We here describe the GSFC-VP1 specimen and relate the assemblage to previous track reports from the Patuxent Formation12,22. The track-bearing surface has very irregular topography (Figs 2–3). Large tracks up about 10 cm deep helped the senior author identify the “discovery track” in outcrop. By contrast 60–70 small tracks and other bioturbation features display relief of only a few millimeters. Conspicuous features of the surface include many wrinkles and tubercle- or bubble-like topographic features, which give the appearance of a solidified mud or gel. The excavation of the specimen described here as GSFC-VP1, is detailed in Supplementary Information SI 1.

Figure 2 (A) “Discovery track” at time of discovery, (B) track-bearing slab in situ soon after discovery, (C) track bearing slab during excavation and jacketing. (D) The “discovery track” after replication, with small tracks around it. See Fig. 3 for orientation of the slab and Suppl info S1 for details of excavation. All photographs taken and compiled by the authors in Adobe Photoshop SC6. Full size image

Figure 3 Photograph (A) and map (B) of replica GSFC-VP1 of whole track bearing surface. Note key to diversity of track morphotypes, and easily recognizable trackways of small theropods (in red), mammals (in blue) and pterosaurs (in green). The large discovery track, of inferred nodosaurian affinity, is situated beside small tracks also interpreted as nodosaurian. Track numbers corresponding to data are given in Supplementary Tables SI 1 and SI 2. See text for further details. All photographs and original map taken and compiled by the authors in Adobe Photoshop SC6. Full size image

While morphologically diagnostic ichnotaxa are attributed with varying degrees of confidence to trackmakers at higher, ordinal or familial levels, trackmakers are rarely inferred at the genus or species level. Thus tracks may be morphologically described, while their trackmaker attribution remains unknown or ambiguous. Understanding the GSFC-VP1 specimen benefits from previously-published, illustrated reports of isolated tracks from the Patuxent Formation12,22. Conversely however, the small size of previously described specimens, prevented study of continuous trackway segments or the association of different track types on large surfaces. The present study provides trackway information for several morphotypes that was previously unavailable.

93% of the previously identified Patuxent tracks12 had footprint lengths less than 18 cm and originated from isolated, reworked ironstone clasts. These were preserved as natural molds (concave epireliefs), casts (convex hyporeliefs), and sub-horizontal cross sections of abraded ironstone clasts: Stanford et al., (2007, Figs 13, 14 and 3A,B respectively)12. By contrast the GSFC VP1 specimen represents an in situ surface larger than any previously discovered. All tracks are natural impressions (concave epireliefs), and we recognize the first unambiguous Patuxent examples of continuous theropod trackway segments. Overall at least eight different track types are preserved representing dinosaurs, pterosaurs and mammals. These are described below using the taxonomic categories shown in Table 1.

Table 1 Ten general track morphotype categories represented in the Patuxent Formation based on the GSFC- VP1 specimen, and previous finds from the same formation. Full size table

Small theropod tracks and trackways

Four small theropod trackway segments each reveal between four and six small, three-toed, moderately mesaxonic (tridactyl) tracks (Fig. 4). We provisionally label these cf. Grallator isp. Track size, shape and step (gait) are remarkably consistent: e.g., mean footprint length 4.64–5.67 cm, step 8.9–9.8 cm for four trackways (Table S1.1). Despite some curvature in trackway T1, T3 and T4 all are oriented more or less to the west with trackway T3 oriented to the southwest. Step measurements indicate very slow speed progression (0.75–0.80 km/hour: Table S1.1). The similarity in size, shape, step, stride and quality of preservation indicates the passage of similar-sized animals at the same time. The unusual right-side rotation of tracks in trackway T1 suggests an atypical, ‘sidling’ gait, perhaps caused by the animal looking to the right as it walked (Fig. 4A).

Figure 4 (A) Theropod trackway T1 with six footprints (T1.1-T1.6), (B) Theropod trackway T4 with five footprints (T4.1-T4.5). A and B both indicate very short steps. (C,D) Isolated tracks with wide digit divarication. Compare with Fig. 3B and Supplementary Fig. SI 1. Note that all tracks in T1 show rotation of middle digit (III) to right. Illustration compiled by the authors from original tracings and in Adobe Photoshop SC6. Full size image

Sauropod tracks

One large sauropod track (Fig. 3) with five distinctive digit traces is interpreted as a left front (i.e., manus) footprint. The overall sub circular shape is generally diagnostic for sauropods as is the closely bundled, equidimensional five toed (pentadactyl) morphology, with digit I having registered a sharper claw-like trace which contrasts with the blunt traces of digits II-IV. The track length (L) is 16.3 cm and the width (W) is ~25.7 cm. The traces of individual digits are unusually well preserved and indicate a columnar, digitigrade manus with blunt unguals. Such well-defined manus digit traces are rarely preserved, but have been recorded in the case of Lower Cretaceous Brontopodus pentadactylus from Korea27.

Ornithischian track

The “discovery track” (Figs 2–3) is triangular in shape with four short but relatively pointed triangular digits. However, the posterior (heel) region is obscured by a smaller track of uncertain affinity. We infer the discovery track (n1) represents a nodosaurian, with the shorter digit trace (left side in Fig. 3) representing digit I of the right pes. The track is wider (FW ~29.0 cm) than long (FL ~22.0 cm). Nodosaurs have a pentadactyl manus, with the traces of digit I most prominent, and those of digit V least prominent12 whereas the pes is typically tetradactyl, and usually longer than wide: FL > FW, often with longer, more clearly-defined digit traces. Thus, we interpret the discovery track as a distorted hind footprint (pes). Tracks here labelled n2 and n3 respectively consist of a larger elongate 4-toed pes with a much smaller, anteriorly situated, transverse 5-toed manus. The manus-pes size difference indicates a high heteropody index. These likely also represent a nodosaurian or other ornithischian. If this interpretation is accepted, this inferred manus-pes set is the smallest yet attributed to an ankylosaurian, with a pes length and width of only 8.1 and 7.8 cm respectively (L/W = 1.04) and manus length and width of 2.9 and 4.9 cm respectively (L/W 0.59). Consistent with this interpretation several “ankylosaurian” manus tracks only 3.0–4.0 cm long were previously reported from the Patuxent Formation12. The shortest pes digit (right side in Fig. 3B) suggests a left manus pes set. These dimensions are of the same order of magnitude as the pes and manus remains of Propanplosaurus sp.21 (manus width ~3.0 cm) and the sharp distal terminations of the pes digit traces seem to mirror the form of the ungual sheath inferred for this taxon.

Pterosaur Tracks

At least one pterosaur manus track (p1, Fig. SI2.1) and four probable pes tracks (p2–p5, Fig. SI2.1) have been identified (Fig. 5). The manus track is ~12.0 cm long and 4 cm wide, and appears to be associated with a paired trace resembling a beak probe mark (Fig. 5A). The inferred pes tracks vary in length from about 7.0- 17.0 cm, with corresponding widths of ~4.0 and 9.0 cm, and the larger track may also be associated with beak probe marks (Fig. 5B). The size range is consistent with that of the sample of isolated pterosaur track reported previously12.

Figure 5 Pterosaur tracks. (A) Line drawing (right) and photo (left) of manus p1 with adjacent probable beak trace. (B) Large pes track p2 with probable beak trace, (C) two pes tracks p3 and p4. (D) Small pes track p5. All photographs and original tracing taken and compiled by the authors in Adobe Photoshop SC6. Full size image

Mammal Tracks

True mammalian tracks are rare in the Mesozoic, with only one named ichnotaxon (Ameginichnus) named from the Jurassic and three (Schadipes isp., Koreasaltipes isp. and Catocapes isp.) named from the Cretaceous: see discussion. A variety of mammal or mammaliform tracks were registered on the GSFC- VP1 specimen surface. We recognize 26 tracks (Figs 3, 6 and Supplementary Fig. SI 1 and Table SI 2) representing at least three morphotypes (Fig. 7), distinguished on the basis of size and morphology. As mammal tracks, representing eutherians and/or metatherians, are rare in the Mesozoic, there is little precedent for identifying them or assigning taxonomic labels. Mesozoic mammal footprints are rarely preserved in trackways and those that are (Ameginichnus isp., Schadipes isp. and Koreasaltipes isp.) indicate hopping gaits. It is impossible to speculate on the gaits of the trackmakers of isolated tracks. However, the Patuxent sample contains a few examples of paired tracks (here named Sederipes goddardensis) which indicate the right and left hind feet in a sitting position. In addition to a pair of pentadactyl tracks illustrated by Stanford et al. 2007 (Fig. 17A)12, but not discussed in detail, we identify another pair (m1 and m2: Fig. 6A) which indicate this behavioral posture, which is also inferred for the pairs m13 and m14 (Fig. 6B), m16 and m17 (Fig. 6D) and possibly for m18 and m19 (Fig. 6E).

Figure 6 Mammal Tracks registered on GSFC specimen GSFC-VP1. (A) Tracks m1-m4 include m1-m2 holotype of Sederpes goddardensis a pentadactyl left –right pair (m1 and m2). (B) Tracks m13-m15 include m14 with pronounced anterior mud rim. (C) m8 and m9, (D) m16 and m17 probably represent a left right pair, (E) m18 and m19 represent a possible pair, (F) m11, (G) m7, an elongate pentadactyl track, (H) m20 and m22, (I) m25 and m26. (J) m23 large pentadactyl track (photo above) with image of similar track described in 200712. Compare with Fig. 3 and SI Fig. 1. All photographs taken and compiled by the authors in Adobe Photoshop SC6. Full size image

Figure 7 Maryland mammal tracks provisionally assigned to morphotypes (A–C). (A) Sederipes goddardensis holotype (top) preserved as pes pair with similar, smaller pes pair12, (B) Moprhotype B, with elongate heel trace, (C) Morphotype C, pentadactyl track with a large divergent posterior digit (top) and similar smaller track12, (D) Schadipes crypticus 11, and (E) unnamed morphotype from Schadipes type locality38, (F) Koreasaltipes jinjuensis holotype, (G) Catocapes angolanus holotype (#3) and paratype (#58)17, (H) unnamed mammal tracks from Tunisia13. Note all tracks are Cretaceous in age and drawn to same scale. Compare A–D with Fig. 6. All track outlines taken from original tracings and compiled by the authors in Adobe Photoshop SC6. Full size image

Morphotype A

At least three pairs of tracks assigned to Morphotype A have been found in symmetrical left and right configurations (m1 and m2, Fig. 6A; m13 and m14, Fig. 6B). Likewise the pair illustrated by Stanford et al. 2007 Fig. (17A)12 (Fig. 7A) resembles the well preserved pair (track m1 and track m2) which show an almost perfect mirror image morphology with the inner digit (I) shorter than the others (II-V), a diagnostic crown mammalian pattern28. A pronounced sediment rim around track m14 highlights a similar pentadactyl morphology. Based on the pair m1 and m2 and the aforementioned pair12 (Fig. 7A) the tracks are about as wide as long (L/W ratio ~1.0). Track lengths may be exaggerated by forward motion, but in all cases track width is between ~4.0 and ~5.0 cm. The sitting-on-haunches posture appears to be typical of the Patuxent sample, and justifies the naming of a new ichnotaxon (Sederipes goddardensis) based on a previously unreported ichnological register of a posture (behavior) diagnostic of small mammals: see systematic section and Supplementary Information.

Morphotype B

Morphotype B as represented by track m7 (Figs 6F and 7B) is an isolated, elongate, pentadactyl track 3.6 cm long and 2.6 cm wide (L/W ratio 1.38). A number of other tracks (e.g., m3 and m26) appear similar. In such small tracks it may be difficult to discern the shorter digit (I) which may make the track appear tetradctyl (e.g. m11, m19).

Morphotype C

A single, large, five-toed (pentadactyl) track (m23 of Supplementary Fig. SI 1) has a distinctive morphology with a short posterolateral “digit” (digit I), and four equidimensional digits (II-V). It is ~11.4 cm long and ~5.9 cm wide. The morphology of m23 bears a striking resemblance to the smaller (FL ~7.0 cm) isolated track described and illustrated by Stanford et al. (Fig. 16, and Figs 6I and 7C herein)12, except the relative lengths of digits II-V are somewhat different. The smaller Morphotype C track has distinct pad impressions resembling those of the extant musk rat and other modern rodents (SI3, Fig. 3.1A,B). As this morphotype has not previously been reported from the Mesozoic, it would warrant description as a new ichnospecies, if a trackway configuration were found. At first sight, this elongate, narrow heeled tetradactyl track resembles a pterosaur pes. But to date all known pterosaurian pes tracks are tetradactyl, not pentadactyl, which is the typical condition in Cretaceous pterodactyloids.