Testing co‐evolutionary hypotheses over geological timescales: interactions between Mesozoic non‐avian dinosaurs and cycads

Abstract

The significance of co-evolution over ecological timescales is well established, yet it remains unclear to what extent co-evolutionary processes contribute to driving large-scale evolutionary and ecological changes over geological timescales. Some of the most intriguing and pervasive long-term co-evolutionary hypotheses relate to proposed interactions between herbivorous non-avian dinosaurs and Mesozoic plants, including cycads. Dinosaurs have been proposed as key dispersers of cycad seeds during the Mesozoic, and temporal variation in cycad diversity and abundance has been linked to dinosaur faunal changes. Here we assess the evidence for proposed hypotheses of trophic and evolutionary interactions between these two groups using diversity analyses, a new database of Cretaceous dinosaur and plant co-occurrence data, and a geographical information system (GIS) as a visualisation tool. Phylogenetic evidence suggests that the origins of several key biological properties of cycads (e.g. toxins, bright-coloured seeds) likely predated the origin of dinosaurs. Direct evidence of dinosaur-cycad interactions is lacking, but evidence from extant ecosystems suggests that dinosaurs may plausibly have acted as seed dispersers for cycads, although it is likely that other vertebrate groups (e.g. birds, early mammals) also played a role. Although the Late Triassic radiations of dinosaurs and cycads appear to have been approximately contemporaneous, few significant changes in dinosaur faunas coincide with the late Early Cretaceous cycad decline. No significant spatiotemporal associations between particular dinosaur groups and cycads can be identified - GIS visualisation reveals disparities between the spatiotemporal distributions of some dinosaur groups (e.g. sauropodomorphs) and cycads that are inconsistent with co-evolutionary hypotheses. The available data provide no unequivocal support for any of the proposed co-evolutionary interactions between cycads and herbivorous dinosaurs - diffuse co-evolutionary scenarios that are proposed to operate over geological timescales are plausible, but such hypotheses need to be firmly grounded on direct evidence of interaction and may be difficult to support given the patchiness of the fossil record.

Biotic and environmental dynamics through theLateJurassic–EarlyCretaceous transition: evidence for protracted faunal and ecological turnover

“…This was based on an apparent ecological shift from higher browsing sauropod-dominated faunas to those composed of more diverse ornithischians. However, numerous recent discoveries in the Late Jurassic to Early Cretaceous interval indicate that such an ecological turnover is not as clearly defined as originally proposed, and the spatiotemporal structure of any such turnover does not support co-evolutionary relationships between herbivorous dinosaurs and either the origin of angiosperms (Barrett & Willis, 2001;Butler et al, 2009a), or diversificiation of gymnosperms (Butler et al, 2009b).…”

How has our knowledge of dinosaur diversity through geologic time changed through research history?

“…This is compounded by a difficulty in assigning a large number of taxa to specific stage bins, which unfortunately excludes many of them from our analyses (Supplemental Information 1). Differences in diversity patterns between sauropodomorphs and ornithischians have classically been interpreted as being due to exclusive competition between the two main herbivorous dinosaurian subtaxa (Butler et al, 2009), with an explosive radiation in ornithischians during the Early Cretaceous resulting from the apparent decline in diversity of sauropodomorphs. In fact, the J/K transition represents a major extinction 'event' for sauropodomorphs, reflecting the decline of non-neosauropods, diplodocoids and basal macronarians Tennant, Mannion & Upchurch, 2016b).…”

“…In particular, to our knowledge, no one has yet tested this potential influence using an occurrence-based tetrapod dataset, such as those available from the Paleobiology Database. This is important, given that a wealth of recent studies, and in particular on tetrapod groups, have focussed on estimating diversity patterns through geological time and interpreting what the potential drivers of these large-scale evolutionary patterns might be (Butler et al, 2009;Butler et al, 2011;Mannion et al, 2015;Nicholson et al, 2015;Benson et al, 2016;Grossnickle & Newham, 2016;Nicholson et al, 2016;Tennant, Mannion & Upchurch, 2016a;Brocklehurst et al, 2017). Many of these studies have employed subsampling methods that are sensitive to changes in the shape of the taxonomic abundance distribution, which we would expect to change in a non-random fashion based on new fossil discoveries through time as they are published (Benton et al, , 2013Benton, 2015) (e.g.…”

Cyanobacteria in Terrestrial Symbiotic Systems

“…Dinosaurs have been proposed as key dispersers of cycad seeds during the Mesozoic, and temporal variation in cycad diversity and abundance has been linked to faunal changes. However, when assessing the fossil evidence, Butler et al (2009) could not find unequivocal support for coevolutionary interactions between cycads and herbivorous dinosaurs.…”