Evolution of ecospace occupancy by Mesozoic marine tetrapods

Ecology and morphology are different, and yet in comparative studies of
fossil vertebrates the two are often conflated. The macroevolution of
Mesozoic marine tetrapods has been explored in terms of morphological
disparity, but less commonly using ecological-functional categories. Here
we use ecospace modelling to quantify ecological disparity across all
Mesozoic marine tetrapods. We document the explosive radiation of marine
tetrapod groups in the Triassic and their rapid attainment of high
ecological disparity. Late Triassic extinctions led to a marked decline in
ecological disparity, and the recovery of ecospace and ecological
disparity was sluggish in the Early Jurassic. High levels of ecological
disparity were again achieved by the Late Jurassic and maintained during
the Cretaceous, when the ecospace became saturated by the Late Cretaceous.
Sauropterygians, turtles and ichthyosauromorphs were the largest
contributors to ecological disparity. Through the Mesozoic, we find that
established groups remained ecologically conservative and did not explore
occupied or vacant niches. Several parts of ecospace remained vacant for
long spans of time. Newly evolved, radiating taxa almost exclusively
explored unoccupied ecospace, suggesting that abiotic releases are needed
to empty niches and initiate diversification. In the balance of
evolutionary drivers in Mesozoic marine tetrapods, abiotic factors were
key to initiating diversification events, but biotic factors dominated the
subsequent generation of ecological diversity.