THE MICROSAURIAN FAMILY STEGOPIDÆ, FROM THE COAL MEASURES OF OHIO.
=Family STEGOPIDÆ Moodie, 1909.=
Moodie, Jour. Geol., XVII, No. 1, 79, 1909.
The chief family characters are the large lacrimal, unknown in other
species of Coal Measures Amphibia, the central position of the orbits, the
general form of the skull, and the peculiar, short, divaricate horns from
the squamosal. If an intertemporal element is present in the skull, which
is suggested as a possibility, the family is further distinct. The type
species is Stegops divaricata Cope from the Linton, Ohio, Coal Measures.
The group seems to be distinct and has no immediate allies, being confined
to the American Coal Measures.
=Genus STEGOPS Moodie, 1909.=
Moodie, Jour. Geol., XVII, 79, 1909.
Type: _Stegops divaricata_ Cope.
This genus has been erected for the reception of the peculiar form
described by Cope as _Ceraterpeton divaricatum_, but there are good
reasons why the form can not be retained in the genus. The position of
the orbits in Stegops (plate 25, fig. 3) is different from _Ceraterpeton_
and the muzzle is rounded, not truncate as in the latter form. The horns
are of a different type and there is no indication of the tabulare
protuberance which is present in _Ceraterpeton_. The sculpturing of the
cranial elements is also distinctive in the present form, consisting of
radiating grooves and ridges; the cranium of _Ceraterpeton_ appears to be
but slightly sculptured. There is no lateral projection from the border of
the skull in Stegops, as there is in the other genus. The structure of the
skull of _Ceraterpeton_ is practically unknown, except in a very general
way, although Andrews (8) was able to make out some of the elements and to
trace the lateral-line canals. A structural comparison is thus impossible,
but on the basis of form alone there are good generic distinctions. The
present genus is apparently distinct from other genera in the presence
of an intertemporal, but additional material will be required before
a satisfactory determination is possible. The genus _Diceratosaurus_
of Jaekel (347) is distinct in the arrangement of the elements of the
cranium, the general form of the skull, and in the two known species of
_Diceratosaurus_ the orbits are located well anteriorly, but in Stegops
they are in the median transverse line of the cranium. The genus _Stegops_
is distinct from _Eoserpeton_ in the smaller size of the prosquamosal,
in the broadly rounded muzzle, in the larger and more posteriorly placed
orbits, and in the presence of an intertemporal bone, or at least in the
elongate character of the postorbital if the intertemporal is not present.
The species on which the genus Eoserpeton is based was first described by
Cope as Ceraterpeton ten n iconic. The form is quite distinct, in spite
of Jacket's protestations to the contrary. The genus _Stegops_ stands
alone among the Carboniferous Amphibia of North America, so far as I am
aware, in the possession of a well-defined lacrimal of the labyrinthodont
type.
=Stegops divaricata Cope.=
Cope, Proc. Am. Phil. Soc., XXII, p. 406, 1885 (_Keraterpeton
divaricatum_).
Moodie, Jour. Geol., XVII, No. 1, p. 79, fig. 22, 1909 (_Stegops_).
Type: Specimen No. 2559 G, American Museum of Natural History. The obverse
of this is No. 12,311, Walker Museum, University of Chicago.
Horizon and locality: Linton, Ohio, Coal Measures.
The skull on which this species is based consists of the impressions
on two slabs of coal, one belonging to the Newberry Collection of the
American Museum of Natural History, No. 2559 G, and the other to the
Gurley Collection of the University of Chicago, No. 12,311. The slab
belonging to the University of Chicago contains the better-preserved
remains, so that the description is based largely on that portion (plate
25, fig. 3). Nearly all of the elements of the skull are determined with
considerable certainty and many important characters in the morphology of
the Microsauria are thus brought out.
[Illustration: Fig. 23.--Skull elements of _Stegops divaricata_ Cope.,
_fr_, frontal; _it_, intertemporal; _j_, jugal; _la_, lacrimal;
_mx_, maxilla; _n_, nasal; _par_, parietal; _pf_, prefrontal; _po_,
postorbital; _pof_, postfrontal; _pmx_, premaxilla; _sq_, squamosal; spt,
supratemporal; _qj_, quadratojugal; _tab_, tabulare; _pp_, postparietal. ×
1.3.]
The skull is oval, elongate, truncate behind, and the quadrate angles
project into sharp horns. The orbits are elongate ovals and their center
lies in the median line which divides the skull transversely. The nostrils
are elongate and have an oblique position. The pineal foramen lies in the
posterior third of the skull.
Teeth are preserved on both maxillæ and premaxillæ. They are simply sharp
pleurodont denticles, and seem to have been fairly abundant. The bones
have been completely carbonized and nothing of the original texture
is preserved, although the details of the structure are beautifully
preserved. (Plate 25, fig. 3.)
The skull is somewhat triangular in its general form. The premaxilla lies
on the anterior border of the cranium, and forms the median border of
the nostril. The suture which separates the maxilla and the premaxilla
is not evident, and it may not be correctly defined in the figure (fig.
23). The nasal is a very large element and is elongate. It unites with
the premaxilla, the lacrimal, the prefrontal, and the parietal. It
is acuminate behind and the point is inclosed by the pref rental and
the parietal. The frontal is quite narrow and elongate, and does not
border the orbit; its posterior boundary is not accurately represented.
The radiations on the surface indicate the extent of the element. The
parietals are remarkable in being smaller than the frontal and nasal. The
pineal foramen is inclosed in the median suture in the anterior third of
the parietals. The parietal has the usual relations of that element. The
postparietal lies posterior to the parietal, but a portion of its bounding
sutures are destroyed. The prefrontal forms the antero-interior border of
the orbit and borders the postfrontal posteriorly, in an unusual manner.
The lacrimal is a large element and is clearly separable from the other
cranial elements. It, with the prefrontal, forms the anterior border of
the orbit. The maxilla is very elongate and forms the larger part of the
lateral border of the skull. Sharp-pointed teeth are present on it and
they may have been pleurodont. The lateral border of the orbit probably
received a portion of the maxilla. The postfrontal and the postorbital
form the greater part of the posterior boundary of the orbit. The
postorbital seems to be divided by a median suture which would indicate an
intertemporal bone, but this is not certain. The appearance may be due to
a fracture. The supratemporal is a large element bordering the parietal
and lies in front of the tabulare. The squamosal is elongate to form the
posterior horn-like extension of the skull. The tabulare is transversely
elongate and has the usual relations. The jugal widens to a fan-shape
backwards, and helps to form the lateral border of the orbit. Its lateral
and anterior boundaries are not assured. The quadratojugal seems to lie as
indicated, although the anterior part of the suture is not distinct. It
is apparently an elongate element and with the maxilla forms the lateral
border of the skull. The base of the skull as restored (fig. 23) is
irregular and may have had a slightly different form.
The genus _Stegops_ is exceptional in the elongate character of the
cranial elements of the single species known. In this respect it recalls
the species _Diceratosaurus lævis_ described below. The large size of the
nasals, frontals, and lacrimals and the small size of the parietals are,
so far as I am aware, unparalleled among the other Coal Measures Amphibia
of North America.
Measurements of the Type.
mm.
Median length of the skull 56
Width across tips of horns 46
Width at base of horns 40
Width across orbits 44
Diameter of the orbit 8
Length of the orbit 15
Interorbital space 16
Length of the nostril 2
Diameter of the pineal foramen 1
Length of the teeth 1.5
Length of the horn from base 7.5
Width of horn at base 4
[Illustration: MOODIE PLATE 15
1. Dorsum of skull of Diceratosaurus punctolineatus (Cope), from the
Coal Measures of Linton. Ohio. Original in the Museum at Berlin
University. × 2. After Jaekel. _po_=postorbital: _fr_= frontal;
_pof_=postfrontal; _j_=jugal; _la_=lacrimal; _mx_ = maxilla;
_ps_="perisquamosal;" _pp_=post-parietal; _pmx_= premaxilla.
2. Ventral surface of the skull of _Diceratosaurus punctolineatus_
(Cope ), from the Coal Measures of Linton, Ohio. × 2. After
Jaekel. _a_=anterior palatine vacuity; _ex_=exoccipital;
_j_=jugal; _mx_=maxilla; _tr_=transverse; _pal_=palatine;
_pt_=pterygoid; _pr_=prevomer; _ps_="perisquamosal;"
_pv_=posterior or suborbital palatine vacuity; _ph_=parasphenoid.
3. Pectoral girdle of _Diceratosaurus punctolineatus_ (Cope),
from the Coal Measures of Linton, Ohio. Original in the
paleontological Museum at Berlin. × 2. After Jaekel. _cli_=inner
side of clavicle; _ic_=interclavicle; _cl_=clavicle (lower side);
_cle_=cleithrum.
4. (e) Cervical or anterior dorsal vertebra of _Diceratosaurus
punctolineatus_ (Cope), from the Linton, Ohio, Coal Measures.
_cv_=caudal vertebra shown with its cap of dermal bone;
_dv_=dorsal vertebra with ribs; _x_=dermal plate on neural spine;
_h_=humerus; _u_=ulna; _r_=radius.
]