Synapsida

Synapsida (from the Greek syn, "together," and apsis, "arch," referring to the single lower temporal arch bordering the skull's lateral opening) is one of the two primary evolutionary branches of Amniota—the sister clade to Sauropsida (reptiles and birds)—comprising crown-group Mammals (Mammalia, ~6,600 living species) and all extinct amniotes more closely related to mammals than to reptiles (stem-mammals, historically and inaccurately called "mammal-like reptiles").

Diverging from Sauropsida in the Late Carboniferous (~318–312 million years ago) as represented by Archaeothyris florensis and Protoclepsydrops from Nova Scotia, non-mammalian synapsids were the dominant large terrestrial herbivores and apex predators on Earth throughout the Early and Late Permian (~299–252 Ma) before suffering severe losses during the Permian–Triassic extinction event. Across a continuous 120-million-year fossil sequence spanning Carboniferous–Permian "pelycosaurs", Middle–Late Permian therapsids (dinocephalians, gorgonopsians, and dicynodonts), and Permian–Jurassic cynodonts (Procynosuchus, Thrinaxodon, Probainognathus, Morganucodon), the synapsid fossil record preserves the most complete macroevolutionary transition in vertebrate paleontology: the step-by-step transformation of an ectothermic, sprawling, multi-boned-jawed amniote into an endothermic, erect-gaited, large-brained mammal with heterodont occlusion, a muscular diaphragm, a secondary bony palate, and three middle-ear ossicles (malleus, incus, and stapes) derived from ancestral jaw-joint bones.

1. The Synapsid Skull Fenestra: The defining osteological synapomorphy of Synapsida is the presence of a single lower lateral temporal opening (infratemporal fenestra) behind each eye orbit, bounded dorsally by the postorbital and squamosal bones and ventrally by a zygomatic arch formed by the jugal and squamosal (or quadratojugal). This opening provided space for the jaw-closing adductor mandibulae musculature to bulge outward during contraction and attach onto the outer cranial wall—giving rise in therapsids and mammals to the differentiated temporalis and masseter chewing muscles.
2. Macroevolutionary Radiation Across Three Evolutionary Grades:

One of the crowning achievements of comparative embryology (formulated by Karl Bogislaus Reichert in 1837 and Ernst Gaupp in 1913) and paleontology (Edgar F. Alliston, Kenneth A. Kermack, Fuzz Crompton, Zhe-Xi Luo, and Jin Meng) is tracing how the bones forming the ancestral gnathostome jaw joint became the delicate sound-amplifying ossicles of the mammalian middle ear:

1. The Ancestral Non-Mammalian Jaw and Single-Bone Middle Ear: In amphibians, sauropsids, and early synapsids (Dimetrodon), the lower jaw contains up to seven bones (the tooth-bearing dentary at the front, plus six postdentary bones: articular, angular, surangular, prearticular, splenial, and coronoid). The jaw hinge itself is formed strictly between the articular bone at the back of the lower jaw (ossified from the posterior end of embryonic Meckel's cartilage, the first pharyngeal arch) and the quadrate bone of the upper skull. Only a single bone—the rod-like stapes (columella) (derived from the hyomandibula of the second pharyngeal arch)—sits in the middle ear cavity, while the angular bone in therapsids develops a thin curved flange (reflected lamina) supporting a mandibular eardrum for hearing ground-borne and airborne vibrations through the lower jaw.
2. Enlargement of the Dentary and the Double-Jaw-Joint Transitional Fossils (Probainognathus, Diarthrognathus, and Morganucodon): Across the Permian–Triassic cynodont sequence (Procynosuchus → Thrinaxodon → Probainognathus → Morganucodon), the dentary bone expanded progressively backward to bear the powerful forces of the temporalis and masseter chewing muscles, while the postdentary bones (articular, angular, prearticular) shrank into a slender rod housed inside a medial groove on the inside of the dentary—allowing the postdentary bones to specialize increasingly in high-frequency sound transmission while being relieved of heavy biting loads.
   • In the Middle–Late Triassic cynodont Probainognathus and the basal mammaliaform Morganucodon (~205 Ma) and Diarthrognathus ("two-jointed-jaw"), fossils preserve both jaw joints functioning side-by-side: a lateral mammalian joint (dentary–squamosal) and an adjacent medial reptilian joint (articular–quadrate)!
3. Final Detachment from Meckel's Cartilage (Hadrocodium, Yanoconodon, and Liaoconodon): In the Early Jurassic mammaliaform Hadrocodium wui (~195 Ma, skull length only 12 mm; Luo, Crompton, & Sun, 2001), expansion of the cerebral hemispheres pushed the squamosal jaw joint outward, detaching the postdentary unit from the dentary. Exquisitely preserved Early Cretaceous Jehol biota fossils from China—Yanoconodon allini and Liaoconodon hui (Luo et al., 2007; Meng et al., 2011)—captured the exact developmental intermediate: the malleus (articular), incus (quadrate), and ectotympanic ring (angular) have detached from the dentary chewing jaw to form a functional middle ear, but remain tethered to the lower jaw by an ossified Meckel's cartilage—identical to the transient embryonic state seen in every human fetus and newborn marsupial joey before Meckel's cartilage resorbs!

Why is Dimetrodon (the sail-backed Permian predator) a synapsid stem-mammal and NOT a dinosaur?

Although popular culture frequently groups Dimetrodon with dinosaurs, Dimetrodon lived during the Early Permian (~295–272 million years ago)—more than 40 million years before the very first dinosaurs evolved in the Late Triassic—and sits on the synapsid (mammal) branch of the amniote tree, meaning you are more closely related to Dimetrodon than Dimetrodon is to Tyrannosaurus rex, Triceratops, or a lizard! Its skull carries the single lower synapsid temporal fenestra, differentiated incisor- and canine-like teeth ("two-measures-of-teeth"), and sphenacodontid cranial sutures shared directly with therapsids and mammals.

Why did the evolution of a secondary bony palate and loss of lumbar ribs occur together in Triassic cynodonts?

In Early Triassic cynodonts such as Thrinaxodon liorhinus (~251 Ma), two anatomical innovations appear simultaneously in the skeleton:

1. Horizontal shelves of the maxilla and palatine bones meet along the midline roof of the mouth to form a complete secondary bony palate, separating the dorsal nasal airway from the oral cavity; and
2. The short ribs on the posterior trunk (lumbar vertebrae) are lost, dividing the trunk into an anterior rib-bearing thorax and a rib-free lumbar abdomen.

Both traits solved the acute respiratory and metabolic demands of endothermy during the hypoxic Early Triassic recovery: losing lumbar ribs allowed a muscular thoracoabdominal diaphragm to contract pistons-like without abdominal wall collapse (eliminating carrier's constraint during running), while the secondary bony palate allowed the animal to breathe continuously through its nose while chewing food with its multicusped cheek teeth (and later allowed infant mammals to suckle milk while breathing!).

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