THE MORPHOLOGY OF THE COAL MEASURES AMPHIBIA.

The anatomy of the Coal Measures Amphibia presents many primitive types of structure. Their organization represents a stage passed through in the ontogeny of higher vertebrates. The animals are similar in a general way, yet so diverse are the modifications which they have suffered under different environmental conditions, that close scrutiny is needed to discern the exact relationship of the forms. Our knowledge of this relationship is based on the structures preserved, which are largely skeletal, since little is known of the soft anatomy (471) of the air-breathing vertebrates of the Coal Measures. The pubis is ossified in the Paleozoic Amphibia later than the ischium and ilium; the carpus and tarsus are cartilaginous; the vertebrÆ consist of a pleurocentrum and two neurocentra, thus paralleling conditions in modern mammalian embryos.

(a) The skull of the Coal Measures Amphibia has (fig. 6) essentially the same structure in the different groups. It is largely formed of bones of intramembranous origin, representing the face bones of the mammalian skull. The skull in life was doubtless a chondrocranium with the membrane bones laid down upon the cartilaginous box containing the sense-organs, as in the sturgeon (Acipenser), where the surface bones of the face were probably originally scales, which later became consolidated into large bony scutes. The membrane bones of the early Amphibia may have been originally derived from scales, but at present nothing is known of this origin; doubtless the elements had an intramembranous origin in the ancestors of the group. Judging from Credner's studies on the series of specimens of Branchiosaurus amblystomus Credner (187), the skull bones do not ossify completely until relatively late in the life of the individual. The skull in the youngest individual figured by Credner (op. cit., Taf. XVI, fig. 1) seems to be largely cartilaginous, with beginnings of separation into the skeletal elements. The manner and time of development and ossification of the skull seems to proceed much as it does in modern amphibians. The condition found in the skull of Cryptobranchus allegheniensis or Necturus maculosus will represent pretty accurately the condition of most of the Coal Measures Amphibia. The face bones in certain forms were sculptured and cut by lateral-line canals.

A median suture divides the skull into two equal regions dorsally. On the sides of this median suture lie pairs of elements which are common to all higher vertebrates. These elements are: the premaxillÆ, nasals, frontals, parietals, and post-parietals. All of these elements vary somewhat in shape and slightly in arrangement, but always occupy the same relative positions. To the side of these elements lie the prefrontal, the postfrontal, the supratemporal, the squamosal, and tabulare, and occupying the margin of the skull are the maxilla, the jugal, the quadratojugal, and possibly the quadrate in a few forms. The parietal foramen occurs usually within the parietal bone, but its position is subject to slight variations and it may occur on the suture between the frontal and the parietal, or even far posterior near the postparietal. The nostrils often lie well forward and are included by the premaxillÆ, nasals, and prefrontals. The orbit is usually well posterior, but it may occur far forward. It is bounded by the prefrontal, the frontal, the postfrontal, the post-orbital, and the jugal. Sometimes the lacrimal is present and has been clearly identified on the anterior margin of the orbit in a few cases.

(b) Sclerotic plates often occur within the orbits, and are not confined to any particular group, though they are quite constant among the Branchiosauria. They are usually delicate, thin, broad plates which evidently overlap and operate as in modern animals. The number varies, as many as 30 occurring within the orbit of one branchiosaur. Between the margin of the orbit and the sclerotic plates there often occur, in the Branchiosauria (186) particularly, small scale-like particles which were doubtless embedded in the heavy skin above the orbit during life.

(c) The palate of the skull is very incompletely known, being indicated in a very few cases. These specimens, however, show that the characters of the palate were quite similar, if not identical, in essential respects with the palate among the European species of the same or slightly later time.

A large cultriform parasphenoid occupies the posterior portion of the palate, on either side of which in some species lies the posterior palatine foramen. On the sides of the anterior prolongation of the parasphenoid lie the vomers (186), membranous bones often bearing minute tubercular teeth, apparently adapted for crushing. The vomers and the maxillÆ, with sometimes the palatine, surround the anterior palatine foramen, which is almost always present; sometimes, however, quite small. The transverse or ectopterygoid unites the pterygoid, a broad plate of thin bone, with the maxilla and jugal.

(d) The teeth of the Coal Measures Amphibia (194) are remarkably similar in the various forms. They are always slender, pleurodont denticles arranged in a single row on the jaws or as tubercular eruptions on the palate bones, with a large pulp-cavity and the enamel often striated. The food of the creatures must have been small Crustacea, worms, insects, and succulent vegetation, such as is the food of the modern Amphibia.

(e) The occiput is formed of partially ossified (465) ex- and basi-occipitals, though these elements are never firmly united by ossific union. Often a pair of condyles occur, one on either exoccipital. The occiput was usually, however, cartilaginous and no trace of its structure is preserved.

(f) The mandible is usually as long as the skull and is slender. It is composed of 6 elements so far as known (465); these are the articular, the surangular, the angular, the coronoid, the dentary, and the splenial. Other elements may be present, but the anatomy of this portion of the animals is not very completely known. The bones are sculptured and cut by lateral-line canals (458) in a few forms. Whether the articular operated on an osseous or cartilaginous quadrate is unknown, though certain specimens seem to indicate an osseous condition for that element. The anterior symphysis was doubtless ligamentous, the halves always separating before fossilization. The dentary always bears a single row of pleurodont teeth, which may vary greatly in size and number.

(g) The hyoid apparatus is well preserved in a few forms (123). Doubtless it was present in all of them, though it has seldom been preserved. The condition of the hyobranchial apparatus in Cocytinus gyrinoides (text-fig. 16) from the Coal Measures of Linton, Ohio, seems to indicate that the species was a perennibranchiate salamander (123). It is well known from the studies of Credner that the European Branchiosauria, in the young, possessed external branchiÆ (187) supported by lateral basibranchials. The gill-arches seem to have been slightly calcified or ossified in a few cases, and they supported denticle-like projections which bore the gill-filaments. When the Branchiosauria had attained a length of 100 mm. or more they lost their gills (187). This change was accompanied by the reduction of the tail, expansion of the pelvis, and increase in ossification of the skull and skeletal elements. Gills have not yet been detected among the American Branchiosauria.

(h) The eye in a few species had a large amount of black pigment, as indicated by the blackening of the stone in the Mazon Creek nodules. Professor Cope (107) thought that this would indicate a nocturnal and crepuscular habit for these vertebrates, since the pigmentum nigrum of the choroid is largely developed. Other than this suggestion nothing is known of the soft parts of the head.

(i) The alimentary canal (text-fig. 7) is beautifully preserved as a cast in three specimens of the American branchiosaur species Eumicrerpeton parvum Moodie (471) from the Mazon Creek beds. The nodules which contain these interesting little fossils measure less than 3 inches in long diameter. The fossil salamanders, about 30 mm. in length, are preserved on their backs and occur as nearly as is possible in the center of the nodule.

If it were not for the fact that the oesophagus became loosened and dropped from its place shortly after death, the alimentary canal would be in place and would immediately recall a freshly dissected specimen of a recent salamander. The anterior end of the oesophagus lies obliquely across the chest region with its tip pointing slightly downward. The length of the oesophagus proper, in one specimen, is only about 3 mm. As it is preserved, the oesophagus lies in a semi-sigmoid curve with the convexity anterior, and enters the cardiac portion of the stomach by a gradual constriction. The stomach is clearly preserved as a distinct sac-like organ with two lobes which correspond to the cardiac and pyloric limbs. It measures about 7 mm. in length by 2 mm. in its greatest diameter. The muscular constriction which divides the organ into pyloric and cardiac divisions occurs at a distance of 4 mm. from the upper end. The pylorus is designated by a rather pronounced constriction which may be partly accidental, although it recalls the pylorus of modern frogs. From this constriction, which lies on the left side of the fossil, as it is preserved, the duodenal portion of the intestine makes a straight course posteriorly to near the anal region, where it takes a sharp bend and curves back to run parallel with itself for the distance of 4 mm. In its upward course the intestine enlarges, and practically the same enlargement continues throughout the remainder of the course to the anus. At a distance of 1 mm. from the anal end, the rectum dilates probably 0.125 mm. to form the cloaca. After the intestine has continued its parallel course for the 4 mm., as above stated, it turns abruptly to the right for a distance of 2 mm. It then runs posteriorly for a short distance, then bends back and under itself to again make a double sigmoid curve, when at a distance of 6 mm. from the anus it assumes a straight course, which it continues to the end.

The anus lies at a level which is approximately that of the lower end of the femur, which is preserved as an impression on the left side of the fossil, thus agreeing in its position with that found in modern Caudata. Lying inside the curve of the stomach and partly inclosed by the oesophagus is a smooth area which may possibly represent the impression of some of the accessory digestive glands, such as the liver. Occurring in this smooth area are numerous fine lines which possibly represent the impressions of blood-vessels; but they are so imperfectly preserved that one can not be sure.

Representatives of several genera of the modern Caudata have been dissected in order to make a direct comparison of the fossil alimentary canal with that of the recent forms. The alimentary tract of Desmognathus fuscus Raf. from the vicinity of Ithaca, New York, resembles in a marked degree that of the fossil form. The nearest approach to the condition there represented is found, however, in an immature branchiate individual, some 47 mm. in length, of Diemyctylits torosus Esch., from a fresh-water pond on Orcas Island in Puget Sound. The presence of this species on the island is very suggestive. It is of extreme interest, too, that the condition represented in the alimentary tract of the fossil branchiosaurs should resemble so closely that of an immature rather than a mature form.

(j) The vertebral column is clearly and readily separable into cervical, dorsal, sacral, and caudal regions. The neck is always short, with from 5 to 10 vertebrÆ, cervical ribs often present. The dorsal region is not long, but varies from 20 to 30 in the constituent vertebrÆ. There is a single sacral vertebra not always to be readily distinguished from those of the dorsal and anterior caudal series. The tail may be very short or extremely long, with neural and hÆmal spines elongate and flattened into an oar-like appendage. The distal caudals are in some species cartilaginous, apparently always so in the Branchiosauria.

(k) The atlas and axis are unknown among the American specimens, but we are able to infer from the structure of the other vertebrÆ what this must have been; and our inferences are partly confirmed by the conditions existing in the European forms (187). The atlas, apparently, consisted of a pair of neurocentral plates which are partly ossified, partly embedded in cartilage, judging from the edges of the plates which have been preserved. The centrum seems not to have been present in the atlas, or if present it was only very slightly developed and quite free from the neural pieces and largely embedded in cartilage. A fairly accurate picture of the condition of the atlas and axis may be seen on examining a cow, pig, or chick embryo (378) in the early stages of vertebral development, which has been cleared by the Schultze method (Amer. Journ. Anat., VII, No. 4, 1908).

(l) The dorsal vertebrÆ, as well as those of the other series, present a primitive character (fig. 8) in the persistence of the notochord (540). Among the Branchiosauria the notochord was not at all or but slightly constricted intravertebrally, but among the Microsauria it was constricted so far that the notochordal remnants in each centrum resemble an hour-glass.

The structure of the vertebrÆ among American forms agrees fully with that outlined by Credner, Fritsch, and others for the European species. The details of structure are so fully given by Zittel (642, pp. 346-353) and by Schwarz (540, 541) that it will not be necessary to state more here as to their structure, since there is nothing new to add concerning the American species.

The temnospondylous vertebra of the same nature and type as exhibited by the Permian forms has its representatives (94, 478) in the Coal Measures. Spondylerpeton spinatum Moodie (478) (plate 4, figs. 1, 2) and Eryops sp. (plate 18, fig. 2) indicate the embolomerous and rachitomous types of vertebral structure. The occurrence of these widely different types of vertebral structure indicates a long history for the group prior to the Coal Measures. This history is further indicated by footprints in the Mississippian and Devonian of this continent.

(m) The ribs (fig. 8) are very diverse in structure and in their mode of articulation (541) with the vertebral column. The characters of the ribs and vertebrÆ constitute the best means of classification of these animals so far discovered. In the Branchiosauria the ribs are always straight, heavy, and short, and articulate intravertebrally upon a large and strong transverse process. They occur throughout the vertebral column. There is a single pair of sacral ribs which are not to be clearly distinguished from the pre-sacral and post-sacral series. The cervical and caudal ribs are shorter than the dorsal series. The branchiosaurian rib is composed almost entirely of perichondral ossification. It presents the same condition as does the humerus of the cow embryo of 2 to 3 inches in length. The ribs of the branchiosaurs are identical in every way with the ribs of modern salamanders and form one of the strong arguments in favor of the relationship of the Branchiosauria to the Caudata. Among the Microsauria the ribs are always long, slender, curved, and intercentral. They may be either single or double headed, but usually the former. They resemble in their characters the ribs of some of the early reptiles and an attempt has been made to relate the Microsauria (469) to the primitive reptiles on this basis. The ribs of the other groups are still unknown. Indeed, representatives of the Temnospondylia and the Stereospondylia are very scanty in the American Coal Measures. One large rib (plate 22, fig. 4) may represent a labyrinthodont, but nothing is known of the species to which the rib belonged.

(n) The pectoral girdle (187) is a very simple and uniform structure, although the details of the association of the elements still remain to be determined. A single, median, usually large and elongate interclavicle occupies the ventral line of the chest. This is morphologically the same element which occurs in the middle line of the chest of the lizards. It is a dermal bone and is usually, especially among the Microsauria (462), highly sculptured. It varies considerably in size and shape, but is remarkably uniform throughout the various groups. Lying anterior to the interclavicle and overlapping its antero-lateral margins lie the two clavicles, which are usually diamond-shaped and are sculptured, dermal bones. The position of the coracoid is still uncertain, and in fact its clear association in the pectoral girdle of these species is still a question. It seems to be constant in the European (186, 251) species and is usually represented by a small rounded plate of bone, which in life no doubt had a large amount of cartilage to form its borders. A cleithrum (285) has been ascribed to one of the Linton, Ohio, species (plate 15, fig. 3) by Jaekel (347), but this needs confirmation. An osseous scapula is usually present, resembling the scapula of modern salamanders, in that it was largely embedded in cartilage. The position of the pectoral girdle is largely a matter of doubt, especially for the American species. After death and before fossilization the girdle was always moved by post-mortem shifting, so that its exact relation to the ribs and vertebral column is still in doubt. Credner (186) has restored the pectoral girdle close behind the head, which would cause an amount of rigidity in the body which probably did not exist.

(o) The arm consists of the humerus, radius, ulna, and 4 digits. The characters of the arm-bones are such as is constant among primitive animals and developing mammals. The osseous portion is perichondral. Epiphyses are totally lacking and it is doubtful if the endochondrium was at all ossified. The digits are often terminated by ungual phalanges, although usually the terminal phalanx was merely embedded in the web of the foot; and among the terrestrial forms a claw was well developed. An osseous carpus is not known in the species from the Coal Measures. Its impression indicates a broad hand, well adapted for swimming.

(p) The pelvic girdle consists uniformly (462) of the ilium and ischium. A small rounded pubis is present in some of the later forms of Amphibia; it is, however, totally absent from the Coal Measures species. The condition of the pelvis is paralleled by the partially grown pelvis of mammalian embryos in which the elements ossify in the order of ilium, ischium, and pubis. The ilium is always the larger of the elements. It supported or was attached to the sacral rib by means of a ligamentous union. The ischium did not ossify completely until the animal was nearly mature. The union between the elements of the pelvis was probably of a loose, membranous sort or else the whole mass was embedded in cartilage; of the two hypotheses the former is the more probable.

The pubis is indicated as a calcified quadrangular plate in a specimen of Amphibamus grandiceps Cope (478) from the Mazon Creek shales, and it is present as a rounded osseous element among some of the Permian forms.

(q) The leg (fig. 21, B) is composed of the femur, tibia, fibula, and 5 digits. The tarsus is usually cartilaginous, a single osseous tarsus (483, 484) being known (plate 23, fig. 1) from America. The distal phalanges may or may not be clawed, depending on the habits of life of the animal. The elements of the leg are ossified in a similar manner to those of the arm.

(r) The ventral scutellation (fig. 9), so commonly present among all groups of Amphibia in the Coal Measures, consists of a series of ossifications or calcifications in the myocommata. Among modern amphibians they occur as thin perpendicular planes of connective-tissue which are sometimes cartilaginous, especially in Necturus, regarded by Wilder (Memoirs of the Boston Society of Natural History, vol. V, No. 9, p. 400, fig. 6, 1903) and by Wiedersheim (605, p. 58) as a homologue or predecessor of the sternum, although Wiedersheim says:

"The sternum appears for the first time in Amphibians in the form of a small variously shaped plate of cartilage situated in the middle line of the chest. It arises as a paired cartilaginous plate in the inscriptiones tendineÆ of the rectus abdominis muscle, and therefore may be looked upon as corresponding to a pair of 'abdominal ribs.' Such cartilaginous abdominal ribs must have been present in greater numbers in the ancestors of existing Urodeles."