SUBCLASS LEPOSPONDYLIA ZITTEL, 1887. COAL MEASURES TO PERMIAN.
(Europe and North America.)
The group is here defined according to the English edition of Zittel's Text Book of Paleontology, 1902, p. 125. "Notochord persistent and enclosed in constricted bony cylinders, hour-glass-shaped in longitudinal section. Teeth simple, conical, hollow." According to Zittel there are two families, the Microsauridæ and the Aistopodidæ. The latter family is dealt with under Aistopoda (p. 76) and it is there shown that the group is in no wise a valid one. The former family is regarded as an order and is fully entitled to that rank. As defined here the subclass Lepospondylia contains but a single order, the Microsauria.
Extinct, terrestrial, aquatic, or semi-aquatic amphibians; skull pitted and grooved by lateral-line canals and by sculpturing marks, or the skull may be smooth; teeth present on most of the palate bones; exoccipitals cartilaginous or calcified, never completely osseous; sclerotic plates sometimes present; skull of various shapes. Vertebræ with notochord largely persistent, hour-glass-shaped; neural spines low or high, or absent; ribs intercentral and single-headed, with an incipient tubercle in some forms; vertebral column differentiated into dorsal and caudal series; cervical series not clearly defined. Limb bones with well-ossified perichondrium, endochondrium partly ossified; epiphyses absent; carpus and tarsus (tarsus osseous in two species) cartilaginous; phalanges clawed or not; digits 4 in hand and 5 in foot. Pubis sometimes calcified but never osseous.
Definition of the Order Microsauria Dawson, 1863. Coal Measures And Permian.
(Europe and North America.)
Lizard-like, sometimes longicaudate, stegocephalous, lepospondylous, ambulatory or legless amphibians; skull bones usually sculptured with pits and grooves; lateral-line canals well developed on skull bones; skull with horns from tabulare and supratemporals or without horns; branchiæ never persistent; sclerotic plates present; orbits usually well forward. Vertebræ hour-glass-shaped; endochondral bone weakly developed throughout skeleton, especially in vertebræ; notochord largely persistent; neural spines low and rudimentary or long, fan-shaped, and highly ornamented. Dorsal series of vertebral column variable; usually from 22 to 30; tail containing sometimes over 75 vertebræ, or tail very short with 2 weakly developed vertebræ; caudal ribs present, in those forms with long caudal series the distal vertebræ sometimes exhibiting 2 pleurocentra. Ribs long and curved, always intercentral in position, single-headed, with at times an incipient tubercle. Pectoral girdle composed of scapulæ, clavicles, coracoids, and interclavicle. Pelvic girdle composed of osseous rod-like ilium, plate-like ischium; pubis cartilaginous, sometimes calcined. Limbs present or wanting or weakly developed, sometimes present in front and wanting behind. Radius and ulna and tibia and fibula free; carpus and tarsus usually cartilaginous; digits 4 in hand and 5 in foot, terminal phalanges sometimes clawed. Phalangeal formula for the hand 2-2-3-2, for the foot 2-2-3-4-3. Abdomen covered with dermal armature composed of osseous or corneous rods or scutes; overlapping scales, fish-like in appearance, sometimes present over the entire body; body also covered with lizard-like scales or naked.
The order Microsauria was established by Sir William Dawson in 1863 ([208]) as a family of "reptiles" for the reception of the genera Hylonomus, Hylerpeton, Smilerpeton, and Fritschia. Hylonomus lyelli is the type species of the order. Dawson ([216, p. 635]) says of the species Hylonomus lyelli Dawson: "It is the type of the genus Hylonomus and of the family Microsauria." The forms which have been referred to the genus Hylonomus, and hence to the order Microsauria, from the deposits of Europe are discussed under Hylonomus.
The Microsauria have been regarded by the writer and others as being ancestral in a sense to some of the later reptiles ([469]), but there seem to be insuperable obstacles in the way of a direct derivation of the reptiles from the Microsauria. One of these obstacles seems to be found in the structure of the hand. In all Microsauria, so far as is known, there is no evidence of more than 4 digits in the hand, while no true primitive reptile possessed less than 5. The carpus of all true reptiles is osseous, while that of the Microsauria is merely cartilaginous. It is possible that the Microsauria stand in some such ancestral relation to the later reptiles as the Crossopterygia ([489b]) do to the Amphibia. The Microsauria had undergone adaptive modifications as to structure and habit, so that they have paralleled many of the groups of reptiles, but their structure is quite different. The evidence, as far as we can see now, points to a close genetic relation between the reptiles and the Microsauria, but that this relation is ancestral I, for one, am not ready to say.