Ostendichnus ichnogen nov. Fig. 3

Diagnosis: large, up to 2-meter-long, bilaterally-symmetrical, bilobed to oval impressions with multiple well-defined digital scratch marks aligned parallel or sub parallel to long axis of the whole trace. Up to 10–15% as deep as long. Traces mostly with a single raised central ridge, separating left and right troughs, which may include complete or partial diagnostic tridactyl theropod tracks.

Type material: holotype Denver Museum of Nature and Science (DMNH) EPV.69705 latex mold and fiberglass replica of large digging trace, within which a diagnostic theropod track occurs. Paratypes DMNH EPV. 69703, EPV. 69704, EPV. 69706 and EPV. 69707 latex molds and fiberglass replicas of large digging traces (Fig. 3 and Supplementary Information 2).

Type horizon and locality: lower part of the Cretaceous Dakota Sandstone, Roubideau Creek, Delta County, Colorado. Information on file with DMNS, CU and BLM.

Derivation of name: from ostendo (Latin) meaning “to show”, or “to display” and ichnos (Latin) meaning “a trace.”

Ostendichnus bilobatus ichnosp nov., Fig. 3

Type material: as for ichnogenus: holotype DMNH EPV.69705 latex mold and fiberglass replica of digging trace number 5.

Type horizon and locality: as for ichnogenus.

Derivation of ichnospecies name: bilobatus meaning two lobes.

Diagnosis: as for ichnogenus.

Description: large, bilaterally-symmetrical, bilobed to oval impressions or scrapes 0.75 to 2.00 m long and 0.50 to 1.25 m wide; depth variable, 5 to 25 cm. Multiple well-defined digital scratch marks align with the whole trace, as does a raised medial ridge defining the long axis of the trace. Some scratch marks have sharp anterior terminations, indistinguishable from typical theropod digit traces. Together with sand crescents, where sediment was pushed back by posterior motion of digits, or thrown posteriorly as a thin apron, the left and right sides of scrapes are defined. In some scrapes complete or partial theropod tracks are recognisable components of the scrapes.

Interpretation of Ostednichnus bilobatus

The sharply-terminated scratch marks found in association with diagnostic theropod tracks represent active theropod scraping or scratching. The most complete theropod tracks include a large Irenesauripus-like27,28 left theropod track on the left side of the holotype scrape (Fig. 3a) and a smaller right theropod track associated with the right side of a shallow paratype scrape (Fig. 3c). The variable size and depth of the scrapes, indicate different levels of activity and persistence by different sized theropods, which based on footprint length had hip heights between ~1.0 and ~2.0 m29 and full body lengths between ~2.5 and ~5.0 m. This implies either two or more different species, or co-occurrence of conspecific adults and sub-adults of quite different sizes. Elapsed time between scraping episodes cannot be estimated accurately, but was likely short given the similar, good preservation of all scrapes.

Interpretation of nest scrape displays

Interpretation of these scrapes as evidence of mating display arenas or courtship ritual sites requires elimination of other possible digging behaviour interpretations unrelated to mating display. We can then demonstrate whether or not the behaviour and resultant trace fossils, are consistent with behaviours of other similar or related species, in this case extant birds.

Possible explanations for the scrapes reported from the Dakota Formation sites in Colorado, are that 1) they are actual nest sites or colonies, 2) they represent evidence of dinosaurs digging for food, water or shelter, 3) they are territory-marking scrapes and 4) they are nuptial display arenas or scrape ceremony sites (Table 1). The nest site explanation is unconvincing because there is no evidence of eggs, eggshells, hatching remains, or the type of well-defined nest rims30 recorded at many nest sites (Fig. 5). Even if eggshell and hatchling remains were removed by parents or taphonomic process, the variable shapes, depths and distribution of scrapes do not conform to the typical shapes and regular-spacing configuration of nests in known dinosaur nest colonies30 or those of extant avians such as gannets or flamingos. It is also difficult to conceive of dinosaurs nesting, incubating and rearing young in scrapes of variable size and depth without obliterating the clear scratch marks and prominent median ridges seen in most examples. Among extant ground nesting colonial birds, such as gannets or flamingoes, nest spacing is highly regular. Nest materials are built into large mounds with preservation potential and there are no signs of irregularly configured scrape marks. Postulating a “failed nest site” is also unconvincing because a site where theropods congregated to scrape, before moving elsewhere, leaves trace evidence essentially indistinguishable from a display arena (interpretation 4). Moreover, it requires that we assume failed nest digging attempts that are not purposeful or integral display phases in the breeding cycle. Such non-display activity would presumably waste energy and might only be explained, tentatively, as bouts of stereotypical scrape behavior for territorial or other purposes such as testing unsuitable substrates that were later abandoned. In this regard nest scrape display is a type of territorial behavior which is better-known, documented and widespread than any scrape behavior unrelated to display.

Table 1 Comparison of digging or scrape trace fossil evidence with the four working hypotheses considered here. √ indicates consistency with one or more hypotheses. x indicates lack of consistency with one or more hypotheses, ? indicates uncertain degree of consistency with one or more hypotheses. Full size table

Figure 5 a 1 -a 3 : maps of Cretaceous sauropod nests from Argentina with cross, section (a4), after30. Note the nest rim in all cases. (b) cross section of Club Gulch dig trace with thin apron of displaced sand. Note lack of sediment rim. (a,b) drawn to approximately the same scale. c: map showing distribution of Kakapo nest scrape bowls, (modified after12). Full size image

Explanations based on digging for food or water also suffer from a lack of consistent and compelling evidence. It is known that elephants and other tetrapods dig for water31,32 or food. However, any successful attempt to dig to the level of the water table would produce pooling that would wash out scrape marks in sandy sediment. Some authors have speculated about the “scratch digging” potential of sauropods33,34. Such notions can be dismissed here because the scratch morphology fits theropods, not herbivorous sauropods. Moreover, despite ~120 known tracksites in the Dakota Sandstone20 the unit was deposited during the sauropod hiatus in North America35 when no sauropod tracks are known. Digging for prey is a plausible theropod activity supported by one diagnostic deinonychosaurid claw trace that penetrated a bioturbated zone of burrows attributed to small tetrapods, possibly mammals36. However, these traces bear no morphological resemblance to the large surface scrapes here described as Ostenichnus bilobatus and there is also no evidence of burrows, or even buried carrion, at any of the scrape sites. Dinosaur burrows are small and very rare. A report of a ~30-cm-diameter, sub-cylindrical burrow containing an adult and two juvenile ornithopod dinosaurs37 could be a case of “denning,” but there is no definitive proof that a dinosaur (presumably the adult) dug the burrow in which it died. The diggers of other possible dinosaur burrows38 have only been inferred.

Could the scrapes be territorial markings ? Mammalian carnivores, notably cats, dig scrapes in unconsolidated sediment along game trails and mark them with urine39. Moreover, these traces have even been labelled “scrapes.” However, they are almost always isolated and often along upland game trails were preservation potential is very low. Unlike ureotelic mammals, reptiles and birds are uricothelic, excreting uric acid as a final product of nitrogenous metabolism. Scent marking of territory is a mammalian trait, not known in water-conserving uricothelic reptiles and birds. This is another reason that searching for water by theropod dinosaurs is an unconvincing explanation for scrapes. In the case of the Dakota Sandstone, representing a coastal plain system with abundant evidence of saturated substrates, surface water was abundant.

Turning to the fourth hypothesis, that the scrapes represent display arenas containing evidence of ceremonial scrapes, we argue that the evidence (Table 1) consistently supports such interpretations. Ornithological literature provides many reports of scrape ceremonies and nest scrape display activity, separate from actual nest site selection and occupation. Supplementary Information 3. A compelling parallel with the Cretaceous scrapes from Colorado was reported for the Atlantic puffin (Fratercula arctica) in the run up to breeding, which thanks to “repeated scratching and kicking produces two parallel furrows on the floor of the burrow [with a] characteristic ridge between [that] becomes worn down and less obvious during the breeding season”8. The puffin is an active digger, although it often occupies and enlarges existing burrows. Unlike a large non-avian theropod it can stand more or less erect in a burrow. The traces produced by digging or scraping: i.e. two parallel furrows, with a ridge between, are strikingly similar to the theropod traces Ostenichnus bilobatus. The ostrich, the largest living bird, much closer in size to the Cretaceous theropod scrape-makers, produces shallow scrapes similar in size (~2–3 meters diameter) to those described here40 but has not been reported to engage in “nest scrape display” away from its chosen nest site, in which incubation obliterates any preservable scrape traces. These two very different birds are among the few whose scrapes have been described or illustrated, even briefly. For most of the many birds whose nest-scrape display behaviour has been documented in detail, the scrape morphology is considered an incidental behavioural bi-product, whereas the ritual ceremonies (movements) have been analysed and classified in detail8,9,10,11,12,13,14, often with intriguing video footage (Supplementary Information 3). At least one vertebrate ichnologist has speculated that theoretically one might find traces made by dinosaurs engaging in display behaviour, possibly even in the act of copulation38. Such conjectures are not new41,42 but they have been, until now, entirely speculative based only on the assumption that coition and pre-copulation courtship, occurs in all tetrapods. However, the point is well-taken as it is the physical trace evidence, mostly ignored by modern ornithologists, that is potentially most important in paleontology38.

Among shorebirds (Charadriidae), nest scrape displays are reported for seven Charadrius species9 including the Wilson’s plover, Charadrius wilsonia9, banded dotterel, Charadrius bicinctus10, the piping plover (Charadrius melodus). Scrape displays occur from the first day a dotterel pair occupies a territory. “A male makes a scrape in sand, shuffling with his breast and kicking backwards. Shallow scrapes are often made that are never used as nests”10. The Knot, Calidris canutus engages in multiple pre-coition “nest scrape displays,” also referred to as “nest scrape advertisement displays”11. The importance of Charadrius scrapes in relation to Cretaceous theropods is that they indicate that extant, ground-nesting shorebirds make multiple scrapes, most of which are never occupied as nests. Such pre-nuptial behaviour is very energetic, expending much more energy than needed to excavate a single nest. However, it is a purposeful part of the mating ritual, not a “failed” nest construction attempt.

The list of substrate scraping birds is long and diverse, even if the morphology of their scrapes is only sporadically recorded (Supplementary Information 3). Studies of the Kakapo Strigops habroptilus, a ground-dwelling, nocturnal New Zealand parrot, demonstrate that it also digs multiple scrapes as part of its courtship rituals12,43. Studies of this species include maps showing how scrapes are irregularly distributed, from 1-5 m apart, within the male’s display area otherwise known as a lek43 (Fig. 5). The lek, or display arena has generated much debate among ornithologists interested in sexual selection. They define both intrasexual leks, where males display to one another and intersexual leks where females observe male displays. The importance of mutual sexual selection has also been discussed in relation to dinosaurs and pterosaurs19. The ornithological literature includes maps of lek territories for many species44,45 and much importance is attached to how sexual selection drives the evolution of size, dimorphism and other characteristics in lekking species. Thus, we must consider whether the Cretaceous theropod congregation sites described here may have been analogous to certain avian lek or arena display sites44,45 as appears to be the case (Supplementary Information 3). The afore-cited studies8,9,10,11,12,13,14 deal with behaviour that is both territorial and sexual, being an integral part of the breeding cycle: specifically pre-nuptial courtship prior to copulation. It is intriguing to imagine the vocalizations of large theropods during courtship scrape ceremonies and copulation (Fig. 6).