski curated the specimen and provided access to other remains in his care. David K. Watkins generously provided nannofossil analysis and dating for matrix collected from the neural canal of the specimen.

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