Parmastega aelidae: Devonian Tetrapod Had Crocodile-Like Lifestyle
Paleontologists have discovered the fossils of a new type of early tetrapod (four-limbed vertebrate) in the Komi Republic. Dubbed Parmastega aelidae, the ancient creature lived about 372 million years ago (Devonian period) and was an aquatic, surface-cruising animal.
“The first tetrapods evolved from fishes during the Devonian period, which ended about 360 million years ago,” said lead author Dr. Per Erik Ahlberg of Uppsala University and colleagues.
“For many decades, our idea of what they were like has been based on just a few genera, chiefly Ichthyostega and Acanthostega.”
“Most other Devonian tetrapods are known only from a few scraps of jaws or limb bones: enough to show that they existed, but not really enough to tell us anything useful.”
“Furthermore, Ichthyostega and Acanthostega lived at the very end of the Devonian period. Some of the fragmentary tetrapods are a lot older, up to 373 million years old, and the oldest fossil tetrapod footprints date back a whopping 390 million years.”
“So Devonian tetrapods have a long early history about which, until now, we have known very little. Parmastega aelidae changes all this.”
The fossilized remains of Parmastega aelidae were recovered from the Sosnogorsk Formation, a limestone formed in a tropical coastal lagoon, which is now exposed on the banks of the Izhma River in the Komi Republic. They are only marginally younger than the oldest fragmentary tetrapod bones.
Fish-like characteristics in some bones indicate that Parmastega aelidae is not only the earliest but also the most primitive of the well-preserved Devonian tetrapods.
“And what a strange creature it is,” the paleontologists said.
“Like other Devonian tetrapods, Parmastega aelidae is vaguely crocodile-like in shape, but its eyes are raised above the top of the head, and the curve of its snout and lower jaw create a disconcerting ‘grin’ that reveals its formidable teeth.”
“A clue to its lifestyle is provided by the lateral line canals, sensory organs for detecting vibrations in the water, which it inherited from its fish ancestors.”
“These canals are well-developed on the lower jaw, the snout and the sides of the face, but they die out on top of the head behind the eyes. This probably means that it spent a lot of time hanging around at the surface of the water, with the top of the head just awash and the eyes protruding into the air. But why?”
“Crocodiles do this today, because they are keeping an eye out for land animals that they might want to catch. We don’t know very much about the land that surrounded Parmastega aelidae’s lagoon, but there may have been large arthropods such as millipedes or sea scorpions to catch at the water’s edge.”
Parmastega aelidae’s slender, elastic lower jaw looks well-suited to scooping prey off the ground, its needle-like teeth contrasting with the robust fangs of the upper jaw that would have been driven into the prey by the body weight.
“The fossil material springs one final surprise: the shoulder girdle was made partly from cartilage, which is softer than bone, and the vertebral column and limbs may have been entirely cartilaginous as they are not preserved,” the researchers said.
“This strongly suggests that Parmastega aelidae, with its crocodile-like head and protruding eyes, never really left the water.”
“Did it creep up on prey at the water’s edge and surge onto the shore to seize it in its jaws, only to then slide back into the supporting embrace of the water? We don’t know.”
“Far from presenting a progressive cavalcade of ever more land-adapted animals, the origin of tetrapods is looking more and more like a tangled bush of ecological experimentation.”
The findings were published in the October 24, 2019 issue of the journal Nature.
Pavel A. Beznosov et al. 2019. Morphology of the earliest reconstructable tetrapod Parmastega aelidae. Nature 574: 527-531; doi: 10.1038/s41586-019-1636-y