| Seymouria | |
|---|---|
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| A fossil ofSeymouria baylorensis displayed at theNational Museum of Natural History | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Order: | †Seymouriamorpha |
| Family: | †Seymouriidae |
| Genus: | †Seymouria Broili, 1904 |
| Species | |
| Synonyms | |
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Seymouria is anextinctgenus ofseymouriamorph from the EarlyPermian ofNorth America andEurope.[1] Although they wereamphibians (in a biological sense),Seymouria were well-adapted to life on land, with manyreptilian features—so many, in fact, thatSeymouria was first thought to be a primitive reptile.[2][3] It is primarily known from two species,Seymouria baylorensis[4] andSeymouria sanjuanensis.[5] The type species,S. baylorensis, is more robust and specialized, though its fossils have only been found inTexas.[6] On the other hand,S. sanjuanensis is more abundant and widespread. This smaller species is known from multiple well-preserved fossils, including a block of six skeletons found in theCutler Formation ofNew Mexico,[7] and a pair of fully grown skeletons from theTambach Formation ofGermany, which were fossilized lying next to each other.[8]
For the first half of the 20th century,Seymouria was considered one of the oldest and most "primitive" known reptiles.[2][3]Paleontologists noted how the general body shape resembled that of early reptiles such ascaptorhinids, and that certain adaptations of the limbs, hip, and skull were also similar to that of early reptiles, rather than any species of modern or extinct amphibians known at the time. The strongly-built limbs and backbone also supported the idea thatSeymouria was primarily terrestrial, spending very little time in the water.[9] However, in the 1950s, fossilizedtadpoles were discovered inDiscosauriscus, which was a close relative ofSeymouria in the groupSeymouriamorpha. This shows that seymouriamorphs (includingSeymouria) had alarval stage which lived in the water, therefore makingSeymouria not a true reptile, but rather an amphibian (in the traditional, paraphyletic sense of the term). At that time, it was still thought to be closely related to reptiles,[10] and many recent studies still support this hypothesis.[11][12] If this hypothesis is correct,Seymouria is still an importanttransitional fossil documenting the acquisition of reptile-like skeletal features prior to the evolution of the amniotic egg, which characterizesamniotes (reptiles,mammals, andbirds).[1] However, under the alternative hypothesis thatSeymouria is a stem-tetrapod, it has little relevance to the origin of amniotes.[13]
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Fossils ofSeymouria were first found near the town ofSeymour, inBaylor County,Texas (hence the name of the type species,Seymouria baylorensis, referring to both the town and county). The earliest fossils of the species to be collected were a cluster of individuals acquired byC.H. Sternberg in 1882. However, these fossils would not be properly prepared and identified asSeymouria until 1930.[15]
Various paleontologists from around the world recovered their ownSeymouria baylorensis fossils in the late 19th century and early 20th century.Seymouria wasformally named and described in 1904 based on a pair of incomplete skulls, one of which was associated with a few pectoral and vertebral elements. These fossils were described by German paleontologistFerdinand Broili, and are now stored inMunich.[4] American paleontologistS.W. Williston later described a nearly complete skeleton in 1911, and noted that "Desmospondylus anomalus", a taxon he had recently named from fragmentary limbs and vertebrae, likely represented juvenile or even embryonic individuals ofSeymouria.[2]
Likewise, English paleontologistD.M.S. Watson noted in 1918 thatConodectes, a dubious genera named byEdward Drinker Cope back in 1896, was likely synonymous withSeymouria.[3]Robert Broom (1922) argued that the genus should be referred to asConodectes since that name was published first,[16] butAlfred Romer (1928) objected, noting that the nameSeymouria was too popular within the scientific community to be replaced.[9] During this time,Seymouria was generally seen as a very early reptile, part of anevolutionary grade known as "cotylosaurs", which also included many other stout-bodied Permian reptiles or reptile-like tetrapods.
Many paleontologists were uncertain aboutSeymouria's allegiance with the reptiles, noting many similarities with theembolomeres, which were unquestionably "labyrinthodont" amphibians. This combination of features from reptiles (i.e. other "cotylosaurs") and amphibians (i.e. embolomeres) was evidence thatSeymouria was central to the evolutionary transition between the two groups. Regardless, not enough was known about its biology to conclude which group it was truly part of. Broom (1922)[16] and Russian paleontologistPeter Sushkin (1925)[17] supported a placement among the Amphibia, but most studies around this time tentatively considered it an extremely "primitive" reptile; these included a comprehensive redescription of material referred to the species, published byTheodore E. White in 1939.[15]
However, indirect evidence thatSeymouria was not biologically reptilian started to emerge by the 1940s. Around this time, several newly described genera were linked toSeymouria as part of the groupSeymouriamorpha. Some seymouriamorphs, such asKotlassia, had evidence of aquatic habits, and evenSeymouria itself had occasionally been argued to possess lateral lines, sensory structures only usable underwater.[15]Watson (1942)[18] and Romer (1947)[19] each reversed their stance onSeymouria's classification, placing it among the amphibians rather than the reptiles. Perhaps the most damning evidence came in 1952, when Czech paleontologistZdeněk Špinar reported gills preserved in juvenile fossils of the seymouriamorphDiscosauriscus. This unequivocally proved that seymouriamorphs had an aquaticlarval stage, and thus were amphibians, biologically speaking.[10] Nevertheless, the numerous similarities betweenSeymouria and reptiles supported the idea that seymouriamorphs were close to the ancestry ofamniotes.
In 1966,Peter Paul Vaughn described an assortment ofSeymouria skulls from theOrgan Rock Shale ofUtah. These remains represented a new species,Seymouria sanjuanensis.[5] Fossils of this species are now understood to be more abundant and widespread than those ofSeymouria baylorensis. Several more species were later named byPaul E. Olson, although their validity has been more questionable than that ofS. sanjuanensis. For example,Seymouria agilis (Olson, 1980), known from a nearly completeskeleton from theChickasha Formation ofOklahoma, was reassigned byMichel Laurin andRobert R. Reisz to theparareptileMacroleter in 2001.[20]Seymouria grandis, described a year earlier from a braincase found in Texas, has not been re-referred to any other tetrapod, but it remains poorly known. Langston (1963) reported a femur indistinguishable from that ofS. baylorensis in Permian sediments atPrince Edward Island on the Eastern coast ofCanada.[21]Seymouria-like skeletal remains are also known from theRichards Spur Quarry in Oklahoma, as first described by Sullivan & Reisz (1999).[22]
A block of sediment containing sixS. sanjuanensis skeletons was found in theCutler Formation ofNew Mexico, as described by Berman, Reisz, & Eberth (1987).[7] In 1993, Berman & Martens reported the firstSeymouria remains outside ofNorth America, when they describedS. sanjuanensis fossils from theTambach Formation ofGermany.[21] The Tambach Formation has producedS. sanjuanensis fossils of a similar quality to those of the Cutler Formation. For example, in 2000 Berman and his colleagues described the "Tambach Lovers", two complete and fully articulated skeletons ofS. sanjuanensis fossilized lying next to each other (though it cannot be determined whether they were a couple killed during courtship).[8] The Tambach Formation has also produced the developmentally youngest known fossils ofSeymouria, assisting comparisons toDiscosauriscus, which is known primarily from juveniles.[23]
Seymouria individuals were robustly-built animals, with a large head, short neck, stocky limbs, and broad feet.[2][19] Even the largest specimens were fairly small, only about 2 ft (60 cm) long. The skull was boxy and roughly triangular when seen from above, but it was lower and longer than that of most other seymouriamorphs. The vertebrae had broad, swollen neural arches (the portion above the spinal cord). As a whole the body shape was similar to that of contemporary reptiles and reptile-like tetrapods such ascaptorhinids,diadectomorphs, andparareptiles. Collectively these types of animals have been referred to as "cotylosaurs" in the past, although they do not form aclade (a natural, relations-based grouping).
The skull was composed of many smaller plate-like bones. The configuration of skull bones present inSeymouria was very similar to that of far more ancient tetrapods and tetrapod relatives. For example, it retains anintertemporal bone, which is theplesiomorphic ("primitive") condition present in animals likeVentastega andembolomeres.[19] The skull bones were heavily textured, as was typical for ancient amphibians andcaptorhinid reptiles. In addition, the rear part of the skull had a large incision stretching along its side. This incision is termed anotic notch, and a similar incision in the same general area is common to most Paleozoic amphibians ("labyrinthodonts", as they are sometimes called), but unknown in amniotes. The lower edge of the otic notch was formed by thesquamosal bone, while the upper edge was formed by downturnedflanges of thesupratemporal andtabular bones (known as otic flanges). The tabular also has a second downturned flange visible from the rear of the skull; this flange (known as an occipital flange) connected to the braincase and partially obscured the space between the braincase and the side of the skull. The development of the otic and occipital flanges is greater inSeymouria (particularlyS. baylorensis) than in any other seymouriamorph.[6]
The sensory apparatus of the skull also deserves mention for an array of unique features. Theorbits (eye sockets) were about midway down the length of the skull, although they were a bit closer to the snout in juveniles. They were more rhomboidal than the circular orbits of other seymouriamorphs, with an acute front edge.[15] Several authors have noted that a few specimens ofSeymouria possessed indistinct grooves present in bones surrounding the orbits and in front of the otic notch. These grooves were likely remnants of alateral line system, a web of pressure-sensing organs useful for aquatic animals, including the presumed larval stage ofSeymouria.[15][19][5] Many specimens do not retain any remnant of their lateral lines,[7][6] not even juveniles.[23] Near the middle of theparietal bones was a small hole known as a pineal foramen, which held a sensory organ known as aparietal eye. The pineal foramen is smaller inSeymouria than in other seymouriamorphs.[8]
Thestapes, a rod-like bone which lies between the braincase and the wall of the skull, was tapered. It connected the braincase to the upper edge of the otic notch, and likely served as a conduit of vibrations received by atympanum (eardrum) which presumably lay within the otic notch. In this way it could transmit sound from the outside world to the brain. The configuration of the stapes is intermediate between non-amniote tetrapods and amniotes. On the one hand, its connection to the otic notch is unusual, since true reptiles and other amniotes have lost an otic notch, forcing the tympanum and stapes to shift downwards towards thequadrate bone of the jaw joint. On the other hand, the thin, sensitive structure ofSeymouria's stapes is a specialization over most non-amniote tetrapods, which have a thick stapes better suited for reinforcing the skull rather than hearing.[9] The inner ear ofSeymouria baylorensis retains acochlear recess located behind (rather than below) thevestibule, and itsanterior semicircular canal was likely encompassed by a cartilaginous (rather than bony)supraoccipital. These features are more primitive than those of true reptiles and synapsids.[24]
Thepalate (roof of the mouth) had some similarities with both amniote and non-amniote tetrapods. On the one hand, it retained a few isolated large fangs with maze-like internal enamel folding, as is characteristic for "labyrinthodont" amphibians. On the other hand, thevomer bones at the front of the mouth were fairly narrow, and the adjacentchoanae (holes leading from the nasal cavity to the mouth) were large and close together, as in amniotes. The palate is generally solid bone, with only vestigial interpteryoid vacuities (a pair of holes adjacent to the midline) separated by a long and thin cultriform process (the front blade of the base of the braincase). Apart from the fangs, the palate is also covered with small denticles radiating out from the rear part of thepterygoid bones.[25]Seymouria has a few amniote-like characteristics of the palate, such as the presence of a prong-like outer rear branch of the pterygoid (formally known as a transverse flange) as well as anepipterygoid bone which is separate from the pterygoid. However, these characteristics have been observed in various non-amniote tetrapods, so they do not signify its status as an amniote.[6]
The lower jaw retained a few plesiomorphic characteristics. For example, the inner edge of the mandible possessed three coronoid bones.[15] The mandible also retained at least one large hole along its inner edge known as a meckelian fenestra, although this feature was only confirmed during a 2005 re-investigation of one of the Cutler Formation specimens.[25] Neither of these traits are the standard in amniotes. The braincase had a mosaic of features in common with various tetrapodomorphs. The system of grooves and nerve openings on the side of the braincase were unusually similar to those of the fishMegalichthys, and the cartilaginous base is another plesiomorphic feature. However, theinternal carotid arteries perforate the braincase near the rear of the bone complex, a derived feature similar to amniotes.[15]
Thevertebral column is fairly short, with a total of 24vertebrae between the hip and skull.[8] The vertebrae are gastrocentrous, meaning that each vertebra has a larger, somewhat spool-shaped component known as a pleurocentrum, and a smaller, wedge-shaped (or crescent-shaped from the front) component known as an intercentrum. The neural arches, which lie above the pleurocentra, are swollen into broad structures with table-likezygapophyses (joint plates) about three times as wide as the pleurocentrum itself. Some vertebrae have neural spines which are partially subdivided down the middle, while others are oval-shaped in horizontal cross-section. The ribs of the dorsal vertebrae extend horizontally and attach to the vertebrae at two places: the intercentrum and the side of the neural arch.[15] The neck is practically absent, only a few vertebrae long. The first neck vertebra, theatlas, had a small intercentrum as well as a reduced pleurocentrum which was only present in mature individuals. Although the atlantal pleurocentrum (when present) was wedged between the intercentrum of the atlas and intercentrum of the succeedingaxis vertebra (as in amniotes), the low bone development in this area of the neck contrasts with the characteristic atlas-axis complex of amniotes.[26] In addition, later studies found that the atlas intercentrum was divided into a left and right portion, more like that of amphibian-grade tetrapods.[7] Unlike almost all other Paleozoic tetrapods (amniote or otherwise),Seymouria completely lacks any bony remnants of scales or scutes, not even the thin, circular belly scales of other seymouriamorphs.[6][8]
Thepectoral (shoulder) girdle has several reptile-like features. For example, thescapula andcoracoid (bony plates which lie above and below the shoulder socket, respectively) are separate bones, rather than one large shoulder blade. Likewise, theinterclavicle was flat and mushroom-shaped, with a long and thin "stem". Thehumerus (forearm bone) was shaped like a boxy and slightly twisted L, with large areas for muscle attachment. This form, which has been described as "tetrahedral", is plesiomorphic for tetrapods and contrasts with the slender hourglass-shaped humerus of amniotes. On the other hand, the lower part of the humerus also has a reptile-like adaptation: a hole known as anentepicondylar foramen. Theradius was narrowest at mid-length. Theulna is similar, but longer due to the possession of a pronouncedolecranon process, as is common in terrestrial tetrapods but rare in amphibious or aquatic ones. Thecarpus (wrist) has ten bones, and the hand has five stout fingers. The carpal bones are fully developed and closely contact each other, another indication of terrestriality. The phalanges (finger bones) decrease in size towards the tip of the fingers, where they each end in a tiny, rounded segment, without a claw. The phalangeal formula (number of phalanges per finger, from thumb to little finger) is 2-3-4-4-3.[8]
Two sacral (hip) vertebrae were present, though only the first one possessed a large, robust rib which contacted the ilium (upper blade of the hip).[15][8] Some studies have argued that there was only one sacral vertebra, with the supposed second sacral actually being the first caudal due to having a shorter, more curved rib than the first sacral.[7] Eachilium is low and teardrop-shaped when seen from the side, while the underside of the hip as a whole is formed by a single robust puboischiadic plate, which is rectangular when seen from below. Both the hip and shoulder sockets were directed at 45 degrees below the horizontal. Thefemur is equally stout as the humerus, and thetibia andfibula are robust, hourglass-shaped bones similar to the radius and ulna.[15] Thetarsus (ankle) incorporates 11 bones, intermediate between earlier tetrapods (which have 12) and amniotes (which have 8 or fewer). The five-toed feet are quite similar to the hands, with phalangeal formula 2-3-4-5-3.[8]
There were only about 20 caudal (tail) vertebrae at most. Past the base of the tail, the caudals start to acquire bony spines along their underside, known aschevrons. These begin to appear in the vicinity of the third to sixth caudal, depending on the specimen. Ribs are only present within the first five or six caudals; they are long at the base of the tail but diminish soon afterwards and typically disappear around the same area the chevrons appear.[8]
Seymouria baylorensis andSeymouria sanjuanensis can be distinguished from each other based on several differences in the shape and connections between the different bones of the skull. For example, the downturned flange of bone above the otic notch (sometimes termed the "tabular horn" or "otic process") is much more well-developed inS. baylorensis than inS. sanjuanensis. In the former species, it acquires a triangular shape (when seen from the side) as it extends downwards more extensively towards the rear of the skull.[8] InS. sanjuanensis, thepostfrontal bone contacts theparietal bone by means of an obtuse, wedge-like suture, while the connection between the two bones is completely straight inS. baylorensis.
Some authors have argued that thepostparietals ofS. baylorensis were smaller than those ofS. sanjuanensis, but some specimens ofS. sanjuanensis (for example, the "Tambach lovers") also had small postparietals. In addition, the "Tambach lovers" have aquadratojugal bone which is more similar to that ofS. baylorensis rather thanS. sanjuanensis. The combination of features from both species in these specimens may indicate that the two species are part of a continuous lineage, rather than two divergent evolutionary paths. Likewise, some differences relating to the proportions of the rear of the skull may be considered to be an artifact of the fact that mostS. sanjuanensis specimens were not fully grown prior to the discovery of the "Tambach lovers", which were adult members of the species.[8]
Nevertheless, several traits are still clearly differentiated between the two species. Thelacrimal bone, in front of the eyes, only occupies the front edge of the orbit inS. baylorensis. Conversely, specimens ofS. sanjuanensis have a branch of the lacrimal which extends a small distance under the orbit. InS. sanjuanensis, much of the rear edge of the orbit is formed by the chevron-shapedpostorbital bone, which is more rectangular inS. baylorensis. The shape of the lacrimal and postorbital ofS. sanjuanensis closely corresponds to the condition in other seymouriamorphs, while the condition inS. baylorensis is more unique and derived.[8]
The tooth-bearing maxilla bone, which forms the side of the snout, is also distinctively unique inS. baylorensis. InS. sanjuanensis, the maxilla was low, with many sharp, closely spaced teeth extending along its length. This condition is similar to other seymouriamorphs. However,S. baylorensis has a taller snout, and its teeth are generally much larger, less numerous, and less homogenous in size.[8] The palate is generally similar between the two species, although theectopterygoids are more triangular inS. baylorensis and rectangular inS. sanjuanensis.[25]
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Romer (1928) was among the first authors to discuss the biological implications ofSeymouria's skeleton. He argued that the robust limbs and wide-set body supported the idea that it was a strong, terrestrial animal with a sprawling gait. However, he also noted that Permian trackways generally support the idea that terrestrial tetrapods from this time period were not belly-draggers, but instead were strong enough to keep their bodies off of the ground. As with other paleontologists around the time, Romer assumed thatSeymouria had a reptilian (or amniote) mode of reproduction, with eggs laid on dry land and protected from the elements by anamnion membrane.[9]
White (1939) elaborated on biological implications. He noted that the presence of an otic notch reduces jaw strength by lowering the amount of surface area jaw muscles can attach to within the cranium. In addition, the skull would have been more fragile due to the presence of such a large incision. As a whole, he found it unlikely thatSeymouria was capable of tackling large, active prey. Nevertheless, the sites for muscle attachment on the palate were more well-developed than those of contemporaneous amphibians. White extrapolated thatSeymouria was a mostly carnivorous generalist and omnivore, feeding on invertebrates, small fish, and perhaps even some plant material. It may have even been cannibalistic according to his reckoning.[15]
White also drew attention to the unusual swollen vertebrae, which would have facilitated lateral (side-to-side) movement but prohibit any torsion (twisting) of the backbone. This would have been beneficial, sinceSeymouria had low-slung limbs and a wide, top-heavy body that would have otherwise been vulnerable to torsion when it was walking. This may also explain the presence of this trait in captorhinids, diadectomorphs, and other "cotylosaurs". Perhaps swollen vertebrae were an interim strategy to prevent torsion, which would later be supplanted by strong hip muscles in later reptiles. The rather undeveloped hip muscles ofSeymouria are in line with this hypothesis. Nevertheless, these vertebrae were inefficient at defending against torsion at any speed faster than a brisk walk, soSeymouria was probably not a quick-moving animal.[15]
Although White consideredSeymoria to be quite competent on land, he also discussed a few other lifestyles. He supposed thatSeymouria was also a good swimmer, since he (erroneously) estimated that the animal had a deep and powerful tail similar to that of moderncrocodilians. However, he also noted that it would have been vulnerable to semiaquatic or aquatic predators, and thatSeymouria fossils were more common in terrestrial deposits as a result of its habitat preferences. Bermanet al. (2000) supported this hypothesis, as theTambach Formation preservedSeymouria fossils while also completely lacking aquatic animals. They also pointed out the well-developed wrist and ankle bones of the "Tambach lovers" as supportive of terrestrial affinities. Despite the strong musculature of the forelimbs, Romer (1928)[9] and White (1939)[15] found little evidence for burrowing adaptations inSeymouria.
Some authors have argued in favor ofsexual dimorphism existing inSeymouria, but others are unconvinced by this hypothesis. White (1939) argued that some specimens ofSeymouria baylorensis hadchevrons (bony spines on the underside of the tail vertebrae) which first appeared on the third tail vertebra, while other specimens had them first appear on the sixth. He postulated that the later appearance of the chevrons in some specimens was indicative that they were males in need of more space to store their internal genitalia. This type of sexual differentiation has been reported in bothturtles and crocodilians. Based on this, he also supported the idea thatSeymouria females gave birth to large-yolked eggs on land, as with turtles and crocodilians.[15] Vaughn (1966) later found a correlation between chevron acquisition and certain skull proportions inSeymouria sanjuanensis, and proposed that they too were examples of sexual dimorphism.[5]
However, Berman, Reisz, & Elberth (1987) criticized the methodologies of White (1939) and Vaughn (1966). They argued that White's observations were probably unrelated to the sex of the animals. This was supported by the fact that some of the Cutler Formation specimens had chevrons which first appeared on their fifth tail vertebra. Although it was possible that genital size was variable among males to the extent of impacting the skeleton, the more likely explanation was that the differences White had observed were caused by individual skeletal variation, evolutionary divergence, or some other factor unrelated to sexual dimorphism. Likewise, they agreed that skull proportions supported Vaughn (1966)'s proposal that dimorphism was present inSeymouria fossils, though they disagreed with how he linked it to sex using a fossil which was considered "female" under White's criteria.[7] The discovery of fossilized larval seymouriamorphs has shown thatSeymouria likely had an aquatic larval stage, debunking earlier hypotheses thatSeymouria laid eggs on land.[10]
Histological evidence from specimens found inRichards Spurs,Oklahoma has provided additional information onSeymouria's biology. A femur was found to have an internal structure characterized by a lamellar matrix pierced by numerous plexiform canals. Rest lines of slow growth are indistinct and closely spaced, but there is no evidence that growth ceased at any time during bone development. Like mostlissamphibians, themedullary cavity is open and has a small amount of spongiosa bone. The development of spongiosa bone is slightly higher that ofAcheloma (a terrestrial amphibian), but is much less extensive than aquatic amphibians such asRhinesuchus andTrimerorhachis.Seymouria's vertebrae are more robust in shape compared toDiscosauriscus, and have a low amount of cartilage despite a high amount of porosity.Seymouria are inferred to have undergone metamorphosis very early in life, likely due to environmental stresses from fluctuating wet and dry seasons.[27]
