A large-scale assessment of skeletal and dental changes in Mesozoic mammals by a team from Oxford University, UK and Macquarie University, Australia has shown that mammal evolution during that period was up to ten times faster in the Middle Jurassic (200-145 million years ago) than it was at the end of it.
- Rates of morphological evolution and disparity are quantified in Mesozoic mammals
- Elevated rates prior to the Late Jurassic support a mid-Jurassic adaptive radiation
- Slower rates characterized the Late Jurassic to the end-Cretaceous
- Early evolution in Theria was exceptionally rapid, but then slowed dramatically
A series of spectacular discoveries have transformed our understanding of Mesozoic mammals in recent years. These finds reveal hitherto-unsuspected ecomorphological diversity that suggests that mammals experienced a major adaptive radiation during the Middle to Late Jurassic. Patterns of mammalian macroevolution must be reinterpreted in light of these new discoveries, but only taxonomic diversity and limited aspects of morphological disparity have been quantified. We assess rates of morphological evolution and temporal patterns of disparity using large datasets of discrete characters. Rates of morphological evolution were significantly elevated prior to the Late Jurassic, with a pronounced peak occurring during the Early to Middle Jurassic. This intense burst of phenotypic innovation coincided with a stepwise increase in apparent long-term standing diversity and the attainment of maximum disparity, supporting a “short-fuse” model of early mammalian diversification. Rates then declined sharply, and remained significantly low until the end of the Mesozoic, even among therians. This supports the “long-fuse” model of diversification in Mesozoic therians. Our findings demonstrate that sustained morphological innovation in Triassic stem-group mammals culminated in a global adaptive radiation of crown-group members during the Early to Middle Jurassic.
This is reminiscent of the so-called Cambrian explosion when newly-evolved multicellular organisms appear to have radiated rapidly into the new niches multicellularity had opened up, evolving different body plans, some of which were to come to predominate and others to go extinct, as they did so.
Of course, by the Mesozoic era, the basic vertebrate body plan was well established so what we have here is variations on the basic mammalian theme of warm-bloodedness, fur for protection and temperature control, and live births. What seems to have happened is that these basic characteristics then opened up a whole new range of possible niches for these early mammals to evolve into; niches that were either unoccupied or occupied by species less able to compete with mammals.
To those used to thinking in terms of evolution progressing at a relatively steady rate, these explosive radiations can look like something other than normal evolution, until the role of the environment in the evolutionary process is taken into account. I have pointed out many times in these blogs that mutations and variations themselves mean nothing unless 'interpreted' by a selective environment and a selective environment can either filter out less advantageous variation or allow through a more advantageous variation in that environment.
But, for an environment to give an advantage to a particular variation, the species must be capable of exploiting it. For example, while there may be masses of food at the bottom of a pond, a small mammal which is going to die of a low body temperature if it dives down for it can't occupy that niche. But, with fur to trap a layer of warm air, any mammal which can do so is going to be at an advantage, so there will now be selective pressure to become better at swimming and able to dive further, for longer. So, we could have a radiation into aquatic mammals.
But, similarly to the Cambrian explosion where there were ultimate winners and losers in an intensive period of evolution, with no clear winners emerging early on, so some of these early mammals appear to have radiated rapidly, only to lose out ultimately to something else. Maybe they radiated into evolutionary cul-de-sacs and became too specialised, unable to adapt to new competition with something that arrived by a different route and still had the potential to adapt.
One such order were the Multituberculates which evolved even faster than the average for the time - up to 13 times faster - to give a wide range of skeletal and dental changes initially, but which settled down to a basic rodent-like body plan by the Late Jurassic with some 100 different known species. But by 130 million years ago, the entire order had disappeared. The Multituberculates existed for about 120 million years becoming the most diversified and, on that test, the most successful of all mammalian orders. Ultimately, however, they were failures.
Evolution is an unemotional, unplanned, vicarious process entirely driven by the environment in which a species finds itself and that species potential for adaptation. An environment which favours one species or one order and which provides multiple opportunities for evolution and diversification, can, because of some entirely fortuitous change, maybe climatic, maybe the presence of a more successful competitor or even a virus, become an environment which dooms an entire order to extinction.
Unless an intelligent designer likes experimenting with different designs and then randomly exterminating them, and did this for millions of years before it thought of humans, there is no way this can honestly be represented as an an example of intelligent design. So, that just leaves creationists with the usual options of ignoring it, or being dishonest.
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