STORIES FROM THE SEASHORE

Exploring A 59-Million-Year-Old Coast On Wyoming’s High Plains

By Anton F.-J. Wroblewski and Bonnie E. Gulas-Wroblewski

HUMID, SALTY FOG HANGS SOGGY

over a coastal lagoon as the tide retreats towards the inland sea’s depths. Flurries of wading birds shimmer across the freshly-exposed muddy flats, frantically probing for holed-up invertebrate delicacies. Hulking mammals plod with implausible grace across the tidal flats, spooking crocodiles into foot-propelled retreats along the bottoms of freshwater channels. The sticky air is full of buzzing, whirling, and darting insects that pause only long enough to daintily deposit their eggs into the languid freshwater of tidal marshes and creeks. The scene, rife with the pulse of ecological drama and dynamism, is south-central Wyoming, 59 million years ago. These ephemeral moments in time are captured not by bones, teeth, shells, or other body fossils but by more subtle clues imprinted in the rock record.

The modern high plains of south-central Wyoming’s Hanna Basin might appear barren on first impression (Figure 1), but it’s amazing what reveals itself when you just take a look around. Walking across the wind-blown, sagebrush flats northwest of Medicine Bow, you’ll see enormous, white windmills, skittish pronghorns, a variety of songbirds and raptors, snow-capped mountain ranges rising in the distance, and low-lying, rust-colored sandstone ridges streaking across the grasslands. For years, we, and many other paleontologists, have wandered these ridges, searching for evidence of past life in the form of fossilized bones, teeth, shell, and even soft parts like skin impressions. Until recently, we all missed the bigger, more vibrant picture of an ancient world hidden in plain sight. We were seeking remnants of dead things when all around us, preserved over countless eons, there were echoes of living, breathing, active animals preserved in the rock. Only after submerging ourselves in the study of trace fossils (i.e., ichnofossils) were we able to recognize these silent witnesses to brief moments from days long past.

Unlike body fossils that constitute the physical remains of dead organisms, trace fossils record the interaction of living organisms and the substrate. Ichnofossils capture the everyday bustle of a rich ecosystem: animals walking, swimming, digging, nesting, feeding, resting, and even pooping as plants extend roots below the surface in search of stability and nutrients. Unfortunately, trace fossils are not as glamorous as body fossils to most of the public, or even most earth

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» CONTINUED FROM PAGE 14 scientists. After all, when’s the last time a fossil burrow or petrified puke was auctioned off by Christie’s for tens of millions of dollars? While not as visually titillating as a creatively-mounted Tyrannosaurus rex skeleton, trace fossils offer unparalleled glimpses into past ecosystems and behaviors as well as valuable information on the ambient physicochemical environment during the time of their making (Ekdale et al., 1984; Curran, 1985; Buatois and Mángano, 2011). Ichnofossils have been used for decades by the energy industry to refine paleogeographic maps of ancient seaways and landscapes and continue to be studied in both the modern and ancient world. Trace fossils can even set back the earliest known date for the origin of various branchings of the tree of life (Wroblewski and Gulas-Wroblewski, 2022). Despite the invaluable and diverse insights we can glean from the study of ichnology, the field continues to be viewed as something arcane and shrouded in mystery by most paleontologists and sedimentologists. Few universities offer courses in ichnology and, in today’s high-tech, data-science-obsessed world of field geology, even fewer students are taught of this low-tech method’s potential to unravel mysteries that can’t be answered by drones or video game-like outcrop models souped up by AI.

In the dusty outcrops of the Paleocene (63-55 Ma) Hanna Formation, trace fossils revealed an unexpected glimpse into coastal plain and lagoon environments that were previously thought to have disappeared with the dinosaurs (Wroblewski and Gulas-Wroblewski, 2021; Wroblewski and Steel, 2022). A plethora of burrows, trails, tracks, and swim traces paint a vivid picture of a previously unknown, thriving coastal ecosystem. The first hints of this seaside world came in 2016 when on a trip to a friend’s dinosaur-themed wedding in Lusk, we discovered natural casts of sea anemone resting traces during a side visit to the outcrop (Figure 2). Siphon traces left by marine clams accompanied the anemone traces and dimpled the top of a meter-thick sandstone body that had accumulated on carbonaceous shale and coal. These strata represent a marine transgression into a coastal coal swamp, just as we see today in the modern Mississippi River delta when subsidence outstrips sediment supply. The scientific consensus had up to this time been unanimous (a rare instance of agreement in the geological community) that by the late Paleocene, southern Wyoming was nowhere near the sea. Countless papers, articles, and conference presentations repeated the mantra that coal swamps and riverine floodplains dominated the landscape (Blackstone, 1975; Lillegraven and Ostresh, 1990; Flores et al., 1999; Hajek et al., 2012; Dechesne et al., 2020). Although reports of tidal deposits and marine trace fossils in the early Paleocene (approximately 65 million years ago) indicated that there was at least some connection to the Western Interior Sea (aka. the Cannonball Sea), by 6 million years later in the late Paleocene, this region was supposedly a fully terrestrial environment hundreds of miles from the closest shoreline (Wroblewski and Steel, 2023). How then did these irrefutably marine trace fossils fit into the picture?

Three years passed as we pondered this seemingly irreconcilable situation while work and other commitments diverted our main attention to other topics. In the fall of 2019, Anton was lucky enough to find himself travelling between Salt Lake City and Cheyenne, enroute to the annual meeting of the Rocky Mountain Section of AAPG. This provided an excuse for a quick trip to the Hanna Formation to revisit the marine mystery traces and to try to make some sense of them. While walking towards the first outcrop, more anemone traces were immediately visible (Figure 3). Insect burrows were mixed into the assemblage, indicating brackish rather than fully marine water. How had he missed these important fossils in all the years he’d been visiting these rocks? From the summer of 1991 until 2001, Anton had worn through countless hiking boots exploring this area, first while working as an undergraduate field assistant and later on his PhD thesis. Although he’d noticed some of the odd shapes and outlines, he’d never recognized their monumental importance. However, nearly two decades of work in the energy industry provided a golden opportunity to study trace fossils in a kaleidoscope of modern and ancient depositional settings, and now he recognized them for what they were. After taking copious notes in his field notebook and photo-documenting the intriguing finds, Anton began walking towards the next ridge over, stratigraphically below the one he was on. It was here that the largest assemblage of Paleocene mammal tracks anywhere in the world stretched out

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The near-vertical sandstone fin rose up above the badlands below, while the grassy, sagebrush plains rolled into the distance behind. The morning sun struck the surface just right, and some apparently irregular depressions and dents in the bedding plane coalesced into three discrete trails with the unmistakable regularity of footsteps (Figure 4). Excitement and panic set in all at once. There were only three other Paleocene mammal trackways reported in the world at that point. Two short ones, made by raccoon- and bear-sized animals in Alberta, Canada, and longer one made by bear-like pantodonts called Titanoides in the ancient coastal coal swamps of what is now Svalbard, Norway (Lüthje et al., 2010; Wroblewski and Gulas-Wroblewski, 2021). The trackways preserved on the slab of sandstone before him were much longer than any of these previously-described trails though. The individual tracks had four or five blunt toes and broad outlines, bringing to mind those made by Nile hippos.

Over the next three summers, detailed mapping, exploring, and measuring of the sedimentary bodies hosting the trackways brought to light at least five individual track-bearing horizons winding over more than half a mile (1 km) along a northwest-southeast transect. Thousands of tracks are preserved as natural casts, original impressions, and under-tracks (beds of sediment deformed downward by tracks made on younger beds or their upper surfaces). Some of these tracks preserve evidence of the animal’s foot dynamics during the walking cycle, while others demonstrate the enormous weight of the trackmakers (Figure 5). Nowhere on Earth is such a diversity of Paleocene mammal tracks preserved. This seemingly barren site tucked away in the badland-scarred grasslands of south-central Wyoming is truly one of a kind.

FIGURE 3: Anton pointing to natural casts of sea anemone traces found in 2019. Possibly mayfly burrows (Fuersichnus) are also present in this bed.

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The Hanna Formation trackmakers frequented emergent or semi-emergent tidal flats, and their tracks are preserved on the tops of sandstone beds that contain anemone burrow casts on their undersides. These animals were at home in coastal lagoons and tidal flats. Large and small tracks occur alongside each other suggesting adults and juveniles walking side by side and providing a rare glimpse into the possible family dynamics of these long-gone creatures.

In addition to these blunt-toed tracks, prints preserving the marks of heavy claws contribute to parts of the uppermost trackway (Figure 6), confirming the presence of another five-toed trackmaker. The most likely suspect is the bear-like pantodont Titanoides, whose fossil bones have been recovered from deposits of the same age in the Bighorn Basin of Wyoming (Gingerich, 1996). Resembling a modern grizzly bear in size and morphology, with sharp incisors and canines and stocky limbs, Titanoides’s use of wet habitats has previously been demonstrated by its tracks in the coal swamp deposits of Svalbard, Norway (Lüthje et al., 2010). Like modern bears, Titanoides probably roamed far and wide in search of food, opportunistically partaking in the coastal buffet provided by tidal flat residents (clams, fish, Coryphodon, etc.) and all sorts of exotic seafood that washed up with the tides and storms.

Smaller, four-toed tracks are less common than the larger, five-toed specimens and may represent the earliest record of artiodactyls or tapiroids (Wroblewski and Gulas-Wroblewski, 2021). Neither mammalian group is known from skeletal remains earlier than the Eocene, but it has been suggested that both had origins in the Paleocene or even Cretaceous, a deep history supported by these tracks (Halliday et al., 2017; Zurano et al., 2019; Wroblewski and Gulas-Wroblewski, 2021). Without skeletal remains to associate with definitive tracks, the identity of these trackmakers will remain controversial and uncertain. However, their presence is intriguing and serves as a reminder of how little we actually know about the early evolution of modern mammalian lineages and how much remains to be discovered.

The more we looked, the more the ancient seaside ecosystem revealed itself. At least two species of ancient wading birds are represented by tracks and probe feeding traces (Figure 7). Avian probe feeding behavior has been described as far back as the Cretaceous and in the Eocene (Falk et al., 2010; Lockley et al., 2020), but this is the first record of this specialized foraging method from the Paleocene and, therefore, fills in an important gap in the fossil record of bird

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