Dental Disparity and Ecological Stability in Bird-like Dinosaurs prior to the End-Cretaceous Mass Extinction

Abstract

The causes, rate, and selectivity of the end-Cretaceous mass extinction continue to be highly debated [1-5]. Extinction patterns in small, feathered maniraptoran dinosaurs (including birds) are important for understanding extant biodiversity and present an enigma considering the survival of crown group birds (Neornithes) and the extinction of their close kin across the end-Cretaceous boundary [6]. Because of the patchy Cretaceous fossil record of small maniraptorans [7-12], this important transition has not been closely examined in this group. Here, we test the hypothesis that morphological disparity in bird-like dinosaurs was decreasing leading up to the end-Cretaceous mass extinction, as has been hypothesized in some dinosaurs [13, 14]. To test this, we examined tooth morphology, an ecological indicator in fossil reptiles [15-19], from over 3,100 maniraptoran teeth from four groups (Troodontidae, Dromaeosauridae, Richardoestesia, and cf. Aves) across the last 18 million years of the Cretaceous. We demonstrate that tooth disparity, a proxy for variation in feeding ecology, shows no significant decline leading up to the extinction event within any of the groups. Tooth morphospace occupation also remains static over this time interval except for increased size during the early Maastrichtian. Our data provide strong support that extinction within this group occurred suddenly after a prolonged period of ecological stability. To explain this sudden extinction of toothed maniraptorans and the survival of Neornithes, we propose that diet may have been an extinction filter and suggest that granivory associated with an edentulous beak was a key ecological trait in the survival of some lineages.

A new species of troodontid theropod (Dinosauria: Maniraptora) from the Horseshoe Canyon Formation (Maastrichtian) of Alberta, Canada

“…With the exception of isolated teeth derived primarily from vertebrate microfossil sites (Larson et al 2010;Larson et al 2016;Larson and Currie 2013;Ryan et al 1998;Torices et al 2014), troodontid material is extremely rare in the Horseshoe Canyon Formation, despite over a century of intensive field collecting. TMP 1993.105.0001 has several distinctive features commonly attributed to troodontids, including a longitudinal groove on the dorsal surface that is defined laterally by a raised orbital rim, a deep, transversely oriented lacrimal buttress of the crista cranii, and a distinct oval olfactory bulb impression offset from enlarged cerebral hemispheres by long olfactory tracts (Currie 1987b;Evans et al 2014 To date, only three troodontid cranial bones have been recovered from the Horseshoe Canyon Formation: a dentary (Currie 1987a) and the two incomplete frontals described here (Eberth et al 2013).…”

“…Data was taken from Larson et al (2016) and updated with data from Torices et al (2014) with modifications to some of the identifications (Appendices 6-7) Torices et al 2014). Dental variables analyzed include 8 fore-aft basal length (FABL), crown height (CH), basal width (BW), mesial denticle length (ADL), and distal denticle length, (PDL).…”

“…Little skeletal material has been described, and skull material is particularly rare (Carpenter 1982;Fiorillo et al 2009;Makovicky and Norell 2004;Russell 1969;Zanno et al 2011), hindering assessments for taxonomic diversity in the group. Despite this, isolated troodontid teeth are relatively common through the Campanian and Maastrichtian of western North America (Cullen and Evans 2016;Larson et al 2016;Larson and Currie 2013). Historically, the genus Troodon has been reported to occur throughout the Campanian and Maastrichtian, and across most of western North America , based on referrals of isolated teeth and the few, and generally fragmentary, available fossils (Zanno et al 2011).…”

The last dinosaurs of Brazil: The Bauru Group and its implications for the end-Cretaceous mass extinction

“…A handful of small avian fossils have been found in the Marília Formation, including material belonging to enantiornithines, a group of primitive birds that thrived in the Cretaceous but did not make it into the Paleogene (Candeiro et al 2012a). As in North America (Longrich et al 2011), it appears as if these birds persisted to the end of the Cretaceous in Brazil but then expired in the same global firestorm that knocked out most of the dinosaurs, but spared a few lineages of the more advanced, better flying, faster growing, seed-eating birds (e.g., Brusatte et al 2015b, Larson et al 2016, which went on to blossom into the 10,000 + avian species still alive today.…”

“…3) With more fossils and a better grasp of the ages and relationships of the formations they are found in, more rigorous statistical studies of dinosaur diversity change will become possible. These have been hugely successful in western North America (e.g., Sheehan et al 1991, Pearson et al 2001, 2002, Fastovsky and Sheehan 2005, Campione and Evans 2011, Brusatte et al 2012, 2015a, Larson et al 2016 and Spain (e.g., Vila et al 2016), and are currently in progress in Romania (e.g., Csiki-Sava et al 2016b). Only through detailed, layer-by-layer sampling, constrained by a robust timescale, can changes in diversity, abundance, and evolutionary rates be calculated over time.…”