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Metoposaurus
Mounted specimen from the Krasiejów Museum in Poland.
When: Mid Triassic (~228 - 216 Million Years Ago)
Where: Europe and North America
What: Metoposaurus is an amphibian that lived on the northern continents during the mid Triassic. It was very large compared to modern amphibians, at 10 feet (3 meters) long and weighing an estimated 1000 lbs (450 kg), but these large amphibians were typical of this time, and for tens of millions of years previously. Its limbs are fairy small and weak for its body size, leading researchers to conclude it spend much of its time in the water. Its head was very well adapted for catching fish, with its dozens upon dozens of needle like teeth. Its head was extremely flat, again typical for these amphibians. A flat head like this would have allowed it to breath and look over the surface of the water with out causing disturbances which would have scared away fish. The flat head of Metoposaurus would also allow it to wait at the surface with out easily being seen by predators on the shore. Large groups of Metoposaurus have been found in some areas, in what appear to be pools that were drying out. These animals clustered together in the last remaining water there was during droughts, but alas, the rains didn’t return again in time.
Metoposaurus fairy typical member of the Temnospondyli. This group is known from most of the condiants from the Carboniferous to the Triassic, with some species making it all the way to the Cretaceous. Temnospondyls include the largest amphibians ever known, some of them easily dwarfing Metoposaurus. All of these large forms still spend a lot of time in the water, shown by a variety of skeletal features, such as the weak limbs seen in Metoposaurus. It is uncertain how the temnospondyls are related to the rest of Tetrapoda. They are farther down the tetrapod line than Pederpes, but past this there is a lot of controversy. Some researchers place them as outside of crown tetrapods (modern amphibians + amniotes), while others place them on the line leading to the modern amphibians (frogs, salamanders, and caecilians). It is also very likely that Temnospondyli is not even a natural group, and that some species are closer to amphibians than others.
Did I mention they have really really flat heads?
Recon by Dmitry Bogdanov (DiBgd)
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Diadectes
When: Early Permian (299-271 Million years ago)
Where: North America
What: Diadectes is a large reptiliomorph. That means that it is on the line towards amniotes (reptiles and mammals), but is not quite there yet. Diadectes is closer than most reptilomorphs, and is found in many studies to be the sister taxonto the amniotes. It is thought to have been amphibious, but spending most of its time on land. The dentition of Diadectes clearly marks it as an herbivore; its front teeth were procumbent(they tilted forward) and its cheek teeth were blunted and fairly wide compared to other stem reptilomorphs. Diadectes was one of the first large terrestrial herbivores, though it was not the only one of its time. Edaphosaurus was a synapsid (‘mammal-like reptile’) contemporary of Diadectes. The herbivorous diet of these two taxa was attained convergently, as there was by this time a large amount of plant material covering the land.
Diadectes looks very reptilian, and in fact some early studies placed it as a true reptile, but odds are the clade is belongs to is not directly ancestral to any of the amniotes. Though Diadectes with its large bulky form and size of up to 9 feet (~3 meters) is convergent on later reptiles such as Scutosaurus, the first true reptiles were small lizard like insect eating forms, such as Hylonomus.
Reconstruction by Matt Celeskey.
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Acanthostega
Mounted specimen on display at the American Museum of Natural History, NYC
When: Late Devonian (~365 million years ago)
Where: The ancient swamps of Greenland, near the Devonian equator.
What: Acanthostega is a basal tetrapod - one of the first vertebrates to develop limbs instead of fins, though it was not fully able to maneuver on land. Its pectoral girdle was constructed much more like that of a fish than a later tetrapod and could not bear the weight of the animal on land. However, it is the first taxon known that did have a pelvic girdle capable of supporting weight and propelling it forwards, this was accomplished by the fusion of either sides of the pelvis to one another ventrally, and a firm contact established between either pelvis and at least two vertebrae - precursors of the fused vertebra that would become the sacrum in later tetrapods. What is the use if only one half of the body is able to be supported by limbs? This type of pelvic girdle most likely did not at first develop for support. This structure also marks a shift from a primarily forelimb driven locomotion mode to one propelled by activity in the hind-limbs. Acanthostega could move in extremely shallow waters by ‘walking’ on the sediment with its hind-limbs, with its forelimbs steering instead of providing the primary movement source.
This interpretation is supported by its jaw structure, which lacks features relating to suction feeding. Rather it is hypothesized this primitive tetrapod would feed by directly biting onto insects and other terrestrial invertebrates that it could reach from the water’s edge. Thus, the predatory mode that characterizes the first terrestrial vertebrates was first developed by an aquatic animal. Later tetrapods were able to emerge from the waters, at least for short periods of time, to hunt prey that were beyond the reach of their ancestors. Another interesting note is that Acanthostega had 8 digits on its forelimb - it took a while in the evolution of tetrapods for five digits on the hands and feet to become established.
