Trampled riverbanks at Tumbler Ridge fill Canada's blank in the Coniacian dinosaur record

Three and a quarter million years is the size of the hole. The Coniacian runs from 89.75 to 86.5 million years ago ^[4], which is 3.25 million years ^[5], and until this report Canada had no dinosaur tracks from any of it ^[6].

The reason becomes plain once you ask where track-bearing rock comes from. Tracks can only be made and preserved in nonmarine, terrestrial sediment ^[11], and between roughly 95 and 84 million years ago, an interval of about 11 million years ^[15], sea level stood high enough to shove shorelines west against the foothills of the Rockies ^[14]. Global sea level in the mid-Cretaceous sat about 100 metres above today's ^[2], under a greenhouse climate with no significant polar ice caps ^[25]. Squeeze the coastal plain and you squeeze the archive. In western Canada, Coniacian rock is marine everywhere except the small Tumbler Ridge area, where one river system built a delta that protruded east into the seaway ^[12]; the terrestrial strata of the Marshybank Formation are what is left of it ^[13].

That makes Tumbler Ridge something close to a control for the sparse-record question. If mid-Cretaceous North America genuinely held few large dinosaurs, the one patch of land-laid rock of the right age should look thin as well. It does not. Ornithopod prints dominate ^[16], narrower clawed tracks interpreted as theropod come second ^[17], and a few rounded prints with four or five indistinct digits may be ankylosaurian ^[18]. The prints survive as natural casts on the undersides of thin flood sands, called crevasse splays, that spread across soft mud ^[20]. The assemblage also resembles what the contemporaneous Frontier Formation of southwestern Montana has yielded ^[22], so this is not one outcrop's quirk.

The other half of the explanation is logistical. Guy Plint noticed the tracks in 1988, and the comprehensive study only began in 2022 ^[8], a wait of 34 years ^[10] that was mostly a helicopter-charter problem. The region was not unknown in the meantime: its Cretaceous terrestrial deposits have produced tracks of giant crocodiles, birds and pterosaurs ^[23], and the area became the Tumbler Ridge UNESCO Global Geopark in 2014 ^[24]. The results appear in Historical Biology ^[9].

The thing a track census does not measure is species. Feet make tracks, and these feet were near water, on mud soft enough to register them, in the rare places a flood happened to bury the record ^[20]. Ornithopod dominance among the prints says which large animals walked the levees and splays of a low, wet delta of shallow lakes, mires and floodplains under dense conifer vegetation ^[21]. It is not a headcount of a fauna. Nor do tracks substitute for skeletons, and the mid-Cretaceous bone record for this interval remains sparse ^[3].