Glen J. Kuban
Originally Published in the MAPS Expo, April 1997
Ichnology, the study of fossil tracks, burrows, eggs, and other trace fossils, has experienced a major revival during the last 15 years. Although once regarded as minor curiosities, vertebrate footprints in particular have become increasingly appreciated as rich sources of information on the behaviors, locomotion, postures, foot anatomy, paleoenvironments, chronologies, and geographic distributions of many prehistoric animals (Lockley, 1991; Lockley and Gillette, 1989; Thulborn, 1990; Farlow, 1987; Kuban, 1994).
Although relatively little work has been done in the application of fossil tracks to questions about extinctions, there is great potential for future work in this area. One major advantage of working with footprints is that in most cases one can be sure the trackway is in its original position, and has not been reworked (redeposited from older or younger sediments). The same cannot be said for disarticulated teeth and bones, and even some trace fossils such as egg fragments, all of which can be readily transported and redeposited. Setting aside birds, all claims of post Cretaceous dinosaurs (Rigby, 1985, 1987; Rigby et al, 1987; Sloan et al, 1986) have been plagued by the possibility of reworking (Argast et al, 1987; Bryant et al, 1986; Behrensmeyer, 1982). However, even a single in situ dinosaur trackway younger than the K-T boundary would provide powerful evidence for Tertiary dinosaurs. As it is, footprints already provide evidence for dinosaurs closer to the K-T boundary than any other kind of evidence.
Horner and Lessem (1993) state that no dinosaur remains are found within about 3 meters of the iridium band taken to represent the K-T boundary in Montana. They call this the three-meter gap, and suggest that it represents perhaps 100,000 years. Some have interpreted this as support for the position that the Cretaceous extinction was a gradual rather than sudden event. However, the 3-meter gap does not exist if one includes footprints. The youngest-known dinosaur fossils so far are actually a set of hadrosaur tracks found by Lockley and Hunt (1995, p. 23) in a sandstone shelf only 37 cm (less than 15 inches) below the iridium layer near Ludlow, Colorado, in the Raton Formation.
This discovery provides obvious proof that at least one group of dinosaurs was alive and well very near the end of the Cretaceous--and narrows the gap of missing dinosaur remains by an order of magnitude.
In fact, it may narrow the gap even more than that if the time frame rather than the sediment thickness is considered. The track bearing layers containing the hadrosaur tracks, which are rich in plant remains and thought to represent a coal swamp or peat bog environment. Based on sedimentation rates in similar environments today, Lockley and Hunt (1995) estimate the 37 cm may represent only a few thousand years or less. Additional hadrosaur tracks have also been found within the 3 meter gap, including a set 59 cm below the K-T boundary at the same site, and another Raton Formation Site about 12 miles south, which contains both hadrosaur and ceratopsian tracks within 1 or 2 meters of the K-T boundary (Lockley and Hunt, 1995).
It is exciting to contemplate the potential tracks have for further clarifying which dinosaurs survived until the close of the Cretaceous, and how sudden or gradual the decline was--not to mention the potential of confirming post-Cretaceous dinosaurs. Indeed, the problem with using body fossils only to study the timing of the K-T extinctions, is that isolated bones and teeth might have been reworked both below and above the boundary, possibly skewing any conclusions about the timing of the event or animals present. However with tracks, this problem is largely eliminated.
Tracks have also been useful in dating earlier Mesozoic events. As noted by Lockley and Hunt (1995), the track record shows a dramatic faunal change turnover at the Triassic-Jurassic boundary. Moreover, in many areas, and especially the east coast of the U.S., the diversity and abundance of dinosaur footprints and other vertebrate traces far exceeds that from body fossils (Weishampel and Young, 1996). Although much work needs to be done sorting out the great confusion of track names accumulated over the years, the potential for further clarifying the Triassic- Jurassic changes and dinosaur evolution in general is great.
Of course, work is also being done on the relevance of invertebrate traces to extinction events, such as Ekdale and Bromely (1984), and Wignall and Twitchett (1996), which help define the status of various invertebrate groups near extinctions boundaries, the environmental conditions associated with them, and perhaps even the cause of the extinction. Nevertheless, when it comes to vertebrates, dinosaurs clearly make the biggest impressions, and studies of their tracks should continue to elucidate the timing and nature of the K-T extinction and other great events in prehistory.
I thank James O. Farlow and Martin Lockley for helpful advice and references, and Ava and Michel Farivar, Gary R. Kuban, and Pat Newhauser for reviewing the manuscript.
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