Seymouriamorphs are a small butwidespread group of early terrestrial choanates. They are known from theLower to Upper Permian and have been found in Europe, Asia, and NorthAmerica (Fig. 1). Six genera and eight species are known. Many additional specieshave been erected, but most of them are not valid (Klembara and Janiga,1993).
Figure 1. Geographic distribution of seymouriamorph localities. These localities are located in Eurasia and North America, but in the Permian, the continents were in different positions and Eurasia was represented by several smaller plates.
Small aquatic larvae bearing external gills are known in someseymouriamorphs (Ivakhnenko, 1981; Kuznetsov and Ivakhnenko, 1981;Klembara, 1995). The adults, when known, appear to have been terrestrial.The skull length of the known specimens ranges from about 6 mm in thesmallest larvea (Ivakhnenko, 1981) to 15 cm in presumably matureindividuals. The monophyly of this group has long been doubted because ofsize and ontogenetic differences between various taxa. Seymouria andKotlassia are only known from relatively large and mature specimens (Fig. 2) whereasall other known seymouriamorphs are represented by larval and small tomid-sized postmetamorphic specimens and were previously calleddiscosauriscids (this is now known to be a paraphyletic group). However,recent discoveries of fairly large specimens of Discosauriscus (Klembaraand Meszáros, 1992) and Ariekanerpeton (Laurin, 1996b) and relatively small specimens of Seymouria (Berman and Martens, 1993) have bridged this gap.
Figure 2. Seymouriamorph skulls in dorsal (A-C) and left lateral (D-F) views.Seymouria baylorensis (A, D). Seymouria sanjuanensis (B, E).Ariekanerpeton sigalovi (E, F). The specimens on which the reconstructionsof S. sanjuanensis and A. sigalovi are based are probably not mature andthese taxa may have reached approximately the same size as S. baylorensis.Modified from Laurin (1995).
The dermal cranial bones of larval and perhaps some juvenile postmetamorphic specimens ofseymouriamorphs bear grooves for the lateral-line organ. This organ iscomposed of ciliated cells that can detect motion in the water. Onlyaquatic vertebrates have a lateral-line. The lateral-line organ is foundin all fishes, most permanently aquatic amphibians, and many aquatic larvaeof terrestrial amphibians. Therefore, juvenile seymouriamorphs werecertainly aquatic. However, the absence of grooves in largepostmetamorphic specimens of seymouriamorphs suggests the lateral-line mayhave disappeared during or after the metamorphosis.
Enigmatic structures consisting of foramina located in deep depressions were recently discovered on some cranial bones of larval specimens of Discosauriscus (Klembara, 1994). These structures have been interpreted as foraminate pits, structures that were previously known in osteolepiforms, but in no terrestrial vertebrate. Comparisons with urodeles suggest that the foraminate pits housed ampullary electroreceptive organs. Similar electroreceptive organs are widespread in vertebrates and are found, among other groups, in sharks, teleosts, and lungfishes. However, in all extant taxa, the ampullary organ is located in soft tissues and leaves no trace on the skeleton. Therefore, it is difficult to prove that the foraminate pits of Discosauriscus housed ampullary organs.
The slender stapes of seymouriamorphs and the presence of a large oticnotch in the temporal area suggest that seymouriamorphs had a tympanicmiddle ear. The otic notch probably supported a large tympanum (ear drum)and the stapes probably transmitted the sounds from the tympanum to thefenestra ovalis (the stapes is involved in sound transmission in alltetrapods that have a tympanum, although in mammals, other bones are alsoinvolved). The tympanic middle ear is an adaptation to hear high-frequencyair-borne sounds (it is not effective underwater). Tetrapods lacking atympanum, like salamanders, can detect low-frequency seismic vibrations(Duellman and Trueb, 1986), but they cannot detect high-frequency air-bornesounds (above 1000 Hz). The presence of a tympanic middle ear inseymouriamorphs and the absence of lateral-line canal grooves in largepostmetamorphic specimens suggest that adult seymouriamorphs wereterrestrial.
The gut contents or coprolites of seymouriamorphs have only been found recently (Klembaraand Meszáros, 1992), and they suggest that seymouriamorphs resorted at least occasionally to cannibalism. Their main diet is unknown. However, their sharp, conical teeth and palatal fangs suggest that seymouriamorphs were predators.
The dermal cranial bones of adult seymouriamorphs are deeply sculptured andexhibit a hexagonal pattern of ridges also found in many temnospondyls (Fig. 3).The larvae lacked this sculpturing but they were covered by shallow pitsand grooves radiating from the center of ossification of the bones. Thepresence of dermal sculpturing suggests that the skin was tightly attachedto the skull. Seymouriamorphs retained all the dermal bones found in mostother groups of terrestrial choanates, and, as in embolomeres and amniotes,the tabular contacted the parietal.
Figure 3. Dorsal view of the skull of a juvenile postmetamorphic specimen of Discosauriscus austriacus showing the dermal sculpturing that characterizes relatively mature seymouriamorphs. Courtesy of Dr. Jozef Klembara.
Seymouriamorphs had a relatively short trunk (24 to 28 presacral vertebrae)and stout limbs. Each vertebra was composed of a neural arch, a large,cylindrical, amphicoelous pleurocentrum, and a small, crescenticintercentrum. The neural arches were swollen, especially in large, maturespecimens. The atlantal (first cervical) arch and pleurocentrum werepaired. This is probably a primitive character because the neural archesand pleurocentra of the earliest terrestrial vertebrates were also paired(they had independent left and right arches and pleurocentra rather than afused median arch and pleurocentrum). Hemal arches were present in thetail. There was usually only one sacral vertebra in small specimens, butlarge individuals occasionally had two. In this case, the posterior pairof sacral ribs was much more slender than the anterior pair, as inSeymouria baylorensis, or only one of the anterior ribs (right or left)contacted the ilium, as in Kotlassia. Ribs were present from the firstcervical to the first few caudal vertebrae. The ribs of the pectoralregion were expanded, presumably to provide better attachment to themuscles that supported the shoulder girdle. The exact number of caudalribs is unknown, but as many as fifteen pairs may have been present(Ivakhnenko, 1981).
The dermal shoulder girdle included a median interclavicle and a pairedclavicle and cleithra. The endochondral shoulder girdle included a scapulaand a coracoid. Like many other terrestrial choanates, seymouriamorphs hadfive fingers and toes. The phalangeal formula was 2 3 4 4 3 or 2 3 4 5 3 inthe hand (depending on the genus) and 2 3 4 5 3 in the foot.
Seymouriamorphs share a few derived characters, but an exhaustive list isdifficult to establish because most taxa are incompletely known. However,all seymouriamorphs have relatively small posttemporal fenestrae (these areopenings on the occiput through which blood vessels enter the skull),although this character is especially noticeable in large, matureseymouriamorphs. Their posttemporal fenestra may have grown with a negativeallometry.
All seymouriamorphs appear to have an otic tube composed of the opisthotic,the prootic, and the parasphenoid.
Most, if not all, seymouriamorphs had a large, transverse dorsal flange ofthe pterygoid (called the lamina ascendens by Bystrow, 1944) that extended to the skull roof.
All seymouriamorphs had a slender stapes that may have been involved inhearing. This character appeared convergently in temnospondyls and inseveral groups of tetrapods.
Evolution and Systematics
Discussion of Phylogenetic Relationships
Seymouriamorphs have long played a prominent role in scenarios andtheories on the origin of amniotes. They were originally believed to beclosely related to amniotes or even to represent the most archaic amniotes(Broili, 1904, White, 1939). The discovery of larval seymouriamorphs withexternal gills contradicts this hypothesis. However, many scientists stillconsider them close relatives of amniotes (Gauthier et al., 1988). Despitetheir long association with the origin of amniotes, seymouriamorphs havelong been considered amphibians simplybecause they are not amniotes, andall anamniotic terrestrial vertebrates were considered amphibians beforeclassifications were modified to include only monophyletic groups(Carroll, 1988). However, seymouriamorphs are not closely related tolissamphibians and are not amphibians in the modern sense of this word.
Some recent studies suggesting that seymouriamorphs are not as closelyrelated to amniotes as previously thought also assert that lepospondylswere more closely related to amniotes than to seymouriamorphs (Carroll,1995). Others (Laurin and Reisz, 1997) argued that seymouriamorphswere not more closely related to amniotes than to amphibians. If thelatter view is correct, seymouriamorphs are not tetrapods because Tetrapodais defined as the crown-group of terrestrial choanates (Gauthier et al.,1989).
Few studies have dealt with the phylogeny within Seymouriamorpha. However,recent work has suggested that Discosauriscus, Ariekanerpeton, andSeymouria are more closely related to each other than to Kotlassia andUtegenia (Laurin, in press). The affinities of Urumqia are difficult toevaluate because this seymouriamorph is poorly known (Zhang et al., 1984),but the presence of rhomboidal ventral bony scales (gastralia) suggests that it is not closely related to Discosauriscus, Ariekanerpeton, and Seymouria.