Sediment grain sizes, internal bedding and rippled top surface of NMV-P231650 imply that sediments were deposited in a fluvial floodplain. This diagnosis is supported by: (1) a uniform thickness to the probable source bed for the tracks, which overlies the Dinosaur Cove bone bed, an overbank splay deposit (Rich et al. 1988; Rich and Vickers-Rich 2000); (2) fine-grained sandy sediment with small-scale cross-bedding (Stow 2006); (3) small invertebrate trace fossils in the top few centimetres of the rock, indicating postdepositional colonization by infauna; and (4) a rippled upper surface imprinted with terrestrial vertebrate tracks. These features are similar to sedimentary structures associated with dinosaur track-bearing surfaces at Milanesia Beach (Martin et al. 2012). Terrestrial vertebrate tracks on the top surface further suggest subaerial conditions, albeit with a relatively moist substrate (sensu Jackson et al. 2010; Martin et al. 2012). However, further evidence of subaerial conditions, such as mudcracks and raindrop impressions, is lacking.
All three tracks are interpreted as either shallow undertracks or poorly defined surface tracks made in moist, fine-grained sand that also deformed sediment within and around the footprints, further obscuring specific features. Undertrack preservation of the tracks is supported by: (1) no skin or digital pad impressions; (2) incomplete track outlines; (3) deformational structures in and outside of tracks; and (4) only partial preservation of ungual impressions (Milàn and Bromley 2006; Martin et al. 2012). Sediment moistness is interpreted on the basis of visual categories defined by Jackson et al. (2010), which match these tracks to those formed in sand with about 30% moisture content. A lower moisture content would have resulted in better defined tracks, whereas tracks made in completely saturated sands would have collapsed on themselves or filled with water, thereby further distorting track outlines (Gatesy et al. 1999; Jackson et al. 2010). We have no evidence of a ‘soupy’ substrate, in which tracemakers immersed and then extracted their entire feet, as described for Late Triassic theropods by Gatesy et al. (1999). Furthermore, one of the tracks (Track 2, Fig. 4) has a prominent plate of sand similar to those formed by tridactyl feet pushing against fine-grained moist sand, which causes a false interdigital ‘webbing’ (Falkingham et al. 2009). However, because all digits on the Eumeralla tracks are definable, these were probably made on an emergent surface. Modern exposure of the fossil tracks on a wave-dominated marine platform may have also eroded these and obscured further anatomical traits. A similar preservational mode was inferred for theropod tracks in the Eumeralla Formation at nearby Milanesia Beach (Martin et al. 2012).
Invertebrate burrows in the uppermost few centimetres of the sandstone, absent below laminated sediments, indicate a probable waning flow that formed both the bedding and allowed for colonization by infauna (Buatois and Mángano 2007; Martin 2009; Martin et al. 2012). This interpretation is further supported by low-angle cross-bedding just below the track-bearing surface. However, without clear evidence of cross-cutting relations, for example a track stepping on a burrow or a burrow crossing a track feature, we cannot definitely say whether the infauna colonized the sediment while it was still subaqueous or after exposure.
Tracks 1 and 2
We interpret Tracks 1 and 2 as avian tracks, based on criteria applied to Mesozoic and Cenozoic fossil bird tracks, as well as modern ones (sensu Elbroch and Marks 2001), using several key features. Lockley et al. (1992), Wright (2006) and Falk et al. (2011) summarized the traits required for distinguishing Mesozoic avian tracks from those of nonavian theropods, which include: (1) relatively small size; (2) thin digits with undefined digital pads; (3) wide (>110 degrees) divarication between digits II and IV (with some exceptions); (4) retroflexed hallux; (5) thin clawmarks; (6) clawmarks curving distally from the main axis of the footprint. Tracks 1 and 2 have nearly all of these traits, save for distally curving clawmarks. Both tracks are slightly smaller compared to some nonavian theropod tracks from the Eumeralla Formation at nearby Milanesia Beach (Martin et al. 2012); have thin digits lacking digital pads; a wide digit II–IV divarication (120 and 95 degrees in tracks 1 and 2, respectively); a hallux impression associated with an anisodactyl foot; and thin (sharp) ungual marks. Of these traits, track size can be variable, whereas the other characteristics are diagnostic of bird tracemakers (Lockley et al. 1992; Wright 2006; Falk et al. 2011). Although Track 1 does not show distally curving clawmarks, the distal end of digit III is likely truncated and ungual impressions are not readily observable on digits II and IV. A thin clawmark, however, is recorded from digit III, bisecting a sediment mound in the middle of the track (Fig. 3C–D), which was probably formed with retraction of that digit as the bird stepped forward. Nonetheless, perhaps the most important qualitative criterion indicating an avian origin for a Mesozoic track is the presence of a well-defined retroflexed hallux. Both tracks 1 and 2 show a hallux, as well as high angles between digits I and II (86 degrees in Track 1, 75 degrees in Track 2), which is typical of other Mesozoic avian tracks (Lockley et al. 1992, 2009; Wright 2006; Falk et al. 2011).
Wright (2006) further distinguished Mesozoic tracks made by wading birds as thin-toed; having high divarication between digits II–IV; with a probable hallux; and webbing. The Eumeralla tracks are thin-toed, have high divarications and well-defined halluces, but evidence for proximal webbing between digits II–III and III–IV is unclear. Shorebirds, in contrast, lack or have a reduced hallux or show full webbing (Elbroch and Marks 2001; Lockley 2009; Falk et al. 2010); these criteria were applied to the interpretation of Early Cretaceous shorebird tracks in Korea (Lim et al. 2000; Lockley et al. 2009, 2012; Falk et al. 2011). Moreover, although the tracks may belong to enantiornithine birds, their overall form and size are similar to those of ornithurines (Falk 2011), specifically birds belonging to the modern clade Ardeidae, such as egrets and herons (Elbroch and Marks 2001; Lockley et al. 2009). If so, these tracks would greatly extend the palaeogeographical range of Early Cretaceous ornithurine footprints (Falk 2011). These tracks would also supplement an Aptian–Albian fossil record in Australia only represented previously by enantiornithines (Close et al. 2009). Ornithurines dominate the Early Cretaceous waterbird body fossil record in China, but are less common elsewhere (Zhou and Zhang 2007).
Both avian tracks are slightly smaller in overall length and width compared to the other track on the same surface, but are significantly larger than most Early Cretaceous avian ichnogenera, such as Jindongornipes and Ignotornis (Matsukawa et al. 2006; Lockley et al. 2009, 2012; Kim et al. 2012). Consequently, we hesitate to assign ichnotaxa to these specimens, despite their gross morphological resemblance to Ignotornis (Currie 1981; Lockley et al. 2009). For instance, the tracks are more than twice the length and width of the holotype and average-sized Ignotornis, described first from Aptian rocks of Colorado, but also reported from the Aptian–Albian of Korea (Currie 1981; Lockley et al. 1992, 2009, 2012; Kim et al. 2006, 2012). A comparable ichnogenus in overall size and form is Hwangsanipes from the Late Cretaceous of Korea, but this ichnogenus shows clear evidence of interdigital webbing (Yang et al. 1995), whereas the Eumeralla tracks apparently lack webbing and are about 20–25% larger. Regardless of ichnotaxonomic considerations, the relatively large dimensions of all three tracks compared to most Early Cretaceous avian tracks support a presence of sizeable enantiornithine or ornithurine bird species in polar environments of Australia.
The lengthy hallux impression in Track 1, coupled with a prominent deformational structure associated with the distal end of digit IV and edges of other digits, further suggests that the total length of this trace was elongated by tracemaker behaviour. We interpret these structures as a result of dragging the hallux through the sediment with forward foot movement, and then placing the remainder of the foot as it stopped abruptly. Such an action would have caused sedimentary structures on the anterior edge of the track analogous to thrust faults and other tectonic features (Graversen et al. 2007), pushing moist, cohesive sand around and in front of the foot. However, when this track is compared to modern flying bird traces (volichnia, sensu Müller 1962 and Reineck 1981), these traits also match those for tracks made by a bird landing after flight (Martin 2013; Fig. 6). This interpretation was likewise applied to avian-like tracks from Argentina (Genise et al. 2009), which were formerly interpreted as Late Triassic – Jurassic (Melchor et al. 2002), but recently reinterpreted as Eocene (Melchor et al. 2013). Falk (2009) likewise interpreted volichnia from Early Cretaceous bird tracks of the Haman Formation of South Korea. In each study, some tracks had elongated hallux impressions, which in modern birds are caused by tracemakers landing with their legs pointing forward (Genise et al. 2009; Martin 2013).
Figure 6. Modern example of volichnia caused by a tricolored heron (Egretta tricolor), a bird with anatomically similar feet (anisodactyl with digit I) and slightly smaller than the inferred Eumeralla tracemaker. A, paired landing tracks (arrow) connected to normal diagonal-walking trackway in bioturbated, moist, muddy fine-grained sand of back-dune area (storm-washover fan), St. Catherines Island, Georgia (USA). B, close-up of offset left–right pair of tracks, caused by planting of left foot followed by right. Note how the left footprint has greater sediment deformation and depth, as well as an elongated hallux dragmark relative to that in the right footprint. Photograph scale in A and bar scale in B both represent 10 cm.
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The ideal method for testing a volichnion hypothesis for fossil bird tracks would be to study an entire sequence of tracks, beginning with the first two formed by landing and ending with the last two made at take-off. Unfortunately, as a single footprint, this specimen cannot be tested in this way. If this right-side track were part of a right–left pair, the left-side track may have been offset far enough ahead that it was not preserved on the sandstone-block surface. Still, we note that the features of this single track are consistent with a volichnion, as observed in modern avian tracks (Martin 2013) and likewise interpreted in fossil tracks (Falk 2009, Genise et al. 2009).
Pressure-release structures associated with Track 2 also provide some evidence of foot movements, with prominent ridges and plates on the anterior and left sides of digits II–IV. These show that pressure was exerted against the sediment forward and left, with an anticlockwise twisting of the foot before its removal from the surface, perhaps as it stepped to its right foot. Track 2 also has a prominent plate of sediment mimicking distal interdigital ‘webbing’, a trait associated with deformation of a fine-grained, water-saturated sediment and imparted by foot movement (Falkingham et al. 2009). Another distinctive feature of this track is a pronounced curvature of digit II, which may have resulted from a bending of the digit as it interacted with the substrate. Posterior to this part of the track is another possible impression of the same digit, but which is nearly linear, further suggesting flexure associated with forward movement of the foot.
If made by the same species of tracemaker, possible sources of variation in dimensions between tracks 1 and 2 could have been through spreading of the digits with forward movement of a foot, whether with landing from flight or interaction of the foot with a soft, wet substrate. These factors would have resulted in a higher divarication angle and overall width of the track, as opposed to a footprint made during normal walking. Lockley et al. (2009) noted similar variations in divarications and track widths for the Early Cretaceous avian ichnogenus Ignotornis, which they attributed to local differences in substrate and behaviour. Alternatively, variations in size and divarications between Tracks 1 and 2 may be biologically linked, such as from different ages or genders of the same species or different species.
Track 3 differs from tracks 1 and 2 in its greater size, smaller digit II–IV divarications and lack of an obvious hallux impression. As a result, we could not confirm with confidence that this was an avian track. However, it conforms well with expected traits of nonavian theropod tracks, particularly those of coelurosaurs, which are normally longer than wide, have narrow digits relative to track length, sharp ungual impressions and relatively low angles of divarication (35–75 degrees) between digits II and IV (Lockley et al. 1992; Lockley 2009, Mateus and Milán 2010). Avian tracks without hallux impressions have been interpreted from the Early Cretaceous, but these have digit II–IV divarications of 100–125 degrees and most are considerably smaller than Track 3 (Currie 1981; Lockley et al. 1992; Lockley et al. 2001; McCrea and Sarjeant 2001; Falk et al. 2010; Kim et al. 2012; Lockley et al. 2012). Hence without further persuasive evidence, we would rather err on the side of caution and interpret this track as that of a nonavian theropod, while also acknowledging an avian affinity. Ornithopods were also ruled out as potential tracemakers for this track and the other two tracks, despite their common representation as body fossils in the Eumeralla Formation, particularly from Dinosaur Cove (Rich and Vickers-Rich 1999, 2000; Rich et al. 2010). This elimination was based on relatively narrow digits ending with sharp clawmarks on all tracks, which are more typical of avian or nonavian theropods (Lockley et al. 1992, Lockley 2009).
Track 3 also has ridges on the anterior sides of digits II and IV, a mound posterior to digit II and an imbricated plate on the anterior-left side of digit III. These structures are consistent with the right foot rotating and lifting just before the tracemaker stepped to its left foot. However, this interpretation remains speculative in the light of how each track is disconnected from a trackway, and hence cannot be tested further.
As was the situation with theropod tracks interpreted from Milanesia Beach (Martin et al. 2012), this track defies an easy ichnotaxonomic designation, owing to its vague outline and lack of anatomical detail. Based on tridactyl tracks illustrated by Lockley (2009), the closest ichnogenera comparable in size and form are Columbosauripus or Irenichnites (Sternberg 1932; Currie 1981). Theropod tracks at Milanesia Beach, interpreted as ornith-omimid or oviraptorsaurid, fell into three statistically distinct size classes, with one significantly smaller than the other two, and the other two differing only slightly but designated as ‘intermediate’ or ‘large’ (Martin et al. 2012). Track 3 would have fallen into the ‘intermediate’-size category applied to the Milanesia Beach theropod tracks, and is nearly identical in form, implying similar tracemakers at each locality. Using the method refined by Henderson (2003) of 4.0 × track length = hip height, and assuming a theropod tracemaker, the calculated hip height for the tracemaker would have been 47 cm. This is smaller than inferred sizes for nonavian theropods represented by body fossils in the Eumeralla Formation, such as Timimus (Rich and Vickers-Rich 1994), an unnamed tyrannosauroid (Benson et al. 2010) and an unnamed spinosaurid (Barrett et al. 2011). Still, it is within the range of sizes calculated for theropod trackmakers at Milanesia Beach (Martin et al. 2012). Ornithomimids, oviraptorsaurids and other small maniraptors have been interpreted in the Eumeralla Formation on the basis of a few body fossils, including some from Dinosaur Cove (Rich and Vickers-Rich 1994; Currie et al. 1996; Benson et al. 2012). Hence, a coelurosaur is reasonable as an inferred tracemaker.