These ideas are not mutually exclusive, of course, and more research is needed. Other evidence suggests that these changes were a byproduct of early mammals’ increasing brain size. Some evidence suggests that the change in the quadrate-articular complex improved hearing. We don’t fully understand why these changes happened. Only in recent years has it become apparent that several lineages of synapsids, including mammals, replaced their quadrate-articular jaw joint with a dentary-squamosal joint. Abbreviations used: a-articular, d-dentary, q-quadrate, s-squamosal. So, over time, the synapsids’ quadrate-articular jaw joint (which the rest of the tetrapods possess) was replaced by a dentary-squamosal joint (which all living mammals possess), while the quadrate and articular migrated, shrank, and became part of the complex of middle ear bones. They became increasingly smaller and eventually migrated into the ear region, where they became the “hammer” and “anvil” of the ear. Soon, the quadrate and articular lost their function in jaw articulation and even their position in the jaw as they evolved. This unusual paired condition did not last long, though. Skull of Probainognathus, an early synapsid. The squamosal bone was positioned alongside the quadrate in the upper jaw, and the dentary was positioned alongside the articular in the lower jaw. Originally the quadrate and articular bones formed the jaw joint, but these synapsids (e.g., Probainognathus) evolved a second pair of bones involved in the jaw articulation. How did this new jawbone configuration evolve?įor reasons we don’t fully understand, several lineages of synapsids - including the one that would eventually give rise to the mammals - began to evolve changes in the jaw joint. But mammals today, including humans, use two different bones, called the squamosal and the dentary, to make this connection. Like birds, crocodiles, turtles, snakes, lizards, amphibians, and most fishes, the earliest synapsids had a bone in the back of the skull on either side called the quadrate that made the connection with the lower jaw via a bone called the articular. Mammal phylogeny from The Tangled Bank, used with permission of the author, Carl Zimmer, and publisher, Roberts & Company, Greenwood Village, Colorado. And of course all mammals (the clade of synapsids still alive today) reproduce using an amnion, and those that lay eggs (e.g., the platypus and echidna) produce amniotic eggs. All reptiles (including birds) have eggs with amniotic membranes (which some lay and others retain inside their bodies until hatching). Synapsids and reptiles are two distinct groups of amniotes, animals that produce young that are enveloped with a membrane called an amnion that prevents desiccation. Sometimes synapsids are called “mammal-like reptiles ” however, that is misleading because synapsids are not reptiles. Information on controversies in the public arena relating to evolutionĪll the animals you see on this evogram are synapsids, the group that gave rise to the mammals.Alignment with the Next Generation Science Standards.The big issues – Pacing, diversity, complexity, and trends.Macroevolution – Evolution above the species level.Microevolution – Evolution within a population.Mechanisms: the processes of evolution – Selection, mutation, migration, and more.The history of life: looking at the patterns – Change over time and shared ancestors.An introduction to evolution: what is evolution and how does it work?.
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