The first reported ceratopsid dinosaur from eastern North America (Owl Creek Formation, Upper Cretaceous, Mississippi, USA)

Occurrence

The tooth described here (MMNS VP-7969) was collected in loose association with the Upper Cretaceous marine Owl Creek Formation (and other units) in northeast Mississippi (Fig. 1). More precisely, it was found out of context in the active fluviatile lag of a modern stream, albeit probably in close proximity to its presumed stratigraphic origins. The pebbly, fossiliferous stream lag contains Pleistocene terrestrial-alluvial, Paleocene marine, and Cretaceous marine fossil float originating from the channel floor and (to a limited extent) the walls. The Paleocene is represented in the area by the Clayton Formation (Fig. 2), the nearest outcrop (preserving the base of the formation) of which is ∼4.3 km upstream (and up-section) from the tooth collection point. Fossil float originating from the Clayton Formation has been limited to fragments of the Paleocene index gastropod Kapalmerella mortoni (Conrad, 1830). Based on the extent of channel length explored thus far, Quaternary alluvium, slumping, vegetation, and water level conceal the underlying Owl Creek Formation (Upper Cretaceous) rather thoroughly, making direct access to the Owl Creek beds very difficult. Although rarely exposed in the stream, these beds crop out intermittently along the channel length between the base of the Clayton and the tooth recovery point. The tooth was retrieved from the stream float within a few meters of the contact between the Owl Creek Formation and the subjacent Chiwapa Sandstone Member of the Ripley Formation at Mississippi Museum of Natural Science (MMNS) locality MS.73.001b (Fig. 1).

Both the Cretaceous and Paleocene units cropping out in the channel contain marine vertebrate fossils, although vertebrate fossils are considerably more common in the former than in the latter. Cretaceous deposits in the area have previously produced dinosaur fossils, and the Paleocene occasionally contains reworked Upper Cretaceous fossils. Based on observations of several short-lived, partial exposures in the greater vicinity (e.g., MMNS locality MS.73.030), a persistent phosphatic fossil assemblage occurs in the uppermost part of the Owl Creek Formation. This assemblage consists largely of a shell bed of locally common, dark, well-lithified phosphatic mollusk and decapod steinkerns along with less frequently occurring fragments of marine vertebrates—most of which are characteristically Maastrichtian (Fig. 3; Table 1; Baird, 1986; Phillips, Nyborg & Vega, 2014; Martínez-Díaz et al., 2016). The upper Owl Creek steinkern assemblage is conspicuously populated by baculitid and scaphitid ammonites not seen elsewhere in the local Maastrichtian section. These same ammonites are common in the stream float that yielded the ceratopsian tooth. The Chiwapa Sandstone is very fossiliferous, as is the basal Owl Creek Formation. However, the suite of Cretaceous fossils in the float is generally inconsistent with the assemblage contained in either of these intervals. The Chiwapa contains crystalline calcite pseudomorphs of mollusk shells, none of which are scaphitid or baculitid ammonites. Also, the highly lithified Chiwapa Sandstone does not surrender fossils to the stream bed in one piece—shark teeth, bones, and even shells shatter as soon as they begin weathering from the surface of the rocky exposure. Where the ceratopsian tooth was recovered, the basal Owl Creek is exposed and deeply weathered and contains mollusk steinkerns; however, it also lacks the kinds of ammonites consistent with the stream float. Of all the sourceable constituents of the modern stream lag, the ceratopsian tooth is most consistent with the average size, specific gravity, and color of the phosphatic fossils and pebbles that populate the upper part of the Owl Creek Formation.

The Owl Creek Formation

The Owl Creek Formation crops out in portions of several states within the former Mississippi Embayment—Missouri, Illinois, Tennessee, and Mississippi (Fig. 1). Local thickness of the Owl Creek Formation is about 12 m, and it is rich in Maastrichtian neritic marine fossils (Stephenson, 1955; Sohl, 1960; Sohl & Koch, 1983, 1986). The Owl Creek Formation in northeast Mississippi is composed of glauconitic, variably micaceous, fine-grained beds ranging from sandy clay to clayey sand that become increasingly calcareous to the south where the mostly siliciclastic facies of Tippah and Union counties (including MMNS locality MS.73.001b) grade into the bedded marls and “dirty chalk” of the Prairie Bluff Formation (Stephenson & Monroe, 1940; Sohl, 1960). Thus, terrigenous input in this part of the outcrop belt decreases toward the more pelagic waters of the gulfward shelf. The Owl Creek sediments on the opposite side of the embayment in Missouri and at the head of the embayment in Illinois are texturally and compositionally similar. Likewise, the formation becomes decreasingly calcareous, and then entirely terrigenous, moving northward into the head of the embayment and nearer to the McNairy delta system.

In the first grand interpretation of Upper Cretaceous sedimentation in the Mississippi Embayment, the depositional sequence in the embayment proper was revealed to consist of sediments mineralogically derived from the Appalachian Plateaus and Blue Ridge Mountains (Pryor, 1960). In that study, the Owl Creek Formation was described as an inner prodelta facies of the McNairy Delta complex, although deposited on top of, and partially reworked from, the lower Maastrichtian McNairy Formation during the very last Cretaceous marine transgression into the embayment. In a sequence stratigraphic model, the lower contact of the Owl Creek with the McNairy Sand or Chiwapa Member of the Ripley Formation represents a transgressive surface. Subsequent beds in the Owl Creek would thus represent sediments associated with a transgressive systems tract followed by progradational beds of a highstand systems tract (Mancini et al., 1995).

A palynomorph assemblage from the Owl Creek Formation across the embayment in Missouri suggests an inner neritic marine environment with high terrestrial input (Eifert, 2009). Angiosperms (Betulaceae, Juglandaceae, Oleaceae, Fagaceae, and Nyssaceae) dominate the assemblage, followed by palm (Areaceae) and cycads (Cycadaceae). A foraminiferal suite from the same samples indicates a hypersaline marsh, and a low-diversity/low-abundance dinoflagellate assemblage is inconsistent with a highstand systems tract (Mancini et al., 1995; Eifert, 2009).

Taphonomy

The discovery of dinosaur remains in marine environments occurs infrequently and typically consists of isolated elements or, more rarely, larger skeletal portions (e.g., partial limb or vertebral associations) shed from a bloat-and-float carcass (Schäfer, 1972; Schwimmer, 1997). In this scenario, the buoyant carcasses of coastal dinosaurs, particularly those originating in riparian habitats of tide-dominated estuaries and deltas, are carried to sea by seasonal or episodic freshets and tides. Dinosaur remains from more distal shelf deposits, particularly the more complete skeletal associations, may result from transport enhanced by maritime storms, such as tropical cyclones. Dinosaur fossils in marine sediments seem to be more commonly encountered, and possess greater taxonomic diversity, as fragmentary yet identifiable bones and teeth from nearshore lag deposits (Schwimmer, 1997).

In addition to being the first dinosaur tooth documented from the Owl Creek Formation, the ceratopsian tooth is the first terrestrial macrofossil ever reported from this unit—much-studied previously for its marine macroinvertebrate content. Although characteristically rich in neritic fossils, the aforementioned terrigenous microfossils suggest a not too distant shoreline (Eifert, 2009). Thus, the occurrence in the Owl Creek of a dinosaur fossil, although rare, is not entirely unexpected.

Still, the Mississippi tooth is, literally, one of only a handful of North American ceratopsian fossils from a marine context. Compared to other types of dinosaurs, hadrosaur bones and teeth are the most common dinosaur fossils from Campanian and Maastrichtian marine sediments (Schwimmer, 1997). A possible explanation for the scarcity of ceratopsian remains versus that of other dinosaur taxa recovered from marine deposits may lie in habitat preferences. A summary of generalized ceratopsian lithofacies associations suggests an affinity for “lacustrine, alluvial, and coastal plain” habitats, at least among Ceratopsidae (Eberth, 2010). Alluvial wetland ecosystems can be separated into riparian (channel margin) and more distal floodplain habitats—clast size decreasing with increasing distance from the channel. A study of alluvial wetland lithofacies in the upper Maastrichtian Hell Creek Formation documents a greater proportional contribution of Triceratops remains (out of seven dinosaur families) to floodplain (muddy) over fluviatile (sandy) deposits. The hadrosaur Edmontosaurus is found with greater frequency in the latter (Lyson & Longrich, 2011). If rivers are the principal conveyor of bloat-and-float dinosaur carcasses to the marine realm, then a possible preference among coastal plain ceratopsids for habitats outside of riparian zones may explain their paucity in marine sediments.

The tooth described here exhibits mechanical abrasion (see Description) ostensibly due to fluviatile transport since its exhumation. Thus, a relatively uneroded condition is presumed for the specimen prior to burial. Not knowing the exact stratigraphic origin of the specimen, or whether it fell loose from an as yet undiscovered partial dentary or was buried in isolation, precludes any further speculation as to its postmortem journey and exactly when it entered the Owl Creek depositional system. Nonetheless, based on the locality’s close proximity to the eastern side of the Mississippi Embayment at the time as well as its nearshore sedimentological context (Figs. 1 and 4), we consider it most parsimonious that the tooth originated from an animal in that region, rather than a carcass that had floated from the direction of Laramidia.

Age

The Owl Creek Formation lies entirely within the upper Maastrichtian (Fig. 2), according to published ammonite stratigraphy (Larina et al., 2016) and non-cephalopod mollusk assemblage zonation (Sohl & Koch, 1986). Planktonic foraminiferan zonation is consistent with the deposits being at least partly (or mostly) within the upper Maastrichtian (Puckett, 2005), although these are likely less reliable than ammonites or dinoflagellates for identifying that lithostratigraphic interval (Larina et al., 2016). Owl Creek dinocyst composition immediately below the K–Pg boundary on the opposite side of the Mississippi Embayment in Missouri supports a latest Maastrichtian age for the uppermost part of the formation (Oboh-Ikuenobe et al., 2012). Finally, at the head of the embayment in southern Illinois, ⁴⁰K/⁴⁰Ar dating of pelletal glauconite in the uppermost Owl Creek Formation yielded an age of 65.7 ± 1.4 Ma (Reed et al., 1977). As indicated above, the exact placement of the tooth within the Owl Creek is uncertain, but associated fossils suggest that it is from considerably closer to the K–Pg boundary (top) than it is to the base of the unit. According to Matt Garb of Brooklyn College (M. Garb, 2016, personal communication), scaphitid ammonite steinkerns in the fossil float accompanying the ceratopsian tooth are almost entirely dominated by Discoscaphites iris (Conrad, 1858; Figs. 3C and 3E), which equates to the uppermost portion of calcareous nannofossil zone CC 26 of Perch-Nielsen (1985) within the latest Maastrichtian (Fig. 2). Thus, we posit that the ceratopsian tooth described here dates to the late Maastrichtian.

Reworking is always a consideration with condensed, phosphatic pebble beds. To date, suspected anachronistic fossils have not been detected at any interval within the Owl Creek Formation. Considering the exceptional condition of the tooth, and that it was collected from modern stream lag below a small waterfall produced by a resistant calcareous sandstone ledge (Ripley Formation, Chiwapa Member), prior to which it had traveled at least several meters across the irregular surface of the exposed sandstone, reworking from a notably older Cretaceous interval prior to entombment in the Owl Creek sediments is highly unlikely.

Referral to Ceratopsidae

The prominent primary ridge and split root of MMNS VP-7969 definitively distinguish it from teeth belonging to other ornithischian dinosaurs present in North America during the Late Cretaceous, such as hadrosaurs, ankylosaurus, pachycephalosaurs, and basal ornithopods, all of which lack these features. This gross morphology, thus, is most consistent with referral to Ceratopsidae. However, to avoid the hazards of “overidentification,” we here examine the phylogenetic distribution of notable apomorphies in MMNS VP-7969 to arrive at the most conservative identification possible. This is particularly important in light of teeth described for Turanoceratops, a non-ceratopsid ceratopsoid from Uzbekistan that also displays some apomorphies historically recognized only in ceratopsids (Sues & Averianov, 2009; Farke et al., 2009). The subject is further complicated by variation across the tooth row in ceratopsids; teeth at the very mesial or distal end differ from those in the middle in the development of some features (Hatcher, Marsh & Lull, 1907).

Biogeographic and paleogeographic implications

The tooth described here (MMNS VP-7969) represents the first reported occurrence of Ceratopsidae from eastern North America (Appalachia). Previous reports of ceratopsians from Appalachia have been from non-ceratopsid neoceratopsians, including isolated teeth from the Aptian-aged Arundel Formation of Maryland and a potential leptoceratopsid from the Campanian-aged Tar Heel Formation of North Carolina (Chinnery et al., 1998; Chinnery-Allgeier & Kirkland, 2010; Longrich, 2016). The dispersal route of these earlier ceratopsians into Appalachia is uncertain, and the overall evidence supports a lengthy geographic separation of Appalachia from Laramidia during the Late Cretaceous (late Cenomanian to latest Maastrichtian, ∼95–66 Ma, Slattery et al., 2015). Although there is some limited biogeographical evidence for occasional connections between Europe and Appalachia during the Late Cretaceous (summarized in Csiki-Sava et al., 2015), no ceratopsids are known from Europe. So, a European origin for the animal associated with the Mississippi tooth is highly unlikely.

We thus hypothesize that the occurrence of a ceratopsid in Mississippi represents a dispersal event from western North America into eastern North America. Significantly, this is the first time that a representative of this previously Laramidian dinosaur clade has been identified from Appalachia. This provides strong biogeographic evidence for a physical connection between eastern and western North America during the late Maastrichtian (Fig. 4).

Because many regions of the former Western Interior Seaway do not have the relevant strata preserved or accessible, the seaway’s extent during the terminal Maastrichtian has been debated (summarized in Berry, in press; Boyd & Lillegraven, 2011; Slattery et al., 2015 and references therein). For instance, ammonite distribution suggests a marine connection from the Gulf of Mexico northward to South Dakota (but not continuous with marine environments around present-day Greenland) up until the Hoploscaphites nebrascensis biozone during part of the late Maastrichtian (Kennedy et al., 1998). In turn, the shared occurrence of the plant Cissites panduratus between Laramidia and Appalachia during the late Maastrichtian supports a subaerial connection between the two land masses during this time, too (Berry, in press). The ceratopsid tooth in Mississippi provides additional evidence consistent with this scenario.

Eastern dinosaurs

Non-avian dinosaurs from Cretaceous deposits in the eastern US have been well publicized (Weishampel & Young, 1996; Schwimmer, 1997). Although few discoveries are complete enough for comprehensive description and precise taxonomic assignment, recent notable exceptions include a tyrannosauroid and hadrosaurid from Alabama (Carr, Williamson & Schwimmer, 2005; Prieto-Márquez, Erickson & Ebersole, 2016a, 2016b). Cretaceous dinosaur finds from eastern North America are not rare, but they are infrequent. Since Cretaceous dinosaur remains were first reported on the east coast in the 1850s, numerous specimens representing several groups, both ornithischian and theropod, have been reported from Mississippi to New Jersey. Most of this material consists of isolated and often fragmentary elements, like the ceratopsian tooth reported herein. Collectively, however, the scattered discoveries across the Gulf and Atlantic Coastal Plain reveal an eastern North American Cretaceous dinosaur bestiary that included six major dinosaur clades. To date, these include hadrosauroids (Langston, 1960; Prieto-Márquez, Weishampel & Horner, 2006; Prieto-Márquez, Erickson & Ebersole, 2016a), ankylosaurians (Langston, 1960; Weishampel & Young, 1996; Stanford, Weishampel & Deleon, 2011), tyrannosauroids (Baird & Horner, 1979; Schwimmer et al., 1993; Carpenter et al., 1997; Carr, Williamson & Schwimmer, 2005), dromaeosaurids (Kiernan & Schwimmer, 2004), ornithomimids (Baird & Horner, 1979; Carpenter, 1982; Schwimmer et al., 1993), and ceratopsians (Chinnery et al., 1998; Longrich, 2016; this paper).

Mississippi’s published fragmentary dinosaur remains currently encompass only hadrosaurs (Horner, 1979) and indeterminate theropods (Carpenter, 1982), although one association of over two dozen elements of a single juvenile hadrosaur has been described (Kaye & Russell, 1973). One of the unassigned theropod pedal phalanges (Carpenter, 1982) was later identified as Mississippi’s first known ornithomimid (Baird, 1986). In addition to previously described Mississippi material (Carpenter, 1982), MMNS possesses unpublished, largely isolated elements of hadrosaurs (the most commonly encountered), nodosaurs (teeth and fragmentary bones), dromaeosaurids (teeth), and ornithomimids (the second most common dinosaur). Except for the ceratopsian tooth, all MMNS Mississippi dinosaur holdings (most of it unpublished) are derived from upper Santonian through lower Maastrichtian deposits. Dinosaurs have been reported (Ebersole & King, 2011) but are otherwise undescribed from the upper Maastrichtian of the Gulf Coastal Plain. Many more dinosaur discoveries have been encountered and substantiated in the Maastrichtian of the Atlantic Coastal Plain, namely from the Navesink Formation in New Jersey (see reviews by Weishampel & Young, 1996; Gallagher, 1997).