.. according to a forthcoming article in the Proceedings of the National Academy of Sciences. Specifically, this paper argues for a trend in increasing morphological complexity, observed in several different parallel lineages of Crustacea.
From the abstract:
The prospect of finding macroevolutionary trends and rules in the history of life is tremendously appealing, but very few pervasive trends have been found. Here, we demonstrate a parallel increase in the morphological complexity of most of the deep lineages within a major clade. We focus on the Crustacea, measuring the morphological differentiation of limbs. First, we show a clear trend of increasing complexity among 66 free-living, ordinal-level taxa from the Phanerozoic fossil record. We next demonstrate that this trend is pervasive, occurring in 10 or 11 of 12 matched-pair comparisons (across five morphological diversity indices) between extinct Paleozoic and related Recent taxa. This clearly differentiates the pattern from the effects of lineage sorting. Furthermore, newly appearing taxa tend to have had more types of limbs and a higher degree of limb differentiation than the contemporaneous average, whereas those going extinct showed higher-than-average limb redundancy. Patterns of contemporary species diversity partially reflect the paleontological trend. These results provide a rare demonstration of a large-scale and probably driven trend occurring across multiple independent lineages and influencing both the form and number of species through deep time and in the present day.
One could argue that the authors have identified a trend towards increasing number of parts in limbs and/or an increasing specialization of limb morphology. Some would argue that this is not an increasing complexity ... it depends on how one defines complexity.
Also, the effect is modest. Limb 'complexity' accounts for about 16% of the variation in species among these clades.
So, how did they measure complexity?
They use a measure of differentiation of limb types, scaled to total number of limbs. (They also looked at total number of limbs and total number of different limb types.)
I think this is a valid measure of something. As I suggest above, this could be specialization of limb type, for instance. I would have avoided the use of the word 'complexity' because it is both baggage-laden and vague. I would have used 'disparity in limb morphology.'
It does indeed seem that there is a series of parallel trends, though, no matter what one calls it. The graph says it all:
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