Comparison of basal synapsids with mammals

The magnitude of the differences in anatomy and physiology between a basal synapsid and a primitive living mammal may be inferred from an examination of their skulls (Hopson 1994). The pelycosaur (Fig. 1.3.10.2a) was an ectotherm, as are living reptiles, with a resting metabolic rate too low to generate sufficient heat to warm its body above ambient temperature. Therefore, its food and oxygen requirements were about one-tenth those of a mammal of comparable size. This is reflected in the simple dentition, adapted for capturing and holding prey items but not for breaking them down into small bits. Also, the internal nostrils, or choanae, lie near the front of the mouth, so that air had to traverse the oral cavity to deliver oxygen to the lungs; if food blocked the air passage, the animal was able to suspend breathing for a few minutes without ill effect.

Synapsid skulls are characterized by an opening behind the eye, the temporal fenestra, which in pely-cosaurs is relatively small. An undifferentiated mass of jaw-closing muscles was restricted to the small space beneath the cheek, attaching primarily to the underside of the broad skull roof. As in all other non-mammalian tetrapods, the lower jaw of the basal synapsid consists of

Synapsid Masseter

Fig. 1.3.10.2 Skulls of (a) an Early Permian basal synapsid, or 'pelycosaur' (based on the ophiacodontid Varanosaurus), and (b) a Recent mammal, the Virginia opossum (Didelphis virginianus), to show the craniodental differences between an ectothermic and an endothermic synapsid. Arrows indicate the path of inspired air; vertical hatching in (b) indicates the bony secondary palate. (c) Posterior half of the skull of an Early Triassic cynodont (Thrinaxodon). (d) Middle ear ossicles of a mammal (Didelphis). Patterning in (c) and (d) indicates homologous elements in the cynodont jaw and the mammalian middle ear. The eardrum of the mammal is held by the C-shaped tympanic bone; possibly the hook-like reflected lamina of the Triassic cynodont also held a tympanum in life. Abbreviations: ang, angular; art, articular; den, dentary; inc, incus; mal, malleus; oss, middle ear ossicles; q, quadrate; ref lam, reflected lamina; sq, squamosal; st, stapes; temp fen, temporal fenestra; tym, tympanic. (From Hopson 1994.)

Fig. 1.3.10.2 Skulls of (a) an Early Permian basal synapsid, or 'pelycosaur' (based on the ophiacodontid Varanosaurus), and (b) a Recent mammal, the Virginia opossum (Didelphis virginianus), to show the craniodental differences between an ectothermic and an endothermic synapsid. Arrows indicate the path of inspired air; vertical hatching in (b) indicates the bony secondary palate. (c) Posterior half of the skull of an Early Triassic cynodont (Thrinaxodon). (d) Middle ear ossicles of a mammal (Didelphis). Patterning in (c) and (d) indicates homologous elements in the cynodont jaw and the mammalian middle ear. The eardrum of the mammal is held by the C-shaped tympanic bone; possibly the hook-like reflected lamina of the Triassic cynodont also held a tympanum in life. Abbreviations: ang, angular; art, articular; den, dentary; inc, incus; mal, malleus; oss, middle ear ossicles; q, quadrate; ref lam, reflected lamina; sq, squamosal; st, stapes; temp fen, temporal fenestra; tym, tympanic. (From Hopson 1994.)

a large number of bones behind the tooth-bearing dentary, with an articular bone forming a hinge-like jaw joint with the quadrate bone in the skull.

The opossum (Fig. 1.3.10.2b), like all mammals, is an endotherm, with an active metabolism generating sufficient heat to provide a constant high optimal body temperature, irrespective of ambient temperature. The mammalian dentition is differentiated into pointed incisors and canines in front for grasping and killing prey, and premolars and molars behind for breaking it down; this vastly increases the surface area available to digestive enzymes and thus increases the rate at which food energy is assimilated. The choanae no longer open directly into the mouth; rather, the air passage is separated from the oral cavity by a bony plate, the secondary palate. Thus, mammals have evolved special features for the rapid delivery of food energy and the uninterrupted delivery of oxygen to their body tissues, in order to maintain an endothermic metabolism.

The jaw muscles of mammals have increased both in mass and complexity, with one portion, the temporalis, attaching around the margins of a greatly enlarged temporal fenestra (now confluent with the eye socket) and a new external portion, the masseter, attached to the cheek bone (zygomatic arch). The lower jaw is formed entirely by the dentary, which bears a posterodorsal coronoid process for attachment of the temporalis and a pos-teroventral angular process for attachment of the mas-seter. These muscles aid in moving the lower jaw from side to side so that the molars can effectively slice and grind the food.

The mammalian jaw joint is a new structure, formed by the dentary with the squamosal bone of the skull. The quadrate and certain postdentary bones of the synapsid lower jaw have been shifted into the middle ear (Fig. 1.3.10.2c,d) where they serve to transmit sound vibrations from the eardrum to the inner ear (Allin and Hopson 1992).

Comparison of the postcranial skeleton of early synapsids with that of mammals also permits inferences about differing activity levels. Pelycosaurs have relatively short sprawling limbs, with joint surfaces permitting only a restricted range of movements. Lateral bending of the trunk helped to increase stride length, but such side-to-side movement also interfered with ventilation of the lungs (Carrier 1987). Thus, pelycosaurs, like living reptiles, probably suspended breathing while running, relying on anaerobic muscle metabolism to sustain a short burst of speed. In contrast, mammals have a more upright stance and more mobile limb joints, permitting the long limbs to move with greater freedom in a fore-aft plane. The trunk bends in a dorsoventral plane, which both increases stride length and aids in ventilating the lungs. Thus, mammals increase respiratory rate with increasing speed and so are able to sustain high levels of aerobic activity over long periods of time.

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Responses

  • Jessica
    Is an opossum a synapsid?
    6 years ago
  • Kirsi Kojonkoski
    Is an Odocoileus virginianus a ectotherm or endotherm?
    6 years ago

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