The Vertebrate Invasion of the Land II: Synapsid amniotes
Amniota: the most recent common ancestor of mammals and birds and all of its descendants. Amniotes represent a major departure in the evolution of full terrestriality in vertebrates, and are the first vertebrates fully to break the dependance on bodies of water.
Who are the amniotes?
- The air-breathing amniotic egg. Enclosed the amniote embryo in a private pond during its development. The egg has:
- A shell (leathery or calcified)
- Three extraembryonic membranes that mediate gas exchange.
- Claws or nails at the end of digits. This novelty allows digging, and burying of eggs.
- Complete loss of water-breathing larvae.
- Complete loss of lateral line system
- Two sacral vertebrae.
- Internal fertilization.
In older literature non-mammalian synapsids are often referred to as "mammal-like reptiles." In the age of phylogenetic systematics, this just doesn't fly. Members of Reptilia are, by definition, not synapsids and vice versa. If you use this archaic term on an exam, I will have to suppress the urge to flunk you. Just thought you should know.
- Synapsida: Mammals and their fossil relatives
- Sauropsida: Reptiles and their fossil relatives
The interval from the Pennsylvanian to early Triassic saw synapsids dominating the world's land faunas. A great diversity of synapsids lived, and inaugurated three major trends in synapsid evol.
- Synapomorphy: Single pair of temporal (infratemporal) fenestrae.
- Biological meaning: Enhanced ability to accomodate powerful jaw muscles.
- Several lineages developed various patterns of fenestration, all aimed at same biological goal. Difference w/ synapsids was that they got there first.
- Among earliest known amniotes are synapsids. (But note, their sister taxon, the sauropsids, must have been present.) Date from Early Pennsylvanian. Synapsids quickly occupied the niches of "large bodied land animals". As such they occupied top of food chain.
- Synapsids also produced first amniote herbivores.
- Enlargement of the infratemporal fenestrae. Ancestrally, the endocranium lies within and beneath the dermal bones of the skull. In synapsids, the dermal bones become integrated with the endocranium, forming a tubular braincase. At the same time, the infratemporal fenestrae enlarge, exposing the braincase.
- Transformation of the bones of the back half of the jaw into acoustic structures.
Exactly when and how is very speculative, but we occasionally luck into pieces of evidence that indicate that this was a prolonged and ongoing process.
- Not all tetrapods have an impedance matching ear. In fact, it seems to have evolved independently at least four separate times in:
- Twice in sauropsids (turtles and their relatives, and the last common ancestor of lizards, crocs, and birds)
- In each group, the the ear structure is distinct. Synapsids are unique in incorporating bones of the jaw. As we climb the synapsid tree, we see these bones become more drastically modified as organs of hearing.
- Regarding jaw bones - Recall that the ancestral jaw joint was formed by the palatoquadrate and Meckel's cartilage. Amond tetrapods, these cartilages ossify into a number of bones. We care about:
- Specialization of regions of the tooth row. The common ancestor of Amniota apparantly had canine teeth, however these were simply enlarged teeth in a row of otherwise similar teeth. Among synapsids, there is a trend toward the specialization of pre and post-canine teeth.
- Behavioral specialization. Somewhere between the last common ancestor of Amniota and that of Mammalia, synapsids acquired
- parental care of young
Caseidae: Including Casea and Cotylorhynchus (above) Pennsylvanian age large herbivores with a very broad flat torsos and improbably small heads. Nevertheless, the synapsid synapomorphy is plainly visible.
Varanopseidae: Small-bodied, primitive looking synapsid predators (see skull. Namre derives from the general similarity of body profile to modern varanoid (i.e. "monitor") lizards. Unremarkable but for the recent discovery of evidence of parental care of young.
Dimetrodon: An early large predator and largest representative of Sphenacodontidae (don't worry). We have speculated on the function of the sail. The rest of the animal was a large (up to alligator-sized) predator with skull and teeth adapted for slicing flesh from prey. In this animal we see two of the synapsid trends well under way:
- The lower posterior edge of the jaw is developed into a thin flange that would vibrate freely when the jaw picked up sound vibrations from the ground.
- The teeth behind the canines, specialized fro slicing prey, are distinctly different from those in front, specialized for nipping or holding prey.
Therapsida: (Permian - Recent) During the Permian, we start to pick up members of a synapsid group that is more highly derived along the trends that we discussed before, Therapsida. This group has many morphological synapomorphies, but for us, a general trend is enough to note:
During the middle and late Permian, therapsids replaced the more basal synapsids as the dominant land fauna, both as predators and as herbivores. They encompass much more diversity than we can address, so here are some high-points.:
- A semi-erect stance and short tail. Not at all lizard-like. This implies that they depended more on the direct movement of the limbs and less on the undulation of the vertebral column for locomotion.
Dicynodontia: (Permian - Triassic) Four facts to note.
- These were synapsid herbivores that had evolved a unique mechanism for cropping plant material. It involved the interaction of a keratinous beak and a pair of long tusks. Earlier members retained non-canine teeth, but the majority of dicynodonts lost all but the canine "tusks". The jaw could be protracted to grasp plant material, then retracted into the mouth. The tusks would force the plants against the beak, where they would be sheared.
- Dicynodonts occurred over wide size range, and during late P and early Tr and were locally the most abundant vertebrate fossils in many localities.
- Their remains are often associated with fresh water.
- They persisted until the end of the Triassic.
We have considered the biological implications of the case of the snuggling dicynodonts in a previous lecture. They were preyed upon by a range of therapsid predators, including:
- Note extremely long canines. These were the first vertebrates to invade the sabre-tooth ecological guild.
- Synapsid trends in specialization of post-dentary bones and enlargement of infratemporal fenestra are evident.
- Extinguished at the end of the Permian.
Cynodontia (Permia n- Recent): The Synapsids that had carried the major trends the farthest. Consider the basal cynodont Procynosuchus.
- Infratemporal fenestra: About as big as it could get. Note tiny braincase.
- The post-dentary bones were weakly attached to the dentary, bore little load during chewing. Largely given over to hearing and presumably supported tympanum.
Procynosuchus marks the starting point of two new trends that mark cynodont evolution.:
- The bones of the palate begin to fold under the choanae to form a secondary palate separating the oral and nasal cavities. Compare the palates of:
- Post canine teeth become increasingly complex, developing precise occlusal surfaces. The eventual result - mammals shed their teeth only once.
The early Triassic saw major changes in dominant land faunas, due largely to the replacement of synapsids by reptiles distantly related to crocodylians and birds as the major large land animals. Many synapsid lineages became extinct at this time. The two important exceptions, confusingly were cynodonts, and dicynodonts.
- In the early Tr., cynodonts had been major land predators. When the croc-like reptiles took over, this was no longer possible, however cynodonts managed to hang on by adding a new trend: miniaturization. They continued to develop the other trends such that by the early Jurassic, we find very small cynodonts that are derived in the direction of mammals.
Morganucodon, (not actually first mammal) an e.g.:
- Infratemporal fenestra enlargement: This really was maximized in the early Triassic and has not progressed since.
- Jaw bones. A new jaw joint has formed between the dentary and the cranium. The old post-dentary bones now are tiny and stuck to the end of the jaw.
- Size: Tiny - Mouse or shrew.
By the Middle Jurassic, any synapsid you saw would be difficult to distinguish from a mammal.
Mammalia (Cretaceous - Recent) appeared in the Cretaceous.