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."
[Illustration: MOODIE PLATE 8
_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."