Temnospondyls - History of Study

History of Study

Temnospondyli was named by German paleontologist Karl Alfred von Zittel in his second edition of Handbuch der Palaeontologie, published in 1888. Temnospondyl remains were known since the early part of the 19th century, however. The earliest described temnospondyl was Mastodonsaurus, named by Georg Friedrich Jaeger in 1828. Jaeger named Mastodonsaurus from a single tooth, and considered it a reptile. Mastodonsaurus means "breast tooth lizard" after the nipple-like shape of the tip of the tooth.

The naming of these first specimens was disputed, however. Leopold Fitzinger named the animal Batrachosaurus in 1837. In 1841, English paleontologist Richard Owen referred to the genus as Labyrinthodon to describe its highly folded or labyrinthine teeth. Owen thought the name Mastodonsaurus "ought not to be retained, because it recalls unavoidably the idea of the mammalian genus Mastodon, or else a mammilloid form of the tooth... and because the second element of the word, saurus, indicates a false affinity, the remains belonging, not to the Saurian, but to the Batrachian order of Reptiles." Owen recognized the animal was not a "saurian" reptile, yet he also referred Jaeger's Phytosaurus to the genus. Although the two genera both have similarly sized conical teeth, Phytosaurus was later found to be a crocodile-like reptile. Additional material, including skulls, firmly placed Labyrinthodon as an amphibian. Jaeger also named Salamandroides giganteus in 1828, basing it on partial occiput, or back portion of the skull. In 1833, he described a complete skull of S. giganteus that had the same teeth as his Mastodonsaurus, making it the first known complete skull of a temnospondyl. Because Mastodonsaurus was named first, it has precedence over the other names as a senior subjective synonym. Batrachosaurus is still used as the name of an unrelated brachyopid temnospondyl.

Mastodonsaurus and other similar animals were referred to as labyrinthodonts, named like Labyrinthodon for teeth that were highly folded in cross section. Owen's "Labyrinthodon Jaegeri" was later found at Guy's Cliffe, England by paleontologist William Buckland. Other specimens were found in the red sandstone of Warwickshire. As more fossils were uncovered in England, Owen depicted these labyrinthodonts as the "highest" form of batrachian and compared them to crocodiles, which he considered the highest form of reptiles. He also noted the large labyrinthodonts of the Keuper (a unit of rocks that dates to the Late Triassic) were younger than more advanced reptiles in the Magnesian and Zechstein, which are Late Permian in age. Owen used these fossils to counter the notion that reptiles evolved from a sequential progression from early amphibians (what he called "metamorphosed fishes").

In addition to Mastodonsaurus, some of the earliest named genera included Metopias and Rhombopholis in 1842, Zygosaurus in 1848, Trematosaurus in 1849, Baphetes and Dendrerpeton in 1853, Capitosaurus in 1858, and Dasyceps in 1859. Baphetes is now placed as an early tetrapod outside Temnospondyli, and Rhombopholis is now considered a prolacertiform reptile.

Later in the 19th century, temnospondyls were classified as various members of Stegocephalia, a name coined by American paleontologist Edward Drinker Cope in 1868. Cope placed stegocephalians in the class Batrachia, the name then used for Amphibia. Stegocephalia means "roof-headed" in Greek, a reference to the wide, flat heads of temnospondyls and other early tetrapods. During this time, paleontologists considered temnospondyls to be amphibians because they possessed three main features: gill arches in juvenile skeletons, indicating they were amphibious for at least the first part of their lives; ribs that do not connect at the underside of the rib cage; and deep pits in the skull that were interpreted as space for mucous glands.

Several suborders of stegocephalians were recognized in the late 19th and early 20th centuries. Animals now regarded as temnospondyls were primarily labyrinthodonts, but some were classified in the Branchiosauria. Branchiosaurs were small-bodied and had simple conical teeth, while labyrinthodonts were larger and had complex, folded dentin and enamel in their teeth. Branchiosauria included only a few forms, such as Branchiosaurus from Europe and Amphibamus from North America, that had poorly developed bones, external gills, and no ribs. Some skeletons of Amphibamus were later found with long ribs, prompting its reassignment to Microsauria (although more detailed studies found it to be a temnospondyl). Soft tissue, such as scales and external gills, were found in many well-preserved branchiosaur fossils from Germany. In the early 20th century, branchiosaurs would be recognized as larval forms of temnospondyls lacking many of the typical features that define the group.

Other animals that would later be classified as temnospondyls were placed in a group called Ganocephala, characterized by plate-like skull bones, small limbs, fish-like scales, and branchial arches. Unlike labyrinthodonts, they did not have parietal foramena, small holes in their skulls behind their eye sockets. Archegosaurus, Dendrerpeton, Eryops and Trimerorhachis were placed in this group and were considered to be the most primitive members of Reptilia. Their rhachitomous vertebrae, notochord, and lack of occipital condyles (which attached the head to the neck) were features that were also shared with fishes. Thus, they were considered a link between early fishes and more advanced forms such as stegocephalians.

Another group called Microsauria was named by Cope in 1868. Cope classified Microsauria as a subgroup of Labyrinthodontia, placing many small, amphibian-like animals within it. Among them were Dendrerpeton, once placed in Ganocephala. Dendrerpeton was later placed as a labyrinthodont with other temnospondyls, but confusion existed for many years over the classification of small amphibians.

By the end of the 19th century, most of what are today regarded as temnospondyls were placed in the suborder Labyrinthodonta. American paleontologist Ermine Cowles Case called it Labyrinthodonta vera or "true labyrinthodonts". The names Stegocephalia and Labyrinthodontia were used interchangeably to refer to the order in which it belonged. The labyrinthodontian suborders Microsauria and Branchiosauria, both of which contain temnospondyls, were distinct from Labyrinthodonta. Within Labyrinthodonta were the groups Rhachitomi, Labyrinthodonti, and Embolerimi. Members of Rhachitomi, such as Archegosaurus and Eryops, had rhachitomous vertebrae with enlarged intercentra that displaced the pleurocentra. Labyrinthodonti, such as Mastodonsaurus, Trematosaurus, and Micropholis, had lost their pleurocentra, and the intercentra made up the entire body of the vertebrae. Embolerimi had intercentra and pleurocentra that were of equal size. Embolomeres are now identified as reptiliomorphs distantly related to temnospondyls.

In 1888, von Zittel divided stegocephalians among three taxa: Lepospondyli, Temnospondyli, and Stereospondyli. He placed microsaurs in Lepospondyli, a group which he characterized as having simple, spool-shaped vertebral centra. Temnospondyli included forms with the centra divided into pleurocentra and intercentra. All members of Stereospondyli had amphicoelous centra composed only of the intercentra. Cope objected to von Zittel's classification, considering the vertebrae of lepospondyls and stereospondyls indistinguishable because each had a simple spool shape. He continued to use Ganocephala and Labyrinthodonta (which he alternatively referred to as Rhachitomi) to distinguish animals based on the absence or presence of occipital condyles.

Temnospondyli became a commonly used name at the turn of the century. Paleontologists included both embolomeres and rhachitomes in the group. Cope's Ganocephala and Labyrinthodonta fell out of use. In 1919, British paleontologist D. M. S. Watson proposed that the evolutionary history of these large amphibians could be seen through changes in their vertebrae. Embolomerous forms in the Carboniferous graded into rhachitomous forms in the Permian, and finally into stereospondyls in the Triassic. More importantly, Watson began using the term Labyrinthodontia to refer to these groups. The name Temnospondyli was rarely used in the decades that followed. Swedish paleontologist Gunnar Säve-Söderbergh removed embolomeres from the group, narrowing its scope to rhachitomes and stereospondyls. His classification of labyrinthodonts was based heavily on characteristics of the skull rather than the vertebrae.

American paleontologist Alfred Romer brought the name Temnospondyli back into use in the later 20th century. Säve-Söderbergh used the name Labyrinthodontia in a strict sense (sensu stricto) to refer to Rhachitomi and Stereospondyli, excluding Embolomeri. Romer agreed with this classification, but used the name Temnospondyli to avoid confusion with Labyrinthodontia in its wider sense (sensu lato). Unlike modern temnospondyl classification, however, Romer included the primitive Ichthyostegalia in the group.