Chipped bone in fossil dung puts wolf-sized scavengers at Ashfall's ash-choked waterhole

An absence counts as evidence only when you can say what would have been preserved had the animal died there. Ashfall clears that bar better than most sites do. Larger animals survived the initial ashfall and then suffocated slowly as dust smothered their food and stayed airborne, and their skeletons were preserved in the ash ^[12]. The first fossil Mike Voorhies found in 1971 was a baby rhino jaw still connected to the rest of its skull and skeleton ^[13]. An animal roughly the size of a wolf ^[6] that died on that surface should be recoverable in comparable condition. So the missing borophagine skeletons are a statement about which bodies were lying there at the end, not about what the ash could hold.

Each link in the identification chain is an inference rather than a specimen. The footprints in the ash are believed to be borophagine ^[1]. The tooth marks on the rhinos, horses and camels that died there seem to match the dogs, according to park superintendent Sam Matson ^[2]. The coprolites carry chips of bone, which Matson says is what identifies the producer as one of the dogs ^[5]. The thing this does not tell you is which species left the marks, how many individuals fed, or how long after the ash settled they arrived. Matson's own sentence stops well short of survival: no skeletons of the scavengers, so we do not know if they survived the event ^[5]. The article's framing goes further, reading the absence as animals that fed and left the scene before dying ^[4].

Then there is the denominator. The park covers 360 acres near Royal, Nebraska ^[17], a structure called the Rhino Barn stands over part of the bone bed ^[19], and Matson estimates another 10 to 15 summers of work in the current excavation area alone ^[16]. That is 55 years of collecting since the 1971 discovery, by my arithmetic ^[21], with new material still turning up every summer on Matson's account ^[22]. Skeletal remains may yet appear, possibly in fossil dens ^[7]. Dens matter here for a specific reason: search effort has concentrated on the death assemblage in the ash, which is precisely where an animal that walked away would not be.

The geology explains why the trap is so rich. Regionally the ash is about a foot thick, some 30 centimetres, but at Ashfall it runs 8 to 10 feet, 2.4 to 3 metres ^[9], which is eight to ten times the regional blanket ^[20], consistent with a depression that held water seasonally and collected drifting ash ^[10]. Note that the waterhole reconstruction also leans on absences, the missing fish fossils and the missing channeling in the strata, but there the negative is paired with positive evidence in the same beds: pond turtles, reed-like stems, diatoms ^[11]. The scavenger case has its own positive evidence in the traces, and no comparable control on the alternative that the bodies sit in ground nobody has opened.

My reading, with its conditions stated: the tracks, tooth marks and bone-chipped coprolites are enough to place bone-crushing dogs on the ash feeding on carcasses, and they are not yet enough to say those dogs outlived the eruption 900 miles to the west in what is now southwest Idaho ^[8].