Chapter XII

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 (489_b_) 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.

The Group _Aistopoda Miall_, 1873, is untenable.

The group _Aistopoda_ was established in 1873 as section IX by the Committee of the British Association for the Advancement of Science, in their "Tabular View of the Classification of the Labyrinthodonts." L. C. Miall (449, 450) was the secretary of the committee, and the report was published in two parts. Two genera were at that time attributed to the Aistopoda, _Ophiderpeton_ and _Dolichosoma_, both described by Huxley from the Coal Measures of Kilkenny, Ireland.

Fritsch (251, pp. 107-126) in 1883 refers to the group as "Familie" and describes 4 genera and 9 species as belonging to the group. Zittel (642, p. 383) refers to the group as "Familie" and places 5 genera in it. Smith-Woodward (Vertebrate Paleontology, 1898, p. 129) refers the Aistopoda to a suborder. In Eastman's translation of Zittel's Paleontology the group is called a family, "Aistopodidæ." Lydekker (393, p. 205) regards the group as a suborder. The writer (469) refers the Aistopoda to an order. The group Aistopoda has been adopted by practically all paleontologists and zoologists who have had occasion to refer to these animals.

Lydekker (393) in 1890 defined the group as follows:

"Body long and snake-like, without limbs, and apparently without
pectoral or pelvic girdles. Vertebræ with elongated centra and aborted
neural spines. Ribs slender, and barbed like those of fishes. Teeth
smooth, without plications of the dentine. External gills probably
persistent."

_Fritschia curtidentata_ Dawson. Above: Bones of skull and anterior extremity, and bony rods of belly. Below: Bones of pelvis and posterior extremity. Nearly natural size. Erect tree, Coal formation, South Joggins, Nova Scotia. Photograph by Dawson, published through the courtesy of Dr. Arthur Willey. Original specimen in the Peter Redpath Museum of McGill University.]

If now we take up a consideration of each of the characters mentioned by Lydekker we find that the first one holds good for all examples of the group. The second character, "without limbs," is not good. Species of _Œstocephalus_, _Ptyonius_, _Molgophis_ all possess limbs; and doubtless Ophiderpeton will be found to possess limbs also, since it has a well-developed pectoral girdle. The limbs in all of these genera are small. The third character, "apparently without pectoral and pelvic girdles," is not at all a good character, since nearly every specimen of some species and almost all species show evidences of pectoral girdles and a few exhibit pelvic girdles. The fourth character, "vertebræ with elongated centra and aborted neural spines," is not a good distinguishing character, since _Amphibamus_, an undoubted microsaurian, possesses the same vertebral characters. "Ribs slender, and barbed like those of fishes" is a character which is common to several widely distinct genera. All Microsauria possess long, slender ribs, and the barbed condition is one which is possessed by only a few, _Thyrsidium_, _Ophiderpeton_, etc., the so-called "barb" being merely a highly exaggerated tuberculum. The teeth of nearly all Microsauria are smooth, so that the character "teeth smooth" is not a good one for a group definition. It has not been possible to examine any of the American specimens for the plication of the dentine, since the forms are so rare and the fossils very fragile. The last character, "external gills probably persistent," is certainly not true for the American species, and the evidence for the European species is negative. Fritsch described and figured (Fauna der Gaskohle, Bd. I, 1883, p. 114, Tafeln 18 and 23) structures which he regarded as supporting structures for the external branchiæ. He says in regard to these structures:

"Bei dem Umstande, dass sie von der Kiemengegend aus sich
büschelförmig verbreiten und man ihren Contakt mit einer Art von
Branchiæ constatiren kann, zweifle ich nicht daran, dass diese
Stäbchen dem Kiemenapparate angehören. Bedenklich ist nur ihre
grosse Zahl und das Vorkommen bis zum 16ten Wirbel und ich erwog
die Möglichkeit, dass diese Stäbchen einem zarten Bauchpanzer
angehören könnten. Da aber weiter im Verlaufe der ganzen Wirbelsäule
nichts Aehnliches vorkömmt, so ist man gezwungen anzunehmen, dass
_Dolichosoma_ sehr grosse lange Kiemenbüschel besessen haben muss."

John Samuel Budgett (79, p. 162), in his discussion of the "Structure of the Larval Polypterus," refers to the above-described specimen of "aistopodous Stegocephali," _i.e._, _Dolichosoma longissimum_ Fritsch, and calls especial attention to the similarity of the external rod of segmented cartilage on the hyomandibular of _Polypterus_ to this structure, to which Fritsch has assigned a branchiate nature in _Dolichosoma_. There is no doubt in the mind of the present writer, however, that the rod of cartilage, referred by Fritsch to the gills, can be other than scutellate rods of the ventral armature, these rods belonging to the armature of the breast or throat. The evidence for this conclusion is furnished by Fritsch himself (Fauna der Gaskohle, Bd. I, plate 18, fig. 11), where all may read in the figure of the specimen the facts of the case. There is quite evidently no justification for Fritsch's conclusion of the branchiate nature of _Dolichosoma_. There is no evidence of any gill-like structure in the American snake-like amphibians of the Coal Measures.

Reviewing, then, the characters of the group which have been assigned by various observers, it will be seen that there is but a single character which holds good: "body long and snake-like." This is totally insufficient for the retention of the group. I therefore propose to abolish the group entirely from zoological classification. It is not even a family. It will, however, be convenient to refer to the snake-like forms as "aistopodous."