THE DINOSAUR THAT CAME IN FROM THE COLD: THE OLDEST DEFINITIVE ORNITHISCHIAN DINOSAURS AND ORIGIN OF THEIR ABRUPT APPEARANCE

One of the most impactful apparent origination events of the entire Early Mesozoic is the abrupt appearance of undoubted ornithischian dinosaurs immediately after the end Triassic mass extinction event (ETE). All contenders for earlier Triassic ornithischians either lack clear apomorphies of that group or have proven to be younger (1, 2). Here we describe the oldest definitive evidence of ornithischian dinosaurs, consisting of trackways of Anomoepus and teeth from eastern North American rift strata that overly the oldest basalt flows and thus just post-dating the ETE. Based on the zircon CA-ID-TIMS U-Pb ages from the stratigraphically-surrounding lava flows (3), and astrochronology of the containing lacustrine strata (4) the age of this ornithischian material ranges from 201.5 to 201.3 Ma (latest Rhaetian to early Hettangian). Anomoepus displays several ornithischian autopod apomophies (5), notably a distinctive, functionally tridactyl pes in association with a pentadactyl manus with sub-equal digits showing it is was made by a non-heterodontosaurid basal ornithischian similar to Eocursor, Lesothotosaurus, or Kulindadromeus. The few teeth found in these deposits (Fundy Basin) (6) are consistent with this interpretation. We posit it is not parsimonious to propose ornithischians evolved in the few thousands or tens of thousands of years between the ETE and these globally oldest occurrences. Rather, we argue that they evolved much earlier in the cold, high latitudes, plausibly the paleoarctic, where there was abundant stable vegetation, albeit with freezing winters (7). And, with apologies to John le Carré, they only migrated to lower latitudes, out of the cold, after their pseudosuchian (and perhaps silesaurid) competitors were extinct.

1) PE Olsen et al. (2011) EETRSE 101:201. 2) MG Baron (2017) Hist. Bio. 38:967. 3) TJ Blackburn et al. (2013) Science 340:941. 4) PE Olsen et al. (2019) PNAS 116:10664. 5) PE Olsen & NG McDonald (2022) in Dinosaur State Park, JO Farlow (Ed) in revision. 6) H-D Sues & PE Olsen (2015) Atlantic Geol. 51:139. 7) PE Olsen et al. (2022) Sci. Adv. 8:eabo6342.