How do you calculate a peak that nobody observed? And why do the most obvious clues lead nowhere?
The pattern was known, but the tool was missing
One pattern appears time and again in the fossil record. An animal group emerges and, soon after its arrival, displays its greatest range of forms. Specialists call this early high disparity. It means that the members of a group differ most strongly in body shape right at the beginning.
According to the team, no method had previously been able to reconstruct such trajectories precisely. So the researchers developed a new one. It combines two established computational models of evolution into a single trajectory.
Accelerator and brake in one model
The first computational model is called ‘Early Burst’. Under this model, a group’s traits diverge very rapidly at the beginning. The second is an Ornstein–Uhlenbeck model, which acts like a brake, keeping the differences within limits afterwards.
Together, the two models trace a curve like a steep mountain path. First, the differences in body shape climb sharply, reaching the summit early in the group’s history. Then the path slopes downwards as the brake pulls those differences back together.
In the skull test, the new model beats almost every rival
The team chose ichthyosaurs as its test case, specifically data on the length of their skulls. The result is clear. In almost every case, the new model described the data better than the other evolutionary models tested.
The calculation places the summit around 231.1 million years ago, in the middle of the Triassic. This figure is the median: the estimate sitting exactly halfway through all the calculated values. By then, only around 12.4 per cent of the lineage’s history had elapsed.
That produces a striking picture of the Triassic seas. Long-skulled ichthyosaurs shared the water with relatives whose heads were markedly shorter. Afterwards, the brake in the model pushes the range of skull lengths back down.
The obvious suspects fail the test
Such an early peak demands an outside trigger. The team therefore searched four datasets for turning points: moments when a trajectory suddenly changes. It examined sea level, sea-surface temperature, rock area and the ages of sedimentary sequences.
Two clues came closest to the summit. The temperature curve has a turning point 237.2 million years ago, while the boundaries of the sedimentary sequences fall at 238 million years ago. Both therefore precede the calculated peak by six to seven million years.
Here, the convenient explanation collapses. None of these turning points coincided with the peak to a statistically significant degree.
The date stands, but the cause remains open
Two points temper the excitement. The study is a preprint, meaning that specialists have not yet reviewed it, so its results remain preliminary. It also relies solely on skull lengths and says nothing about overall body shape or the number of species.
Even so, the new model delivers something that was missing before: a calculated date for a pattern that repeatedly appears in the fossil record. For ichthyosaurs, that date is around 231.1 million years ago in the Triassic, when only about 12.4 per cent of their evolutionary history had passed.
According to the model’s median, ichthyosaur skull lengths spread farther apart around 231 million years ago than at any other time. What triggered this early surge remains unresolved. In this analysis, at least, neither the ocean nor the climate provides a matching signal.
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