Module 2 Flashcards
Early Tetradpop Relationships, Origin of the Synapsida; Early Synapsids, Mammaliaformes, and Mammals
True or false? Synapsids are not true mammals, but instead still considered dinosaurs.
False. Synapsids are neither true mammals nor dinosaurs. They are mammal-like reptiles.
Describe the origins of mammals, from the first tetrapods to the innovation of the amniotic egg.
During the Devonian period (~400 mya) when the first Tetrapods make their journey to life on land, the environment is warm and humid. This allows for the delicate, soft skinned, gill-having tetrapods to survive on land. This period is also characterized by extremely high oxygen levels, due to the establishment of land plants since their debut 505 mya, allowing for these creatures with primitive, underdeveloped lungs to survive.
By the Permian period 286mya (Formation of Pangea; right before the Triassic period and our first true mammals), it has become much drier and cooler. For the amphibian/reptile-like mammals to stay moist and have viable offspring, they underwent some major evolutions relevant to modern day eutherian mammals. These adaptations were scaled skin and the amniotic egg (so they could lay eggs on land and not limited to the water).
Identify the selective pressures on the planet present giving rise to the clade Synapsida and the “mammal-like reptiles”.
What innovations were needed (and when did they appear) for vertebrate life to survive in a fully terrestrial environment?
Amphibian/reptile-like mammals adapted scaled skin and the amniotic egg (so they could lay eggs on land and not limited to the water) to stay moist and have viable offspring. For example, the Dimetrodon has scales to avoid desiccation and a large fin on its back that is highly vascularized, likely to better absorb heat from the sun in thermoregulation.
Other key innovations during this time period include: thoracic breathing (ribcage expansion in breathing that will give way to diaphoretic breathing later on), water-conserving kidneys (loops of henley), internal fertilization, and the beginnings of teeth differentiation (competitive edge for predators).
Contrast the innovations made by the Synapsids to other reptilian groups.
Synapsids and reptiles both developed the amniotic egg and scaled skin to avoid desiccation in this new dry, hot climate. This is what differentiates Amniota from amphibians and other ancestral tetrapods.
Synapsids specifically evolved the infratemporal fenestra, which are hole(s) in the skulls of all mammal-like reptiles excluding turtles, developed for lighter skulls and more room for greater muscle mass. This adaptation was crucial for synapsids to hold their heads up outside of the low-gravity, aquatic environments of their ancestors.
What was the environment like when the first reptiles walked the earth, prior to the formation of Pangea, the first mammals, or the first dinosaurs.
The early Carboniferous period was relatively warm and wet with diverse terrestrial flora, including ferns and cycads (prehistoric “trees”).
What are tetrapods? What is the lobe-finned fish? How are these related?
Tetrapods are four-legged vertebrates that were first to walk on land. They/we evolved from fish in the late Devonian period and would soon give rise to amphibians.
The lobe-finned fish is a living example of the early tetrapod species that moved onto land. A type of Lobe-finned fish is the lung fish, which still possesses both gills and lungs.
We can observe the evolution from a Ray-finned fish to the lung fish to modern living tetrapods (such as humans) through the evolution of limbs. Over time, we witness not only the elongation of bones but also the growth of additional bones (increase in digits)!! We describe 3 stages: pectoral fin, transitional foot-like structure, and then the hind limb.
Therefore, this tells us that by the late Devonian period “mammal” ancestors are on land.
Define what makes a true mammal.
Much of the literature and discussions refer to Therapsids as mammals. However, when we are discussing “true mammals” specifically, Synapsids-Therapsids-Cynodonts are other tetrapods.
Mammalia are characterized by hair, mammary and skin glands, molars and jaw action designed for shearing, and derived mammalian skeletal characteristics. Early mammalian groups arose in the Late Triassic period (followed by the Monotremes, marsupials, then eutherians–in that order).
Prototheria and Theria (monotremes, marsupials, and eutherians) differ from early mammalian groups by having three ear ossicles, modified vertebrae, and longer bones.
Marsupials diverge from monotremes (Prototheria) by possessing a modified brain case and vertical lymphatic membrane.
Lastly, “true mammals” (Eutherians or placental mammals) possess chorioallantoic placenta; longer gestation; and brown adipose tissue. These are the defining features for a “true mammal”.
Describe the changes leading to the first of the mammaliaformes and then the first true mammals (Mammal).
One of the skeletal features characteristic of mammals is the pelvic and pectoral girdles which allows for the movement of limbs underneath the body rather than out the side as seen in reptiles. These skeletal characteristics were derived from therapsids, specifically the Dinocephalians clade (these bear-sized therapsids with body structure similar to that of a rhino and thick skulls). This is among other mammal features derived from early cynodonts during the Triassic period. Cynodonts have enlarged dentary bones, reduced post-dentary bones, postcanine teeth that are well developed and a complete secondary palate. Leading to the several skull and teeth acquired features that are retained in Mammalia.
List of Cynodonts (in order of lineage).
Cynodontia is a clade of eutheriodont therapsids that first appeared in the Late Permian, and extensively diversified after the Permian–Triassic extinction event. Mammals are cynodonts, as are their extinct ancestors and close relatives, having evolved from advanced probainognathian cynodonts during the Late Triassic.
