DETERMINANTS OF EARLY SURVIVAL IN MARINE ANIMAL GENERA

Recent work has shown that animal genera, after being geographically restricted at their inception, tend to expand in geographic range as they age, then contract in range to become restricted once again before their extinction. Other properties, for example environmental breadth and species richness, show similar trajectories. The extent to which this "rise-and-fall" pattern reflects the simple expectations of a random walk, ecological interaction, or other factors remains unknown. But, given that about half of marine animal genera are confined to their stage of first appearance, why do some genera and not others even have a long-term history to speak of?

Using occurrence data from the Paleobiology Database, we find that the initial rate of expansion of geographic range has the strongest effect on survival beyond the stage of first appearance, followed by environmental breadth, with the effect of species richness weakest on average. Patterns of selectivity vary substantially over the Phanerozoic, with environmental breadth relatively more important in the early Paleozoic than later, and species richness more important during the Cenozoic than earlier. During the Mesozoic, survival is only weakly selective with respect to range, breadth, and richness; it therefore seems that there is a considerable stochastic element to early survival during this time span.

By following genera beyond their stage of first appearance, we find that selectivity of survival with respect to all factors strengthens as cohorts age and genera become progressively more differentiated from one another in range, breadth, and richness. This may help account for a previously identified statistical effect of genus age on the chances of survival.