Pelycosaurs

Pelycosaurs (Order Pelycosauria) were basal synapsids that became important during the Early Permian. The predatory finback Dimetrodon and the herbivorous finback Edaphosaurus are among the best known pelycosaurs. Finbacks had large sails along their backs made from long neural spines covered with vascularized skin. The fins were used for thermoregulation allowing the animal to absorb heat energy from the Sun or radiate heat from the body. The sails of finbacks may indicate that synapsids were not yet endotherms.

Dimetrodon means “two measures of teeth” and refers to its heterodont condition. Unlike reptiles Dimetrodon had shearing teeth as well as canines for tearing. Dimetrodon had four legs and a long tail. The four limbs of Dimetrodon were splade to the sides of the body giving it a sprawling gate.

Edaphosaurus was a primitive pelycosaur and represents one of the earliest known tetrapod herbivores. Edaphosaurus had teeth specialized for chopping up plant material. Herbivory represents an important evolutionary innovation in digestion, as it requires hosting a community of bacteria within the gut that can help chemically process the cellulose. Prior to organisms like Edaphosaurus all vertebrates were carnivores and detritivores.

The Edaphosaurus and Dimetrodon genera would evolve into different species during the Permian. Both genera would evolve into species that reached over 3 meters in length. This made Dimetrodon one of the largest predators of the time.

Therapsids

Pelycosaurs became extinct by the Late Permian, but a second radiation of synapsids, the therapsids (Order Therapsida) would come to dominate the landscape. Therapsids are more advanced synapsids from which all mammals evolved. Evolutionary trends included a more upright posture, with legs tucked more directly beneath the body, enlarged temporal fenestrae to accommodate larger, more powerful jaw muscles, and increased heterodonty with teeth differentiated into incisors, canines, and molars. Therapsids evolved into a variety of carnivorous and herbivorous forms. We will focus on several groups.

Dicynodonts (Suborder Dicynodontia) were the dominant herbivores in the late Permian. Dicynondont means “two dog teeth” and refers to their two large tusk-like canines. Dicynodonts had an almost toothless mouth equipped with a strong beak-like structure. A sliding jaw joint allowed them to process tough vegetation. The body was barrel-shaped with short tails and legs. Dicynodonts had a semi-sprawling gate and dimensions ranged from rat to hippo size. At a length of approximately 1 meter, Lystrosaurus was a pig-sized dicynodont of the Late Permian and Early Triassic. Lystrasaurus is the most famous survivor of the Late Permian mass extinction. Lystrasaurus spread worldwide and made up to 95% of the faunas in some areas. This low biodiversity is a sure sign that a major crisis had taken place. Lystrosaurus fossils of the same age are found in Africa, India, China, and Antarctica providing biological evidence that the continents were once joined into the landmass Pangea. Dicynodonts continued to radiate in the Triassic. Kannemeyeria was a 3-meter long ox-sized dicynodont. Kannemeyeria specimens are found in Africa, India, and South America indicating these landmasses were once connected. Members of the family Kannemeyriidae were the dominant herbivores of the Triassic.

Gorgonopsians (Suborder Gorgonopsia) were the dominant carnivores of the Late Permian. Gorgonopsians were wolf and bear-sized predators with large canines and strong jaws. Arctognathus, a saber tooth-like gorgonopsian had the ability to open its jaw 90 degrees. The bite of this animal secured prey with large canines. The jaw could then shift forward, allowing the incisors to meet and remove chunks of flesh (Benton, 2005, p. 129).

Cynodonts (Suborder Cynodontia) form a clade if mammals are included. Cynodont means, “dog tooth” and refers to their dog-like teeth. Cynodonts are the most derived synapsids having differentiated teeth, larger braincase, secondary palate, a more upright posture, as well as an ear and jaw structure like mammals. Cynodonts like all protomammals laid eggs. The cynodonts were weasel to dog sized predators, although Cynognathus was bear-sized (Prothero, 1998, p. 383). Thrinaxodon from the Early Triassic of Africa and Antarctica was a highly derived, carnivorous, cat-sized therapsid. Thrinaxodon had an erect posture with strong hindlegs and was probably capable of running fast. Only the thoracic vertebrae bore ribs, which had broad flanges, so the body was divided into a thoracic and lumbar region. The location and structure of the ribs indicate that Thrinaxodon breathed with a diaphragm. Thrinaxodon had a secondary palate and its teeth were set only in the margins of the jaw and almost fully differentiated into incisors, canines, and molars. Thrinaxodon also possessed small pits on their snouts, which may indicate the presence of whiskers or hair (Prothero, 2007, p. 276). So, multiple lines of evidence suggest that Thrinaxodon was an endotherm.

Becoming a Mammal

A variety of synapsid fossils document the evolution of early amniotes to mammals as reflected in changes to skeletal structure. Many of these changes in skeletal structure may reflect the development of a new method for controlling body temperature (Dixon, 1988, pp. 184-185). Reptiles and early protomammals were cold-blooded or ectotherms. Ectotherms rely on external sources for body heat. Ectotherms may seek the sun, shade, or different water temperatures to control body temperature. Endotherms also exhibit behaviors to adjust body temperature but also generate heat from within the body. Endotherms like birds, mammals, and the later therapsids rely on a fast metabolism fueled by the quick and frequent processing of food for their internal source of heat energy.

Teeth became increasingly differentiated a condition called heterodont dentition. Reptiles replace their teeth continuously throughout their life. Therapsids evolved to replace teeth less often. Today mammals replace deciduous teeth once, while molars are never replaced. Changing teeth less often allows opposing teeth on the upper and lower jaws to form crests and valleys creating a precision bite. The temporal fenestrae increased in size and the back of the skull became larger with the sides bowed out, which accommodated larger jaw muscles. Mammal teeth and jaws could process food more efficiently. Well-chewed food is digested more quickly helping to fuel a warm-blooded metabolism.

Biting, chewing and hearing in synapsids were affected by changes in jaw structure. Early amniotes, synapsids, and reptiles had several bones making up their jaw. These organisms hear with their lower jaws as the sound is transmitted from the jaw joint into the middle ear. In the lower jaw the dentary bore the teeth and was connected to the articular bone, which formed a jaw hinge with the quadrate bone of the skull. The quadrate and articular bones not only formed the jaw hinge they also made contact with the stirrup or stapes of the inner ear.

Evolutionary trends in synapsids include an enlargement of the dentary and a reduction in the size of the articular and quadrate bones. The enlargement of the dentary continued until it came into contact with the squamosal bone, forming a new joint. The dentary bone also developed the coronoid process, which became an extra attachment point for jaw muscles. The dentary/squamosal jaw joint is a characteristic of mammals. The transition from the articular/quadrate joint to the dentary/squamosal joint is recorded in both the fossil record and embryological development of mammals.

Diarthrognathus (“two jaw joint”) is a synapsid that had both jaw joints functioning side by side, the articular/quadrate and the dentary/squamosal. The articular and quadrate eventually became detached from the jaw to form the bones of the middle ear. The quadrate came into contact with the stapes or stirrup, which had been in the ear since the early tetrapods. The quadrate became the incus or anvil and the articular became the malleus or hammer. The malleus, incus, and stapes make up the auditory ossicles that transmit vibrations from the tympanic membrane (eardrum) to the oval window of the inner ear. A recent fossil find provides further evidence for this sequence of events. Yanoconodon, from the lower Cretaceous of China, had the middle ear bones still connected to the lower jaw. So, this organism transmitted sound with its jaw/middle ear bones (Prothero, 2007, p. 280). Early in our embryonic development these bones start out as part of the jaw, but are transferred to the ear later in ontogeny (Prothero, 1998, p. 381). Interestingly, the layout of bones in the ear of Yanoconodon is the same as cartilage precursors in mammalian embryos before the ear and jawbones separate. The ontogeny (development of an organism) can sometimes reflect the phylogeny (evolutionary history) of a species.

The idea that ontogeny recapitulates phylogeny was first formulated as the Biogenetic Law by German zoologist Enrst Haeckel (1834-1919) in 1866. Haeckel’s law stated that an organism’s development followed the same path as its evolutionary history. Although the original form of his law is now rejected, embryological development is used to help build evolutionary histories of species.

A faster metabolism not only requires more efficient methods of processing food, it also requires better breathing. An abrupt reduction in the rib cage of later protomammals indicates that a diaphragm closed off the front part of the body cavity, which houses the lungs and heart. The diaphragm and upper body cavity allows larger lungs to be filled and emptied more rapidly. Thus, greater amounts of oxygen enter the bloodstream. Quicker digestion and increased availability of oxygen are needed to accelerate metabolism. Breathing was also enhanced by the evolution of a secondary palate, which is a shelf of bone that separates the air passage from the mouth. In reptiles and early amniotes, the nasal passage opens in front of the mouth, so they must hold their breath while swallowing. Later therapsids and mammals were able to breathe while retaining food in their mouth, allowing them to chew food for longer periods of time. Thus, the secondary palate helps animals to better process their food for quicker digestion.

Therapsid limbs and joints were modified to change from a semi-sprawling gait to an erect gait. The pelvic and pectoral girdles along with the limbs became structured to tuck the limbs under the body. The backbone came to support an up and down flexure instead of a side-to-side bending. These limb improvements had a greater potential for fast movement. Several mammalian characteristics are not so well preserved in the fossil record.

Mammals are often defined as homeothermic endothermic amniotes with hair. Mammals have a high metabolism, a four-chambered hear, a diaphragm, and a sophisticated brain with an enlarged neocortex. Most mammals, except for the egg-laying platypus and echidna, bear live young, which females nurse with milk. Mammals require greater parental care than reptiles. Mammals grow rapidly after birth, but slow to a terminal adult stage. This is in contrast to most other animals, which grow continuously throughout their lives. These features do not fossilize well, but skeletal structures may provide indirect evidence for mammalian physiology and reproduction (Prothero, 1998, p. 380). The braincase can be a clue to the enlargement of the neocortex. Fossil bones can leave indicators of an organism’s growth rate. And as we have already discussed, clues to metabolism may also be revealed by skeletal structures.

The largest mass extinction occurred at the end of the Permian around 250 million years ago. Seventy five percent of tetrapod families went extinct. At the same time 50% of marine families died out. These family level losses indicate that 80 to 96% of all species went extinct (Benton, 2005, p. 133). Some believe the mass extinction could have been caused by an asteroid impact, like that hypothesized for the KT event. The Siberian Traps occurred at this time and represent a flood basalt that may have changed the Earth’s atmosphere and temperature. Some believe that an asteroid impact could have triggered the massive volcanic activity. Whatever the cause therapsids, like other organisms, were hit hard. A few therapsids would recover from this mass extinction and come to dominate the landscape until the Mid-Triassic when rapidly evolving archosaurs, especially the dinosaurs, would displace them. Therapsids declined and went extinct in the Cretaceous, but not before giving rise to the first mammals. Mammals would stay shrew size throughout the 150 million years of the dinosaur reign. As the dinosaurs declined at the end of the Cretaceous mammals would diversify and come to dominate the landscape, giving their synapsid ancestors the “last ghostly laugh” (Dixon, 1988, p. 184).