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Article

Giant Bird Tracks (Family Gastornithidae) from the Paleogene Chuckanut Formation, Northwest Washington, USA, with a Review of Gastornis Distribution

by
George E. Mustoe
Geology Department, Western Washington University, Bellingham, WA 98225, USA
Foss. Stud. 2025, 3(1), 4; https://doi.org/10.3390/fossils3010004
Submission received: 27 December 2024 / Revised: 29 January 2025 / Accepted: 18 February 2025 / Published: 27 February 2025
(This article belongs to the Special Issue Advances in Palaeontology—Feature Papers to Celebrate the Inaugural Issue of Fossil Studies)
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Abstract

:
Giant Paleogene groundbirds named Gastornis have long been known from Europe, with similar fossils from North America being placed in the genus Diatryma. A more recent discovery in China is evidence that these birds had wide geographic distribution. The name Gastornis is now generally considered to be the name that has historical precedence. Historically, Gastornis has been interpreted as being a fierce predator, but anatomical and isotopic evidence suggests that the giant birds were herbivores. Gastornithid tracks preserved in Lower Eocene fluvial sediments of the Chuckanut Formation in northwest Washington State, USA, support the herbivore interpretation. These tridactyl footprints preserve broad triangular toenails rather than talons. The Chuckanut Formation gastornithid tracks have been given the ichnotaxonomic name Rivavipes giganteus Mustoe et al. (2012). In 2024, two important new discoveries were made. These are a trackway that preserves three adult tracks, and two tracks left by a gastornithid chick.The adult bird trackway has stride and pace distances that are consistent with the short lower limb bones (tarsometatarsals) observed in Gastornis skeletal remains. The reproductive strategies of gastornithids remain enigmatic; the evidence consists of numerous egg shell fragments found at sites in France and the newly discovered Chuckanut tracks.

1. Introduction

On 9 January 2009, heavy rain triggered a huge landslide that swept down the southeast slope of a high ridge in the Mount Baker foothills in western Whatcom County, Washington, USA. Informally known as the Racehorse Creek Landslide [1], the massive slope failure exposed a multitude of blocks of Lower Eocene sedimentary rock containing abundant fossils. These included foliage impressions, invertebrate trace fossils, and a diverse variety of bird and animal tracks. These included about 20 spectacular tridactyl footprints left by the giant groundbird Gastornis (=Diatryma). These tracks were described by Mustoe et al. [2], who gave them the ichnotaxonomic name Rivavipes giganteus.

1.1. Landslide History

The Racehorse Creek area has a long history of landslide activity, including a gigantic prehistoric slide (Figure 1 and Figure 2) [3,4]. Fossil occurrences are found in situ in bedrock and in material transported as colluviums. Because of forest and soil cover, fossil discoveries are mostly limited to fresh exposures produced by landslides, stream erosion, and the construction of logging roads. Rapid weathering rates result in the degradation of most fossil sites, and the 2009 Racehorse Creek Landslide provided an outstanding opportunity for paleontology research. The lower terminus of the slide debris was a short distance upstream from Racehorse Falls (Figure 3); the torrent water-laden debris scoured the downstream channel, revealing the gastornithid trackway in the streamside bedrock, and excavating and transporting the sandstone block that preserves two juvenile tracks.

1.2. Gastornithid Footprints at Racehorse Creek Landslide

The first giant bird footprint was discovered by Keith Kemplin on 22 May 2009, on a day when a Western Washington University team of geologists was conducting preliminary investigations of the landslide (Figure 4 and Figure 5). In 2010, the track-bearing slab was transported offsite by a helicopter under contract with the WWU Geology Department, where it is now a featured display (Figure 6). This specimen (WWU TR-66) is the holotype for Rivavipes giganteus Mustoe et al. [2]. Several other tracks are shown in Figure 7.

1.3. Gastornithid Tracks from Racehorse Creek

On 12 December 2022, three R. giganteus footprints were discovered on the surface of a sandstone bed exposed on the banks of Racehorse Creek just downstream from Racehorse Falls (Figure 8). These footprints provide the first stride and pace data for a gastornithid. In March 2024, a sandstone block containing two tracks of a gastornithid chick was discovered about 100 m downstream from the outcrop containing the adult footprints (Figure 9). Because the 30 kg block was not found in situ, the exact stratigraphic position could not be determined.
The rocks bordering Racehorse Creek are topographically separated from the original Racehorse Landslide locality (Figure 1 and Figure 2), but the inclination of the strata suggests that the track sites may be approximately age-correlative (Figure 2). The goals of this report are to describe the new track discoveries, compare them to the earlier Chuckanut specimens, and discuss these fossils in the context of previous research.

1.4. Previous Gastornithid Track Reports

In 1859, large tridactyl bird footprints discovered in Late Eocene gypsum beds of the Paris Basin were attributed to Gastornis. Unfortunately, the tracks were not illustrated or described in detail, and the identity of the track-maker remains enigmatic [8]. Ichnologic uncertainty was generated by the controversy surrounding the discovery in 1992 of an alleged Diatryma footprint in Eocene strata in the Green River gorge in King County, Washington. A detailed report by Andors [9] concluded that the imprint is not a trace fossil. In contrast, the authors of a subsequent publication [10] asserted that the imprint is a probable Diatryma track, and it was given the ichnotaxonomic name of Ornithoformipes controversus. The latter investigation was based on the examination of a resin cast of the specimen, which did not preserve lithological characteristics. The original specimen is presently on exhibit at the Geology Department, Western Washington University, Bellingham, WA, USA. The re-examination of the specimen [2] supported the possibility that the imprint is a pseudofossil.

2. Geologic Setting

The Chuckanut Formation comprises beds of Paleogene conglomerate, arkosic sandstone, siltstone, and coal that unconformably overlie Paleozoic and Mesozoic metamorphic basement rocks. Chuckanut Formation sediments were deposited on a broad floodplain that existed prior to the Late Tertiary uplift of the North Cascade Range, during a time when meandering rivers flowed westward from source areas near the present Washington/Idaho border (Figure 10) [11,12,13]. The main outcrop belt occurs in western Whatcom and Skagit Counties, but isolated exposures exist along fault zones that connect the main outcrop belt on the west side of the Cascade Range with the Swauk Formation in central Washington. Correlative strata also extend north into British Columbia, where they are called the Huntingdon Formation. This dispersed outcrop pattern is evidence of a large Paleogene basin that was dissected by strike-slip faulting [14]. The track fossils described in this report were all collected from beds exposed in the Mount Baker foothills west of Bellingham, Washington, where gently dipping strata reveal bedding plane surfaces favorable for the detection of footprints.

2.1. Age and Stratigraphy

Extensive soil and forest cover restricts the measurement of continuous stratigraphic sections, but the Chuckanut Formation is one of the thickest non-marine sedimentary formations in North America. Estimates of total thickness of the formation in the main outcrop belt range from 3000 m [15] to 8300 m [14]. Mapping by Breedlovestrout [16] suggest a minimum thickness of 4000–5000 m.
The age range of the Chuckanut Formation ranges from the Late Paleocene to the Early Eocene, based on radiometric ages from ash interbeds [16]. Several schemes have been proposed for dividing the formation into stratigraphic members [5,14,16]. The basic architecture is a three-component system composed of the Late Paleocene–Early Eocene Bellingham Bay Member, overlain in the Mount Baker foothills by the Early Eocene Slide Member, the unit that hosts all known fossil footprints. The third component is the Middle Eocene– to possibly LateEocene Padden Member, which is perhaps separated from the older members by an unconformity. The stratigraphic position of several minor members remains uncertain (Figure 11).

2.2. Chuckanut Formation Paleontology

The Chuckanut Formation preserves abundant plant fossils. These consist of two principal components: leaf remains preserved in carbonaceous overbank deposits, and sandstone casts of driftwood that occur in ancient sandbars. The study of Chuckanut Formation fossils dates to 1841, when James Dwight Dana collected specimens during the coastal investigations conducted by the Wilkes Exploring Expedition. Preliminary descriptions of fossil plants [18,19,20,21] were followed by more detailed studies [14,16,22,23].
The region’s warm humid Paleogene climate is evidenced by the abundance of palm frond imprints and tree fern fronds (Figure 12). The only known vertebrate body fossils are the carapace of a trionychid turtle [24] and the partial skeleton of a fish, genus Phareodus [17]. Invertebrate trace fossils are abundant in Chuckanut Formation strata, including the Racehorse Creek locations [25].
Quantitative paleoclimatic parameters based on angiosperm leaf characteristics are indicative of humid subtropical conditions [14,27]. These estimates are evidence that the fossil footprints preserved in the Mount Baker foothills were made by creatures that inhabited the northwest Washington lowlands during the warmest part of the Cenozoic Era (Figure 13).

3. Materials and Methods

Research studies began soon after the 2009 landslide event, including investigations of the slide mechanism [1] and intensive paleontology research that continues to the present. Studies of Chuckanut Formation fossil footprints from other sites in the Mount Baker foothills [14,30,31,32] provided a regional perspective. At these localities, tracks were photographed in situ, with tracings made on clear vinyl sheets to record geometry. Three Rivavipes track slabs were collected by backpacking; the best example was transported by helicopter to a nearby road, where it was transported by truck to Western Washington University for public display. For slabs that were too large to transport, replicas were made using silicone or latex molds. Since 2009, several Rivavipes tracks have been obliterated by erosion, one was destroyed by an inept extraction attempt by an unauthorized amateur, and several footprint slabs remain at the site. Specimens, molds, and related materials are archived at the Western Washington University Geology Department in Bellingham, WA, USA.

4. Results

Multiple R. giganteus tracks provide an abundance of morphometric data. With the exception of the newly discovered juvenile tracks, the footprints are generally similar in size. Interdigital angles are variable, even among two or more tracks made by a single individual. These variations suggest that the toes were sufficiently flexible to be able to adjust to substrate conditions or variations in the direction of travel. The Racehorse Creek Landslide tracks are typically preserved where a thin fine-grained sediment layer overlays a well-sorted sandstone bed. This sedimentology allows tracks of shorebirds and small mammals to be preserved on the same bedding plane that contains the footprints of the giant groundbird. The outlines of the Racehorse Creek track appear to be evidence of very different sediment conditions. These broad, rather indistinct tridactyl footprints may be under-tracks, where erosion has removed an upper layer of sediment. Alternately, they may be diffuse footprints created when the giant bird was walking on firm wet sand, without a finer-grained surface layer suitable for accurately preserving footprint morphology.

4.1. Morphometric Data

Measurements of Chuckanut Formation Rivavipes giganteus footprints are shown in Figure 14, with numerical data listed in Table 1. The geometry of the Racehorse Creek trackways is shown in Figure 15.

4.2. Locomotion Evidence from the Racehorse Creek Trackway

The speed of travel is commonly a function of stride rate and stride length, where the stride rate is the number of strides taken during a given time interval, and stride length is the distance covered in each step. The walking or running rates of birds are different depending on their lifestyle. Locomotion kinematics investigations have commonly been carried out using birds or animals that were photographed while they are walking on a treadmill [33].
Cursorial birds achieve running speed by increasing their stride rate; graviportal (non-running) birds walk faster by increasing their stride length [34]. These kinematics tend to be applicable regardless of the size of the bird. For example, large cursorial birds like ostrich (Struthio camelus), emu (Dromaius novaehollandae), and rhea (Rhea americana) rely on stride frequency changes to achieve greater running speed [34].
Analytical data from extant species are of limited use for interpreting the kinematics of fossil footprints, because these trace fossils provide no record of the stride rate. However, because the stride rate typically has a high correlation with leg length, anatomical information may be determined from skeletons of the track-maker. For Gastornis, the short length of the metatarsal bones suggests that these giant birds were incapable of fast locomotion (Figure 16) [35,36,37,38,39]. Angst et al. [40] proposed that the locomotion characteristics of groundbirds can be calculated based on the length-to-width ratio of the tarsometatarsus.
A related line of ichnologic evidence comes from the relatively short stride lengths measured for the Racehorse Creek tracks. During bipedal walking, forward motion occurs when the creature’s body vaults forward over the legs during each step, a kinematic condition known as inverted pendulum walking. The slowest walking speeds are only achievable with small pace lengths and low step frequencies [41].
All R. giganteus tracks found in the Chuckanut Formation are plantigrade, a morphology that is characteristic of a slow walking speed. This pattern is evidence of a locomotion style where, at all times, one foot is in full contact with the substrate. In contrast, for bipedal and quadrupedal track-makers, footprints made during running are likely to be digitigrade, and there are times when neither foot has ground contact. This phenomenon was first documented for horses in 1878 by Eadweard Muybridge [42].
For quadruped trackways, the distance between right and left tracks provides an indication of body width. For bipeds, trackways tend to be narrow because the track-maker was walking erect, placing one foot in front of another [43].
Fossstud 03 00004 g016
Figure 16. Gastornis anatomy. (A) Skeleton of Diatryma gigantea from Wyoming, USA, as reconstructed by Matthew and Granger [44], redrawn by Buffetaut and Angst [39] to show shorter lengths for lower limb bones (tarsometatarsi). (B) Foot bones of the Wyoming specimen, AMNH 60169, American Museum of Natural History, New York, USA.
Figure 16. Gastornis anatomy. (A) Skeleton of Diatryma gigantea from Wyoming, USA, as reconstructed by Matthew and Granger [44], redrawn by Buffetaut and Angst [39] to show shorter lengths for lower limb bones (tarsometatarsi). (B) Foot bones of the Wyoming specimen, AMNH 60169, American Museum of Natural History, New York, USA.
Fossstud 03 00004 g016

4.3. Paleoenvironment

The paleoenvironment of the track-bearing strata can be considered based on a variety of factors. R. giganteus commonly occur on a bedding plane that contains footprints left by perissodactyl mammals and small shorebirds (Figure 7C). Probable mammalian track-makers are Hyracotherium and a small tapiroid, two animals that were abundant during the Eocene. Both mammals have distinctive footprints comprising four-toe manus and three-toe pesanatomy [31]. Although the association of the coexistence of Gastornis (=Diatryma) bones was originally assumed to be evidence that the giant birds were predators [44], an alternate explanation is that the tracks resulted from diverse species that walked together on the riverbank sandbar, akin to the animals that communally visit modern waterholes [45]. The attractionof the riverbank may have been to have the opportunity to get a drink ofwater; streamside vegetation may have been an additional attraction to herbivores. Shorebirds were likely eager to consume larvae of aquatic insects, the most abundant source for invertebrate trace fossils in Chuckanut strata at Racehorse Creek [25].
Sediments associated with the ancient meandering river were subject to facies changes caused by channel migration. These transitions are evident at both the Racehorse Creek Landslide site and the streamside exposures near Racehorse Falls. Sandstone blocks in the landslide talus field contain three separate examples of crocodilian footprint. These are typically “swim tracks” where the reptiles were traveling in shallow water [26]. The strata along Racehorse Creek likewise record channel migration. The three-track Rivavipes trackway occurs in proximity to sandstone blocks that contain molds of driftwood logs, a feature that is common to Chuckanut Formation sandbar deposits (Figure 17). A fish fossil is presumed to have come from a layer that overlies the track-bearing bedding plane [17]. A higher stratigraphic layer exposed just above Racehorse Falls has yielded fossilized unionid pelecypods [14].
These facies transitions may partially explain why footprint fossils have been found at only a few locations in the Chuckanut Formation: most outcrops consist of sediments that were deposited under environmental conditions that were not favorable for terrestrial walking. And even in terrestrial habitats, lithologic conditions may have been unfavorable for track preservation. Structural geology also played a major role: the discovery of track fossils in the Mount Baker foothills is favored by gently dipping strata that expose bedding plane surfaces. In most Chuckanut Formation outcrops, steep bedding inclination means that outcrops are likely to expose bedding edges, where footprints on bedding plane surfaces are not likely to be recognized.
A Gastornis reconstruction created by Marlin Peterson, a scientific illustrator, is shown in Figure 18. The plants shown in the painting are all based on leaf fossils from the Racehorse Creek Landslide site.

5. Discussion

Gastornis fossils have attracted the attention of scientists for 170 years, beginning with the flurry of attention generated by the 1855 reports of the first discovery [46,47,48]. Taxonomic issues persisted until the 1990s, when Gastornis began to be generally accepted as being congeneric with Diatryma. Subsequent investigations have made great progress in elucidating the paleoecology, paleogeography, and evolutionary history of these giant groundbirds.

5.1. Previous Research

Knowledge of the existence of Paleogene giant groundbirds began with the discovery of a tibiotarsus and femur in Lower Eocene strata at Meuodin, near Paris, France, by Gaston Planté. These remains were described as Gastornis parisiensis [47,48]. The subsequent discovery of additional bones in Paleocene sediments of the Paris basin led to a purported skeletal reconstruction of Gastornis [49,50], and the genus name became widely accepted among European paleontologists. Additional remains were reported from Belgium [51] and the UK [52]. These discoveries generated widespread attention by paleontologists.
Other giant bird fossils were found in North America, beginning with a tarsometatarsus discovered in Early Eocene sedimentary rocks in New Mexico [53]. This creature was named Diatryma gigantea. Marsh [54] reported giant bird bones from New Jersey, with the proposed taxonomic name Barornis regens. The name was revised to Diatryma ajax [55], andlater to D. regens [35,36]. A nearly complete Diatryma skeleton from Wyoming [44] was named as the new species D. steini.
The result of these discoveries on two continents led to a taxonomic dichotomy that lasted for nearly a century: in North America the genus name Diatryma has been used to describe fossils that would be named Gastornis in Europe. The taxonomic uncertainty can be traced to the anatomical differences between the Gastornis skeletal reconstruction of Gastornis by Lemoine [49,50] versus the Diatryma skeleton envisioned by Matthew and Granger [44].Confusion has increased due to the naming of European discoveries as Diatryma. These sites include Switzerland [56], France [57], and Germany [58,59,60]. It was not until 1992 that the Gastornis reconstruction was recognized to be flawed by the accidental inclusion of fish and reptile bones [61], producing a reconstruction that included an elongate reptilian skull with toothed mandibles. The enigmatic taxonomy of Gastornis has been subject to refinement: vertebrate remains from Lissieu, France, and Egetkingen, Switzerland, have been reconsidered as phorusrhacids rather than gastornithids [62].
The close similarities between Gastornis and Diatryma were described in 1992 in two papers that appeared in the same volume [37,61]. In this publication, two American authors, L.D. Martin (Natural History Museum of Los Angeles County in California) and Allison Andors (American Museum of Natural History in New York) independently suggested that the two forms may be congeneric. Martin concluded that the anatomical similarities were indicative that both Diatryma and Gastornis could be placed in the family Gastornithidae, but he expressed caution in considering that the two forms were congeneric. Andors was clearer in his statement that the two genera were equivalent, but he continued the tradition of calling the North American fossils as Diatryma.
The move to use Gastornis as the preferred name has been led by European paleontologists. Mayr [63] synonymized Diatryma with Gastornis, but this taxonomic declaration was made without an examination of type specimens. Buffetaut [64,65,66] and Mikovsky [67] had previously suggested that Diatryma and Gastornis are congeneric, with the latter name having precedence. This taxonomy has gained wide acceptance. One example is the informal renaming of Diatryma gigantea as Gastornis giganteus by Bourdon et al. [68]. Once a contemporary gastornithid pioneer [35,36,37,38], in the early 1990s, Dr. Andors chose to leave paleornithology to transition to a new career as a university administrator. The study of gastornithids is now led by the distinguished French vertebrate paleontologist Eric Buffetaut and his colleagues, particularly his former student Delphine Angst, e.g., [8,39,40,62,63,64,65,66,69,70,71,72,73,74,75,76,77].
Lemoine’s fanciful 1881 reconstruction included supposed wing bones (actually non-avian) that led him to envision Gastornis with paddle-like limbs that indicated swimming capability, being aquatic rather than terrestrial. Over the years, Gastornis (=Diatryma) has been hypothesized to belong to diverse avian families. At the time of first discovery, Gastornis was considered to be a type of giant duck [47,48,78] or heron [79]. Lartet, quoted by Owen [80] suggested alliance with wading birds, and Valenciennes [81] suggested the albatross as a relative; Andrews [82] suggested the new family Paleognathae. The acknowledgement of Gastornis as a flightless groundbird originated with Bonaparte [83]. Martin [61] asserted that the Gastorniformes lacked any special relationship with any modern avian group. This interpretation echoed the earlier views of Owen [80], who noted the dissimilarities between the tibia of Gastornis pariesiensis Hébert and comparable bones from modern birds. In contrast, Andors [35,36] considered the Gastorniformes to be a sister group of the Anseriformes.

5.2. Paleoecology

The ecological role of Gastornithids has been controversial. These large terrestrial birds were long considered to be fierce predators, an evolutionary replacement for theropods whose extinction at the close of the Cretaceous left an empty niche. The occurrence of Diatryma bones in North American strata that also contained remains of the early “dawn horse” Hyracotherium gave rise to the belief that these giant birds preyed upon these mammals, though there is an equal possibility that these animals peacefully coexisted. The authors of a study on the biomechanics of the Diatryma jaw supported the carnivore hypothesis [84].
Several lines of evidence contradict the fierce predator interpretation. Andors [35,36,37,38] noted anatomical characteristics of gastornithids that appeared to contradict a predatorial lifestyle. These features include the lack of a hooked beak (a feature found in all raptors), and short lower limb length that would have inhibited fast locomotion.
As described later in this report, the first discovery of gastornithid footprints in the Eocene Chuckanut Formation of northwestern Washington, USA, revealed the imprints of broad triangular toenails rather than sharp talons [2]. The interpretation of these giant groundbirds as vegetarians was further supported by isotopic dietary evidence and anatomical structure [72].

5.3. Geologic and Geographic Range

The earliest known occurrence of Gastornis dates to the Middle Paleocene, but fossil evidence from Europe suggests that large terrestrial birds were present several million years before the great KT extinction. Fossil bones of Gargantuavis have been found at several Late Cretaceous sites in southern France and Spain [85,86,87,88,89,90]. Although the body forms of Gastornis and Gargantuavis are very different, the presence of the members of the latter genus in the Late Mesozoic is evidence that the evolution of giant groundbirds did not depend upon the extinction of dinosaurs.The paleogeographic distribution and temporal range of the gastornithids provide clues for their evolutionary dispersal [74]. Gastornis was present in Europe from the beginning of the Late Paleocene, persisting to the Middle Eocene (Figure 19). Diatryma (=Gastornis) is known to have lived in North America during the Early Eocene (Figure 20). The presence of gastornithid remains in the Lower Eocene of Ellesmere Island in the Canadian Arctic [91] suggests that gastornithids may have traveled a land bridge connecting Europe and North America as part of the faunal interchange that occurred when a land bridge connected the two continents during the Early Eocene. The only known gastornithid fossil found outside of North America and Europe is a tibiotarsus fragment from Lower Eocene strata at Honan, China [92] (Figure 21).

5.4. Other Fossil Evidence of Gastornithids

Gastornis is most commonly identified from skeletal elements, particularly limb bones. However, two other types of fossils associated with these giant birds are egg shell fragments and tracks. This report presents descriptions of well-preserved tridactyl footprints from Lower Eocene rocks in western Washington. Other discoveries comprise two occurrences where the putative gastornitid tracks cannot be substantiated.

5.5. Eggs

Paleogene eggshell fragments discovered in southern France in 1957 were assigned to the form genus Ornitolithes [94,95,96,97]. A later paper [98] provided a detailed review of subsequent studies that investigated the microstructure of these shell fragments and collected additional specimens. Their results confirm the earlier speculations that the eggs represent Gastornis. Morphometric characteristics indicative of the body size of the egg-layer are consistent with body sizes based on Gastornis skeletal remains.
The reproductive dynamics of Gastornis remain unknown. The estimated egg volume of the French fossils is 1330.4 cm3, which is comparable to that of the largest ostrich egg size range [99].Although the egg sizes of Gastornis and Struthio are similar, their egg-laying habits may be dissimilar. Ostriches (Struthio camelus) typically form small flocks, where egg-laying and chick monitoring are communal enterprises [100]. The social hierarchy comprises a dominant male with a cohort of three to five hens. Nests consist of shallow pits whose main function is to keep eggs from rolling away. Each nest may contain 20 or more eggs, up to a maximum of about 50. Ostriches inhabit savannas, grasslands, and shrub lands of Africa, but they can survive even in deserts. Like mammals that inhabit open grazing environments, their rapid running speed provides a key defensive strategy in the presence of predators; ostrich velocity may reach 64 km/h. These habitat characteristics may be quite different from the environments where Gastornis flourished. A key difference is that Gastornis fossils typically occur in riverbank habitats during an era when climatic conditions were warm and humid. Plant communities were likely to be semi-tropical or warm temperate, at a time when grasslands were not yet an evolved ecosystem. Under these habitat conditions, there was no incentive for Gastornis to possess rapid locomotion. Likewise, the shallow communal sandpit nests that were suitable for arid environments may have been ineffective for protecting eggs in a shady forest environment. Perhaps, the nesting strategy was like that of rhea and emu, large groundbirds that line the nest cavity with leaves or other plant matter. Flock or herd behavior may be an asset for birds or mammals that dwell in open areas, where direct sight lines allow coordinated movement, and where group structure offers protection against predators. For forest-dwelling Gastornis, a more solitary or single-pair lifestyle may have been an advantage.
The Chuckanut Formation gastornithid tracks include the only known example of juvenile footprints. The size of the Gastornis chick can be roughly estimated based on track measurements. Pace distances are not useful because the two juvenile tracks are so close together that they presumably do not record a normal walking pace. Track size is a preferable parameter for estimating body size. Adult footprints range in length from 260 to 285 mm (Table 1), with a mean length of 266 mm. Assuming an adult height of 2.13 m (7 ft) as indicated by the skeletal Gastornis reconstruction by Matthew and Grainger [44], the ratio of total height to footprint length suggests that the 6 cm length of the chick tracks represents a total height of 0.48 m (1.57 ft).This is equivalent to ~23% of the adult height.
The age of the Racehorse Creek chick is speculative, but evidence from modern giant groundbirds provides possible clues. For example, ostrich eggs have incubation times of 42–46 days [101]. Ostrich hatchlings are commonly 25 cm in height, with weights of 1 to 1.2 kg. They grow rapidly, with height gains of 25–30 cm per month for the first six months. Chicks reach adult height in about one year [99,100,102]. This comparison suggests that the Gastornis chick may have been several months old.

5.6. Possible Sexual Dimorphism

The scarcity of fossil evidence precludes determining whether male and female gastornithids were different in body size. Observations of extant and recently extinct groundbirds do not provide a reliable basis for conjecture. One study of farmed ostriches (Struthio camelus) revealed that males had average weights of 119.2 kg, versus 122.3 kg for females [103]. Greater rhea (Rhea americana) likewise show little variation between males and females (animaldiversity.org). In contrast, the extinct moa (Dinornis) of New Zealand had extreme sexual diversity, the largest females having 280% of the body mass and 150% of the height of the largest males; adult males typically weighed 34–85 kg, compared to up to 240 kg for females [103].
The Chuckanut Formation footprints provide no clear evidence regarding possible sexual dimorphism. At the Racehorse Creek Landslidesite, track fossils are preserved in blocks of sedimentary rock that are scattered over a wide area, but this distribution is the result of material deposited during the slope failure. Track-bearing blocks generally have similar petrologic character, with footprints commonly being preserved in a thin layer of fine clastic sediment that is underlain by a thick bed of arkosic sandstone. This combination allows tracks from small shorebirds to be preserved on the same bedding plain as the gastornitid footprints. Perissodactyl mammal tracks are also locally present. Rock blocks that contain Gastornis tracks were mostly found the ridge crest on the southern boundary of the landslide, where they can be traced to a stratigraphic position on the lower part of the head scarp. The general similarity in size of the tracks suggests the possibility that they represent fragmentation on a single trackway. Alternately, they may represent tracks made by multiple birds, all of similar size.

6. Summary

Gastornithid tracks are preserved at two nearby sites in the Chuckanut Formation of northwestern Washington State, USA. The Racehorse Creek Landslide strata, where giant bird tracks were discovered in 2009, include an ash bed that gives a U/Pb radiometric age of 53.7 Ma (Lower Eocene) [16]. The track-bearing strata of the Landside site and the Racehorse Creek bed sites show approximately 1.5 km of geographic separation, but the inclination of the strata suggests that these locations are approximately correlative in age (Figure 2). The Chuckanut Formation gastornithid tracks have been given the ichnotaxonomic name Rivavipes giganteus Mustoe et al. [2].
The giant bird tracks preserved in the Lower Eocene fluvial sediments of the Chuckanut Formation in northwest Washington State, USA, support the interpretation that Gastornis was a herbivore, not a fierce carnivore. These tridactyl footprints preserve broad triangular toenails rather than talons. In 2024, two important additional discoveries were made. These are a trackway preserving three adult tracks and two tracks left by a gastornithid chick.The adult bird trackway has stride and pace distances that are consistent with the short lower limb bones (tarsometatarsals) observed in Gastornis skeletal remains. The reproductive strategies of gastornithids remain enigmatic; the evidence consists of numerous egg shell fragments found at sites in France and the newly discovered Chuckanut tracks.

Funding

This research involved no external funding.

Data Availability Statement

Fossil specimens, molds, and related materials are archived at the Western Washington University Geology Department in Bellingham, WA, USA. Please contact the author for more information.

Acknowledgments

W.W.U. Research Associate Dave Tucker organized the “Bird Herd” volunteers who arranged the helicopter recovery of the 540 kg slab that became the type specimen for Rivavapes giganteus. Keith Kemplin served as a Research Assistant during the field work for this study. The Racehorse Creek trackway was discovered by Andrew Olive and Jason Miller, who quickly notified the author so that the trackway could receive scientific attention. Mark Mikkelson collected the slab containing the juvenile tracks, and generously donated the specimen so that it could become an addition to the W.W.U Geology Department museum.

Conflicts of Interest

The author declares no conflicts of interest.

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Figure 1. Location maps showing sites where gastornithid footprints have been found in northwest Washington, USA. (A) Regional geography. (B) Geologic map of Racehorse Creek area. Strike and dip data from [5,6].
Figure 1. Location maps showing sites where gastornithid footprints have been found in northwest Washington, USA. (A) Regional geography. (B) Geologic map of Racehorse Creek area. Strike and dip data from [5,6].
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Figure 2. Colorized lidar image of Racehorse Creek area, showing a giant prehistoric landslide and the much smaller 2009 Racehorse Creek Landslide. Gastornithid track locations are marked with red circles. The inclination of strata suggests that although the landslide and creek-side localities are topographically separated, the stratigraphic difference may be relatively small. Photo adapted from an open-source image [7].
Figure 2. Colorized lidar image of Racehorse Creek area, showing a giant prehistoric landslide and the much smaller 2009 Racehorse Creek Landslide. Gastornithid track locations are marked with red circles. The inclination of strata suggests that although the landslide and creek-side localities are topographically separated, the stratigraphic difference may be relatively small. Photo adapted from an open-source image [7].
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Figure 3. Racehorse Falls, late summer low-water-flow conditions. Sedimentary bedrock is exposed in the streamside outcrops, but rugged terrain, abundant plant growth and thick soil cover limit opportunities for finding fossils in most Chuckanut strata.
Figure 3. Racehorse Falls, late summer low-water-flow conditions. Sedimentary bedrock is exposed in the streamside outcrops, but rugged terrain, abundant plant growth and thick soil cover limit opportunities for finding fossils in most Chuckanut strata.
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Figure 4. Head scarp of Racehorse Creek Landslide, a near-vertical face with a height of ~35 m. Sandstone blocks in the foreground have continued to become detached from the face, which originated in the 9 January 2009 landslide.
Figure 4. Head scarp of Racehorse Creek Landslide, a near-vertical face with a height of ~35 m. Sandstone blocks in the foreground have continued to become detached from the face, which originated in the 9 January 2009 landslide.
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Figure 5. The first giant bird track (WWU-TR-66) that was discovered at Racehorse Creek Landslide. (A) Photo of the author with the in situ slab. Photo by Keith Kemplin, used with permission. (B) Chalk-marked outline of the tridactyl footprint, and three small shorebird tracks.
Figure 5. The first giant bird track (WWU-TR-66) that was discovered at Racehorse Creek Landslide. (A) Photo of the author with the in situ slab. Photo by Keith Kemplin, used with permission. (B) Chalk-marked outline of the tridactyl footprint, and three small shorebird tracks.
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Figure 6. Display of gastornithid trace fossils at the Western Washington University Geology Department, Bellingham, Washington, USA. (A) Display Gastornis tracks from Racehorse Creek Landslide. These are a two-track slab, WWU-TR-57, and the holotype, WWU-TR-66. (B) Local school children explore the type specimen. 2013 photo courtesy of David Tucker.
Figure 6. Display of gastornithid trace fossils at the Western Washington University Geology Department, Bellingham, Washington, USA. (A) Display Gastornis tracks from Racehorse Creek Landslide. These are a two-track slab, WWU-TR-57, and the holotype, WWU-TR-66. (B) Local school children explore the type specimen. 2013 photo courtesy of David Tucker.
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Figure 7. Three additional Rivavapes giganteus track specimens. (A,B) Epirelief tracks. (A) Specimen WWU-TR-059. (B) Specimen WWU-TR-57. (C) Plaster replica of track WWU-TR-059, showing the association of the giant bird track with perissodactyl mammal tracks and small shorebird tracks. Specimens A and B are on display at the WWU Geology Department; Track C remained in situ, and was destroyed in 2023 when an unauthorized collector attempted to chisel out the track from the large matrix block.
Figure 7. Three additional Rivavapes giganteus track specimens. (A,B) Epirelief tracks. (A) Specimen WWU-TR-059. (B) Specimen WWU-TR-57. (C) Plaster replica of track WWU-TR-059, showing the association of the giant bird track with perissodactyl mammal tracks and small shorebird tracks. Specimens A and B are on display at the WWU Geology Department; Track C remained in situ, and was destroyed in 2023 when an unauthorized collector attempted to chisel out the track from the large matrix block.
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Figure 8. Racehorse Creek gastornithid tracks. (A) The arrow shows the location of the track-bearing sandstone bedding plane. Racehorse Falls can be seen in the background. (B) Photo of tracks on the day of the January 2024 discovery, when the outcrop was wet from rain. Photo by Andrew Olive, used with permission. (C) Photo taken by the author on 7 October 2024, showing three footprints outlined with chalk.
Figure 8. Racehorse Creek gastornithid tracks. (A) The arrow shows the location of the track-bearing sandstone bedding plane. Racehorse Falls can be seen in the background. (B) Photo of tracks on the day of the January 2024 discovery, when the outcrop was wet from rain. Photo by Andrew Olive, used with permission. (C) Photo taken by the author on 7 October 2024, showing three footprints outlined with chalk.
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Figure 9. Pair of sediment-filled footprints from a gastornithid chick, discovered at Racehorse Creek by Mark Mikkelson in May 2024.
Figure 9. Pair of sediment-filled footprints from a gastornithid chick, discovered at Racehorse Creek by Mark Mikkelson in May 2024.
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Figure 10. Simplified sketch of Chuckanut Formation depositional environments. Sediments of the Chuckanut Formation were deposited along a meandering river that flowed west from headwaters near the present Idaho/Washington border. Granitic bedrock provided a source for arkosic sandstone, which is a dominant constituent of the Chuckanut Formation, where it represents point bars and crevasse splays. Fluvial deposits typically occur as linear ribbons that follow the course of the river, combined with silt-rich deposits that accumulated during episodic flood events.
Figure 10. Simplified sketch of Chuckanut Formation depositional environments. Sediments of the Chuckanut Formation were deposited along a meandering river that flowed west from headwaters near the present Idaho/Washington border. Granitic bedrock provided a source for arkosic sandstone, which is a dominant constituent of the Chuckanut Formation, where it represents point bars and crevasse splays. Fluvial deposits typically occur as linear ribbons that follow the course of the river, combined with silt-rich deposits that accumulated during episodic flood events.
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Figure 11. Generalized stratigraphy of the Chuckanut Formation main outcrop belt showing the approximate position of Racehorse Slide/Racehorse Creek track sites. Figure adapted from [17].
Figure 11. Generalized stratigraphy of the Chuckanut Formation main outcrop belt showing the approximate position of Racehorse Slide/Racehorse Creek track sites. Figure adapted from [17].
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Figure 12. Semi-tropical plant fossils from Racehorse Creek Landslide. (A,B) Impressions of palmetto fronds, Sabalites cambelli. (C) Tree fern frond, Cyathia pinnata. Photo reprinted from [26].
Figure 12. Semi-tropical plant fossils from Racehorse Creek Landslide. (A,B) Impressions of palmetto fronds, Sabalites cambelli. (C) Tree fern frond, Cyathia pinnata. Photo reprinted from [26].
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Figure 13. Paleotemperature plot for the Cenozoic Era, showing the warm climate that existed when the Racehorse Creek tracks were made. Graph adapted from [28,29].
Figure 13. Paleotemperature plot for the Cenozoic Era, showing the warm climate that existed when the Racehorse Creek tracks were made. Graph adapted from [28,29].
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Figure 14. Outlines and geometric data for single and multiple R. giganteus footprints from Racehorse Creek Landslide and Racehorse Creek. Tracks labeled A, B, C refer to specimens that preserve two or three tracks in proximity on the same bedding plane.
Figure 14. Outlines and geometric data for single and multiple R. giganteus footprints from Racehorse Creek Landslide and Racehorse Creek. Tracks labeled A, B, C refer to specimens that preserve two or three tracks in proximity on the same bedding plane.
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Figure 15. Sketch showing the geometry of three tracks on a sandstone bedding plane exposed in the streambank a few hundred meters downstream of Racehorse Falls.
Figure 15. Sketch showing the geometry of three tracks on a sandstone bedding plane exposed in the streambank a few hundred meters downstream of Racehorse Falls.
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Figure 17. Sandstone cast of driftwood log (marked with red arrow), just downstream from R. giganteus trackway (blue arrow).
Figure 17. Sandstone cast of driftwood log (marked with red arrow), just downstream from R. giganteus trackway (blue arrow).
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Figure 18. Reconstruction of an adult Gastornis and three chicks in an ancient landscape based on Chuckanut Formation plant fossils and sedimentary rock types. Painting by Marlin Peterson, 2011. Reprinted in color from previously published grayscale image [2].
Figure 18. Reconstruction of an adult Gastornis and three chicks in an ancient landscape based on Chuckanut Formation plant fossils and sedimentary rock types. Painting by Marlin Peterson, 2011. Reprinted in color from previously published grayscale image [2].
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Figure 19. Gastornis localities in Europe. 1. Walleck; 2. Rivecourt; 3. Cernay, Berru, and Louvois; 4. Mesvin; 5. Meudon; 6. Monthelon; 7. Saint-Papoul; 8. Croydon; 9. Geiseltal; 10. Messel. Egg shell fragments have been collected from localities in southern France and northeastern Spain. The time chart on the right shows the temporal range of Gastornis fossils. Map redrawn from [74].
Figure 19. Gastornis localities in Europe. 1. Walleck; 2. Rivecourt; 3. Cernay, Berru, and Louvois; 4. Mesvin; 5. Meudon; 6. Monthelon; 7. Saint-Papoul; 8. Croydon; 9. Geiseltal; 10. Messel. Egg shell fragments have been collected from localities in southern France and northeastern Spain. The time chart on the right shows the temporal range of Gastornis fossils. Map redrawn from [74].
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Figure 20. Gastornis (Diatryma) localities in western USA.
Figure 20. Gastornis (Diatryma) localities in western USA.
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Figure 21. Early Eocene (50 Ma) continents, with arrows showing possible dispersal routes for gastornithids from Europe to North America and Asia. Base map adapted from [93].
Figure 21. Early Eocene (50 Ma) continents, with arrows showing possible dispersal routes for gastornithids from Europe to North America and Asia. Base map adapted from [93].
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Table 1. Morphometric data for R. giganteus tracks.
Table 1. Morphometric data for R. giganteus tracks.
Racehorse Creek LandslideRacehorse Creek
(Linear Dimensions in mm; Angles in Degrees)
WWU
TR-57 A		WWU
TR-57B		WWU
TR-58B		WWU
TR-58C		WWU TR-59BWWU TR-66WWU-TR-67WWU-TR-68WWU TR-72WWU
TR-73
A		WWU
TR-73
B		WWU
TR-74
A
Width 1245225~230~230265280230225*28528560
Length 228526228028524525026028528523026060
Digit II-IV angle6667646684868280849710292
Digit III max. width4852504858475055559511021
Interdigital angles30, 3632, 3532, 3232, 3443, 3848, 3845, 3742, 3842, 4249, 4854, 4843, 49
Length left digit 3239210200220190192190200200707011
Length right digit 3c. 205c. 200c. 200-222210195228*826011
Heel pad imprint max. depth181816161532302212NDNDND
1 Width measured as distance between apices of digits II and IV. 2 Length measured from proximal margin of heel pad to apex of digit 3. 3 Digits II and IV cannot be reliably identified on the basis of length or interdigital angle relative to digit 3. Measured angles refer to left and right position of digits as shown in Figure 12. * Incomplete track. ND = not determined.
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MDPI and ACS Style

Mustoe, G.E. Giant Bird Tracks (Family Gastornithidae) from the Paleogene Chuckanut Formation, Northwest Washington, USA, with a Review of Gastornis Distribution. Foss. Stud. 2025, 3, 4. https://doi.org/10.3390/fossils3010004

AMA Style

Mustoe GE. Giant Bird Tracks (Family Gastornithidae) from the Paleogene Chuckanut Formation, Northwest Washington, USA, with a Review of Gastornis Distribution. Fossil Studies. 2025; 3(1):4. https://doi.org/10.3390/fossils3010004

Chicago/Turabian Style

Mustoe, George E. 2025. "Giant Bird Tracks (Family Gastornithidae) from the Paleogene Chuckanut Formation, Northwest Washington, USA, with a Review of Gastornis Distribution" Fossil Studies 3, no. 1: 4. https://doi.org/10.3390/fossils3010004

APA Style

Mustoe, G. E. (2025). Giant Bird Tracks (Family Gastornithidae) from the Paleogene Chuckanut Formation, Northwest Washington, USA, with a Review of Gastornis Distribution. Fossil Studies, 3(1), 4. https://doi.org/10.3390/fossils3010004

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