November 14, 2017 at 04:21AM

Today I Learned: 1) Mammals were dominant as large land animals *before* the dinosaurs. Well, not really -- but our non-dinosaurian ancestors were. Mammals are part of a group called the synapsids, which began as lizard-like creatures with a slightly weird skull. These were the biggest, baddest, I *think* most common large animals (amniotes, technically) during the Permian Period, and "reigned supreme" for some 46 million years. The Permian-Triassic extinction hit the synapsids really hard, as is typical for large-bodied groups in a mass extinction, and their extinctions left ecological niches open for large animals that the dinosaurs filled nicely. Only three clades of Synapsids survived; of these, one died out later in the Triassic, one survived as a large herbivore alongside the dinosaurs, and one shrank really quickly down to rat size and began specializing in insect predation. The last group, known as eucynodonts, would eventually survive the late Cretaceous extinction event and eventually give rise to modern mammals. There's something of an open question about why dinosaurs (properly, the immediate ancestors of the dinosaurs) were able to take over from the Synapsids. One theory is that the post-Permian world was relatively arid, and dinosaurs had better adaptations for low-water environments (uric acid excretion being a key example). This theory is rather controversial, though -- after all, there are some mammals today that have adapted extremely well to arid environments, so why wouldn't the old Synapsids? 2) Another mammal fact -- there's a theory called the Nocturnal Bottleneck that says that many of the common features of mammals *are* common features of mammals because our last common ancestor was nocturnal, and was for quite some time. Evey mammal that isn't nocturnal had to evolve that way from the nocturnal ancestor. The major categories of evidence for the nocturnal bottleneck are: mammals, as a whole, have highly developed hearing and smell, but not great vision (especially in color); mammals have fur, tissues specialized in rapid heating, and extremely active mitochondria, which would help stay warm at night; mammals don't have particularly good UV protection; and burrowing behavior appears to be a basal mammalian trait. 3) Here's a nice evolution number to know -- it takes about 10 million years for a dormant mammalian gene to become completely unrecoverable by random disabling mutations.