Fossil-record rates put an early speciation burst at the start of the archosauromorph radiation

This reconstruction reports a pattern, and the cause is a separate finding. Speciation was high early in the archosauromorph radiation, then fell, while extinction rates climbed ^[7]. Studies built on family trees of the same group have generally produced that same early-burst shape, and the processes behind it have stayed unclear ^[4]. Nothing in the Phys.org account of the work suggests the phylogenetic version of the pattern was an artifact of how those trees are built; the fossil-based rates, published in Proceedings of the Royal Society B, recover the same shape ^[6][7].

Roland B. Sookias, the paper's first author, set out the candidate explanations to Phys.org ^[18]. "One hypothesis is that this pattern occurs because 'niche space,' the different ecological opportunities available, gets filled up," he said ^[10]. The second comes from the unified neutral theory of biodiversity and biogeography that Hubbell published in 2001: early on there are more individual animals per species, so speciation events outnumber extinctions, and once species are numerous each one has a smaller population and is likelier to go extinct ^[11]. The third is body size, since larger animals generally reproduce more slowly than smaller ones ^[12]. "We already know that during radiations, the maximum size of animals tends to get bigger, probably simply by chance because they usually start off small, so we also might expect speciation to slow down because there are more big animals," Sookias said ^[13].

The rising extinction rate is where those accounts can be pulled apart. Niche filling, as Sookias described it, is a claim about ecological opportunity for new species ^[10]. The neutral mechanism predicts extinction going up as species multiply and each population thins ^[11]. The reconstruction has extinction going up ^[7]. The account does not report a model comparison, the reconstructed rate values, or the uncertainty around them ^[19].

It also carries two dates for the same event: about 252 million years ago in its own framing ^[1], and "some 255 million years ago" in Sookias's quoted remark on the end-Permian extinctions ^[9], three million years apart ^[17].

Sookias tied the reason for caring to the present. "Understanding these radiation events thus helps us understand the origins of the amazing biodiversity around us and predict the effects of future mass extinction events, like the one humans are causing now," he said ^[15]. The timescales under that inference are enormous. The era these reptiles founded ran from 252 to 66 million years ago ^[3], a span of 186 million years ^[16], and the interval studied here reaches from the late Permian to the Early Jurassic ^[14].