Despite intense interest in and field studies of early Cenozoic mammalian radiation and dispersal patterns, Paleocene mammal trackways are exceedingly rare with only three previously reported worldwide. The early Paleocene Sarjeantipes1 from Alberta, Canada is a medium-sized, five-toed ichnotaxon that superficially resembles prints made by modern raccoons (Procyon lotor). A heavily-eroded 11.2 m long trackway, also from Alberta, Canada, was laid down by large mammals in the late Paleocene. Although the prints are reported to be wide-gauge2, they might represent a parallel pair of narrow-gauge tracks. A narrow-gauged trackway from the late Paleocene of Spitsbergen, Norway is the ichnotype of Thulitheripus svalbardii and has been attributed to large pantodonts traversing a continental coastal mire3.
Here we report a newly discovered, aerially-extensive series of late Tiffanian (58 Ma) mammalian trackways, dispersed across multiple stratigraphic intervals and traceable for 1,032 m along a belt of well-exposed siltstone and very fine-grained sandstone outcrops. Uniquely preserved in brackish-water delta complexes within a restricted marine embayment or lagoon, these trackways attest to the recurrent use of marine habitats by medium- to large-bodied mammals during the late Paleocene. A minimum of two mammalian taxa are identifiable: one associated with relatively large, narrow-gauge, five-toed tracks, and the other with medium-sized, four-toed tracks. This direct evidence of marine habitat utilization by early Paleocene mammals predates the earliest mammalian skeletal remains preserved in marine sediments4 by 9.4–20 million years and highlights the potential for ichnological data to identify previously unknown taxa and their ecological adaptations.
Geological Context
The Paleocene-Eocene (63 − 53 Ma) Hanna Formation in the Hanna Basin (Wyoming, U.S.A.) contains a mosaic of fluvial, lacustrine, and brackish-water paleoenvironments5,6. The track-bearing interval occurs in the lower portion of a newly recognized, 216 m thick, brackish-water member of the formation (Fig. 1). Fossil plants and pollen collected from surrounding deposits indicate a late Paleocene age (upper P5-P6 palynostratigraphic zone: 58 Ma) for the tracksite6. The bulk of the formation below the brackish-water unit consists of siltstone, fine-grained sandstone, and pebble to cobble conglomerate weathering into badlands topography. Crayfish burrows preserved within siltstones demonstrate seasonally low paleo-water tables and the development of well-drained soils7. Transition to the unnamed, brackish-water member is marked by a lithological shift to carbonaceous shales, coals, and prominent, rusty-orange sandstone and siltstone ridges. This unit is further differentiated by the presence of definitively marine ichnofossils, including Thalassinoides, Siphonichnus, Bergaueria, Arenicolites, Gyrochorte, Ophiomorpha, Skolithos, Cylindrichnus, Palaeophycus, and Rhizocorallium, preserved in silty and very fine-grained sandy delta front, tidal flats, and lagoonal deposits.
Hundreds of prints are preserved in at least five discrete horizons within two separate, silty to very fine-grained sandy delta lobes (Fig. 2). The delta complex is 12 m thick, with individual lobes composed of 1–2 m thick packages of coarsening-upward clay, silt, and very fine-grained sand. A 1.5-2 m thick and 113 m wide distributary channel separates the northwestern portion of the trackway from the southeastern. Wavy and lenticular bedding dominate prodelta and delta front successions with current ripples, climbing ripples, and small-scale trough and planar cross-bedding more common towards the proximal delta front. In addition to the mammal footprints, heavily-burrowed, contorted siltstone beds are marked by an abundance of polychaete and bivalve traces, supporting their interpretation as tidal flat environments. The presence of Bergaueria (sea anemone burrows), Rhizocorallium (polychaete burrows), Gyrochorte (polychaete trails), Siphonichnus (marine bivalve burrows), and other marine-derived ichnofossil suites throughout the silty beds is definitive evidence of marine or brackish-water influence during deposition of these strata8,9,10,11,12,13,14,15.
Description Of Tracks And Trackways
Along the 1,032 m transect of track-bearing strata, four footprint morphotypes can be recognized: 1) round, amorphous depressions on bedding surfaces, 2) moderately- to well-preserved, five-toed prints preserved on bedding surfaces, 3) well-preserved, four-toed prints preserved on bedding surfaces, and 4) natural load casts penetrating underlying, heterolithic strata and exposed in cross-sectional view. Siltstone beds contain prints preserved with clearer outlines and toe impressions than those in fine-grained sandstone beds. The lower surface area to volume ratio of individual sand grains compared to silt results in lower grain-to-grain adhesion and a decreased ability for the substrate to retain an imprint, causing prints to be poorly preserved in sandstone compared to those in siltstone beds.
Individual five-toed imprints are mostly poorly-defined, but measure on average between 15–20 cm long and 15–22 cm wide with specimens ranging up to 24.5 cm long and 25 cm wide, approximating the upper size range of the North American Brown Bear (Ursus arctos)16 (Fig. 3). Four larger toes are directed forward, while a smaller, fifth toe is angled nearly 90° to the others. Two of the four clearest bedding plane prints are broader than long (21.5 x 15 cm and 19 x 14 cm), one is equally broad and long (18 x 18 cm), and a fourth appears longer than broad (37 x 15 cm), but exact measurements of the latter are hampered by the incomplete register of the foot along its posterior margin and possible slide-in mark (Fig. 3A, B). A particularly deep track preserves possible evidence of a direct register gait in which the apparently narrow-clawed manus impression is partially or completely obliterated by the pes (Fig. 3F, G). Alternatively, the imprint may be an artifact of the deep penetration and withdrawal of a blunt-toed foot triggering inward collapse of more saturated, deeper sediment, giving the impression of narrower digits towards the bottom of the print.
Clearly distinguishable from the larger, five-toed tracks are prints bearing four distinct, equal-sized digits. One 11 cm-wide x 11.5 cm-long print with four toe imprints exhibits a drag mark representing forward and downward movement of the foot (Fig. 3D, E). A possible partial imprint of another, forward-facing toe is visible but is obscured by the third toe from the left, a pattern consistent with a direct register gait. The print is situated within a trackway consisting of six amorphous prints with a bearing of 94o. A roughly parallel trackway (bearing 97o) of four prints 58 cm to the north of these tracks also includes a four-toed imprint (Fig. 3J, K). The two middle toes in this 13 cm-wide x 12 cm-long track are tightly appressed and initially appear to be a single digit. Both four-toed prints were likely made by the same mammalian taxon, and differences in the spread between the median toes of the prints in each trackway resulted from varying toe placement, probably reflecting differences in substrate moisture.
Where exposed in cross-section, the larger, five-toed prints display characteristics of footprints made by heavy vertebrates in water-saturated, weakly-consolidated sediment20,21,22(Fig. 4). Although poor preservation precludes detailed analysis of print morphology in the cross-sectional examples, evidence for the direction of travel is afforded by the downward rotation of the anterior portion of the foot as the toes press deeper into the substrate during forward propulsion3. Distinct toe imprints are rare in deeply-penetrating footprints and natural casts, but five blunt toe marks can be recognized in a single 17.25 cm long track (Fig. 4A, B).
The stratigraphically-lowest trackway is located to the northwest of the distributary channel (Fig. 2). A broadly lenticular, silty sandstone bed hosts thousands of Bergaueria at its base (Fig. 4H). Polychaete and bivalve burrows (Skolithos and Siphonichnus, respectively) are distributed across the surface of this bed, indicating continuation of marine conditions after deposition of the sediment (Fig. 4I). Mammalian footprints originate in a siltstone horizon 25–30 cm above the base of the bed and penetrate as deep as 39 cm into the underlying silty claystone (Fig. 4D). Tracks are exposed in plan-view as well as in cross-section with several preserved in both views, confirming that surface-only tracks are not simply anomalous depressions.
Southeast of the distributary channel, a continuous, 356 m-long exposure of several trackways exhibits planform (Fig. 5A, B) and cross-sectional imprints that penetrate and deform underlying strata. Four separate horizons are identifiable, each representing flood-delivery of siltstone and very fine-grained sand onto the brackish-water delta front, mouth bars, and tidal flats. The uppermost horizon is traceable for 230 m and contains hundreds of individual prints arranged into more than 20 distinct trackways, although portions are obscured by vegetation cover or damaged by weathering. The majority of prints with discernable toe impressions are located in this horizon, their enhanced preservation attributable to the rheology of the very fine siltstone in which these tracks were imprinted. Evidence of social behavior exists in some of the clearer trackways. For instance, a pair of tracks consisting of a large (average = 17.5 cm wide x 14.8 cm long) and a 70% smaller (average = 12 cm wide x 11 cm long) set of prints, walking in tandem to the southeast (105o bearing) may represent an adult and juvenile or a sexually-dimorphic pair traversing together for approximately 3 m (Fig. 5B).
A well-exposed, 7 m-long, planform trackway composed of amorphous and five-toed morphotypes is located proximal to the northwestern side of the distributary channel and includes parallel tracks of at least three individuals preserved in very fine-grained sandstone (Fig. 5C). This trackway appears to be the highest stratigraphically, but the distributary channel forms a stratigraphic barrier that impedes its direct correlation to any of the trackways to the southeast (Fig. 2).