The Affinities of the Enigmatic Theropods of the Arundel Clay Facies (Aptian), Potomac Formation, Atlantic Coastal Plain of Maryland

Thomas R. Lipka

Introduction

The Arundel Clay Facies as exposed at the Muirkirk site in northern Prince Georges County, Maryland (Laurel Quadrangle, USGS 1:24,000, 1979) contains a polygeneric assemblage of terrestrial, aquatic and riparian organisms that inhabited a fluviodeltaic depositional environment of back swamps and oxbow lakes during Aptian time (Brenner, 1963; Glaser, 1969; Wolfe and Pakiser, 1971; Doyle and Hickey, 1979, Hickey and Doyle, 1977; Doyle and Robbins, 1977; Robbins, 1991). No articulated dinosaurian remains have ever been found, but loosely associated remains are known. The three major clades of Dinosauria are represented by a limited number of bones and more abundant teeth; Sauropoda, by Astrodon johnstoni, Leidy (formerly Pleurocoelus nanus and P. altus; Ornithischia by Priconodon crassus (Marsh) and possibly Tenontosaurus sp. (Galton and Jensen, 1979; Kranz, 1987; Weishampel, 1990; Weishampel and Young, 1996); a new ornithischian with ceratopsian affinities based on two recently discovered teeth (this volume); Theropoda by "Allosaurus media" Marsh (cf. "Dryptosaurus medius" Gilmore), "Coelurus gracilis" (Marsh) and "Dryptosaurus? potens" Gilmore (cf. "Creosaurus potens" Lull); and ornithomimids by "Ornithomimus affinis" (Lull).

In nearly a decade of work at the Muirkirk site I have assembled a considerable "tooth fauna" representing the aforementioned Dinosauria, of which the theropod component will be discussed. I have informally grouped them according to absolute size into two morphotypes: the smaller (< 20 mm long) coelurosaur-like teeth and the larger carnosaur-like teeth. Both morphotypes exhibit morphology that are often diagnostic to generic and specific levels. Two examples of such taxa are discussed below. Though these are preliminary findings, it will be shown that in the first case, a generic and possible specific identification is possible. In the second case, the teeth may be diagnosable to either familial or generic level.

The establishment of theropod taxa based on tooth morphology alone is still controversial. This applies especially to carnosaur teeth that are considered highly derived (T. Holtz, written comm.). Currie et al (1990) demonstrated that diagnosis to generic or even specific level is possible, using Late Cretaceous Judithian theropod teeth from Canada which also contain abundant shed teeth and disarticulated skeletons. Judithian theropod teeth from Canada which also contain abundant shed teeth and disarticulated skeletons. Currie et al. (1990, p. 107) also pointed out "the potential for stratigraphic determination, extending temporal and geographic ranges and allowing paleoecological statements to be made on the relative diversity and abundance of certain taxa." It is with these goals in mind that I hope to elucidate the taxonomic uncertainties of the Arundel theropod remains. Data gleaned from work derived from contemporaneous strata of the American Western Interior augment this study. I also utilize genera currently known from these western faunas as "proxy taxa" with which to test the hypothesis that the Arundel Clay Dinosauria are a closely related fauna. For example, some of the first Arundel tooth morphotypes (the coelurosaur-like teeth) will be shown to bear such striking similarity with those of Deinonychus antirrhopus known from the Cloverly Formation of Wyoming and Montana (Ostrom, 1969, 1970, 1990) and recently reported from the Antlers Formation of Oklahoma (Brinkman et al., in press), that it is reasonable to conclude that many of these coelurosaur teeth are referable to this taxon. Similarly, the second morphotype containing the large carnosaur-like teeth appears to bear strong similarity to teeth described for the Antlers and Trinity formations allosauroid Acrocanthosaurus atokensis (Stovall and Langston). Given the Aptian-Albian age of these horizons and including reports from the lower Cedar Mountain Formation of possible acrocanthosaur material (Kirkland et al., 1997; Kirkland, 1996; Kirkland and Parish, 1995). it is also reasonable to infer that the Arundel form may be conspecific with the western form.

ABBREVIATIONS

METHODS AND MATERIALS

THE ARUNDEL DEINONYCHUS

FIGURE 1. SEM photograph of USNM 497727. Probable deinonychid lateral tooth exhibiting velociraptorine autapomorphies: laterally compressed, strongly recurved mesial keel, distal keel with large denticulation relative to the mesial denticles. SEM by Randall Nydam, University of Oklahoma, Norman. Scale bar equals 100 um.

A single well-preserved, slightly asymmetrical crown interpreted as a premaxillary tooth was recovered (USNM 497720). It is more incisiform, wider labio-lingually, and has offset carinae. Mesial denticles proceed from the tip of the tooth to roughly one-half the length of the keel and also display a denticle size disparity on the mesial and distal keels but are less pronounced as in the case for deinonychid lateral teeth. This morphology is consistent with that described by Ostrom (1969) and Currie (1995) for Deinonychus premaxillary teeth. Although none of the teeth just described (as well as those awaiting description) can be positively associated with each other as originating from the same animal, the morphological similarities of the Arundel specimens with those of Deinonychus are striking and cannot be ignored. It thus seems likely that the Arundel deinonychid teeth represent an East Coast occurrence of D. antirrhopus.

Skeletal remains in addition to teeth belonging to Deinonychus antirrthopus have recently been reported from the Antlers Formation of southeastern Oklahoma (Brinkman et al., in press). This provides a direct link between the Cloverly and Antlers formations and also reinforces the notion that acrocanthosaurs and deinonychids were wide ranging Aptian contemporaries with a more widespread distribution than previously recognized. Deinonychids are also indicated in other contemporaneous rocks such as the Trinity Group of Texas (e.g., Thurmond, 1974; Winlker et al., 1988; Jacobs, 1995), the Cedar Mountain Formation of Utah (Kirkland and Parrish, 1995; Kirkland, 1996; Kirkland et al., 1997) and Lower Cretaceous strata of Arizona (Thayer and Ratkevich 1996), and these occurrences consist of isolated deinonychid-like teeth. Except for the dromaeosaurid Utahraptor (Kirkland et al., 1993), these specimens have yet to be described in detail so it is not possible to comment on them at this time. Thus the possibility exists for using deinonychids as an index taxon for correlative purposes and biogeographic comparisons. At the very least, my findings may extend the geographic and temporal range of Deinonychus.

In both the Cleverly and Antlers formations, Deinonychus teeth and skeletal elements have been found in close association with remains of the euornithopod dinosaur Tenontosaurus tilletti, providing compelling evidence for an intimate predator-prey relationship (Ostrom, 1969; Forster, 1984; Maxwell and Ostrom, 1995; Maxwell and Witmer (1996). Additionally, Maxwell and Ostrom (1995) noted that deinonychid material was found secondarily in close association with the remains of the nodosaurid Sauropelta, which also suggests that Deinonychus preyed on nodosaurs. Currently, no Arundel deinonychid teeth have been found directly associated with elements of potential prey species. The euornithopod Tenontosaurus sp. is believed to occur in the Arundel Clay, but evidence rests solely on a single partial dentary tooth, USNM 244564 (Galton and Jensen, 1979; Kranz, 1987; Weishampel, 1990; Weishampel and Young, 1996). A new ornithischian tooth morphotype was recently discovered and given the name "Magulodon muirkirkensis" (Kranz, 1997), a taxon that is currently a nomen nudum. A second tooth found by the author is also referable to this dinosaur, which is now believed to possess ceratopsian affinities (see Lipka, Chinnery and Brett-Surman, this volume). The best known ornithischian occurring in the Arundel Clay is the nodosaurid Priconodon crassus (Marsh). Described at the same time as "Allosaurus medius" and "Coelurus gracillis," again based on teeth, no skeletal or dermal armor remains are known for Priconodon. Thus, while extremely meager and possibly circumstantial, the occurrence of Deinonychus prey species in the Arundel together with Deinonychus teeth tends to support the overall hypothesis and indicate conspecificity with the Cloverly and Antlers fauna. In conclusion, many of these small coelurosaur-like teeth from the Arundel Clay are likely referable to Deinonychus antirrhopus.

OTHER COELUROSAUR REMAINS

ARUNDEL ACROCANTHOSAURS

Unlike the confusing situation that exists for the Arundel carnosaurs, new fossils and data continue to shed light on the Arundel's carnosaurian contemporaries, that, as in the case of the deinonychids, may help to elucidate the carnosaurian affinities of the Arundel theropods. Until very recently, the allosauroid Acrocanthosaurus atokensis (Stovall and Langston, 1950) was known only on the basis of two incomplete skeletons, which are the holotype and paratype for this taxon. No teeth were reported with either type specimen. Harris (1998) described a more recently discovered, more complete acrocanthosaur from the Trinity Group of northeast Texas (SMU 74646), which included two nearly complete teeth. A fourth acrocanthosaur skeleton from the Antlers Formation of Oklahoma is now known. Until recently it had been in private hands, and little is known about it except that it is nearly complete and still under study.

The second Arundel theropod tooth morphotype, the carnosaur-like teeth, consist of the original specimens of Marsh and Lull and now include recent additions by myself (USNM 497718, 497722, 497723, 497724, 497725 and 497726) and others. Marsh's type (USNM 4972) reportedly measures 30 mm in length, 15 mm wide and is 7 mm thick labio-lingually (Marsh, 1888). Bibbins (1895, fig. G) and Lull (1911b, pl. xiv, fig. 1) both cite a tooth (GC 3121) then owned by one Charles Coffin (one of the original land owners where Arundel remains were first discovered), that is reportedly twice the size of Marsh's type. Lull (1911b) also reported two other larger crowns under GC 5685, but they have yet to be compared with the new material.

Of these newest additions, only USNM 497722, 497723, 497724 and 497725 are relatively complete exclusive of the roots. USNM 497718 and USNM 497726 are partial crowns representing the apical portion of one tooth and the distal lower one-third to one-half of another much larger tooth. Both mesial and distal denticulation are preserved to varying degrees on the remaining specimens. These newest additions have yet to be directly compared with the original material and are still being studied. Comparisons have been made, however, with data reported by Harris (1998) of SMU 74646 (1-1 and 1-2, both probably lateral teeth), of the Texas Trinity specimen and with a cast of a partial acrocanthosaur lateral tooth, OMNH 51788, from the Antlers Formation, site V708, and the results reported here are based on these comparisons.

Harris (1998) noted features on these two teeth that may prove to be diagnostic for acrocanthosaurs, which are: 1) small size of denticulation with respect to the overall size of the tooth, 2) denticle counts of just under 11 denticles per 5 mm midway down each keel, 3) cartouche-shaped distal denticles with slightly compressed bodies at their bases and denticle axes perpendicular to the axis of the distal keel, 4) distal denticles slightly larger (0.6 mm) than mesial denticles (0.5 mm), 5) mesial denticles are parallelogram-shaped with axes inclined with respect to the mesial keel becoming more inclined toward the tip, 6) denticles decrease in size towards the tip, and 7) "apical denticulation:" contiguous keels with denticles continuing over the tip.

In general, USNM 497725 compares closely with SMU 74646 1-1. The latter specimen measures 84 mm crown length, 19.5 mm labio-lingual width and has a fore-to-aft basal length (FABL of Farlow et al., 1991) of 31 mm, whereas USNM 497725 is slightly smaller at 58 mm in length by 13 mm labio-lingually by ~ 22 mm FABL. Both specimens are slightly recurved, with the Maryland specimen exhibiting an apparent 15 mm long wear facet on its lingual surface. USNM 497725 also lacks the apical portion of the tip (as do all of these new specimens), also probably due to wear precluding the possibility of determining the most telling feature of acrocanthosaur teeth, apical denticulation (feature 7 above). Mesial denticulation also proceeds approximately three-fifths of the length of the tooth from the tip, and the carinae are slightly offset, indicating that the Arundel tooth is possibly a right premaxillary tooth. SMU 74646 is believed to have been a more mesial lateral tooth but is otherwise indeterminate as to position in the maxilla or dentary (Harris, 1998). In most other respects of gross denticle morphology (denticle sizes were not measured but compared visually) and denticle count (~ 12/cm). USNM 497725 appears to exhibit many of the other features discussed above. USNM 497724 (40 mm long by 10 mm labio-lingual by 20 mm FABL), compares closely to OMNH 51788 (25 mm long by 7 mm labio-lingual by 14.5 mm FABL) with the exception that the Antlers specimen is more recurved, and the Arundel morphology shows denticulation along the entire mesial keel, unlike the Antlers tooth. One smaller specimen, USNM 487723 (24.5 mm long, 7 mm labio-lingual, and 10 mm FABL), more closely resembles the Antlers tooth even in terms of breakage pattern of the apical portion of the crown except for an apparent slight offset of the carinae and poorly preserved distal denticulation, which makes an absolute comparison impossible.

Two fragmentary specimens of possible acrocanthosaur affinities are worth mentioning. The first, USNM 497718, is only a 10 mm long apical portion if a larger crown with mesial and distal denticles preserved but lacks the apex of the tip. Otherwise, it seems to compare well with the larger Arundel teeth. USNM 497726 is a ~ 56 mm long fragment of a much larger crown that consists of only the distal part of the distal keel, and it too lacks the apical portion of the tip. With a labio-lingual width of ~ 14.5 mm and an estimated overall length of 80 mm exclusive of the root, and an estimated FABL of > 40 mm, USNM 497726 would have been nearly the same size as OMNH 74646 (1-1)

It thus seems nearly certain that Acrocanthosaurus is indicated by the available tooth material. The few skeletal remains associated with "D. potens" are another matter, and have not been studied by this author. J. Harris (pers. comm.) expresses doubt that the skeletal material is referable to Acrocanthosaurus. If this is so then either of two scenarios is likely: 1) at least two large carnosaurs are indicated by the remains as previously cited, or 2) that the carnosaur teeth may possibly be associated with the skeletal material currently listed under "Dryptosaurus potens." If the latter, then a temporal problem exists as "Dryptosaurus" is a genus of Late Cretaceous (Maastrichtian) age--a roughly 30 million year span! ln that case it would be more parsimonious to retain Lull's "Creosaurus potens" as the preferred name until more material becomes available.

DISCUSSION

A possible taphonomic scenario might involve transport of old, already disarticulated and scavenged carcasses into the Arundel oxbows and back swamps during flood events, along with logs and plant debris which accumulated over time. Occasionally becoming subaerially exposed during periods of drought or low riverine flow, as evidenced by diagenetic siderite (Glaser, 1969), the remains would undergo continued biochemical attack, scavenging and decomposition. With re-burial within these anoxic, acidic bogs, resumed chemical and anaerobic microbial breakdown of whatever skeletal material remaining would destroy all but the largest bone material. Rapid burial probably accounts for the few smaller vertebrate remains preserved.

The abundance of teeth presents an interesting taphonomic problem and is related to teeth being composed of enamel which is harder than bone, thus making them more resistant to chemical breakdown. Ostrom (1970) noted in his review of the original collection of Arundel material that there was evidence of stream abrasion. However, material that I have recovered from the Muirkirk site seems to show little evidence of this. This suggests that if washed in as hypothesized, then the carcasses were derived of the local fauna and deposited locally from a short distance away.

Palynological data establish the Aptian age of the Arundel Clay, and also show that the Arundel Clay is a local comformable facies of the underlying Barremian-age Patuxent "Formation" (Brenner, 1963; Robbins, 1991). The Arundel Clay facies is also important for paleobotanic reasons. Above the Arundel-Potapsoco contact (e.g., Aptian-Albian boundary) is one of the earliest records of definite angiosperm pollen (Brenner, 1963), and, together, the tripartite Potomac Formation records this floral turnover from a gymnosperm-pteridophyte dominated flora to an increasingly angiosperm-dominated flora. From a European biogeographic standpoint, the Arundel fauna is slightly younger than the English Wealden and probably correlates more closely with the Lower Greensand unit. Thus, faunal interchange between the Arundel fauna, the western American faunas and European faunas must have occurred up until Eurasia was completely separated from North America. During the Early Cretaceous, the paleo-Appalachians would have been a more formidable obstacle to direct east-west faunal migration than this range is today because 115 million years of erosion has since taken place. The possibility exists then that the Arundel fauna is the result of a mixture of migration and relict precursors of organisms which antedate the breakup of Pangea. This may help to explain the presence of large allosauroids almost simultaneously occurring on four separate continents of Gondwana and Laurasia. Migration of North American and European taxa may have involved two routes up until the Aptian, dictated by the northern terminus of the Appalachian range. Some taxa may have migrated into the Western Interior, thus avoiding the Appalachians, while others migrated southward along the east coast of the US. Over time they would have eventually been cut off from Eurasia when eustatic sea levels rose, submerging the land bridge(s), and with continued continental drift. With the Appalachian chain blocking direct western migration, minimal mixing via the northern "Appalachian trail" may have occurred, resulting in relative isolation of the Arundel fauna from its western counterparts.

CONCLUSIONS

While progress is being made in terms of fossil recovery, virtually every aspect of the Arundel Clay facies (geologic, paleontologic and stratigraphic) of the Potomac Formation of Maryland requires review and restudy. This paper is a first step in addressing this problem, and subsequent studies are planned. Work continues at the Muirkirk site primarily by surface collection. Currently plans are in the works for screen washing the Arundel Clay in hopes of recovering still more material in the manner outlined in Cifelli et al. (1996). This should improve recovery of smaller, more easily overlooked material as well as increase the chances of recovering the first Arundel mammalian remains, if present.

ACKNOWLEDGMENTS

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