July 9. Labyrinthodonts

Labyrinthodont tooth cross-section

Labyrinthodonts’ name means maze-tooth for the incredibly convoluted infoldings of their tooth enamel. They were tetrapods, which had evolved from the lobe-finned fishes, and while they began during the Devonian, it appears that they evolved rapidly during the Mississippian and Pennsylvanian. During the Pennsylvanian, they developed better and better adaptations to living on land.  

Broadly speaking labyrinthodonts were amphibians, which return to water to reproduce. But their subgroups include variations that were more fish-like and perhaps entirely aquatic, as well as some that are more reptile-like, with stronger legs, branched toes, thick skulls, and maybe even skin that helped retain water during excursions on land.

Labyrinthodonts came in many sizes, ranging from salamander-like animals a few centimeters long to 9-meter monsters similar in size and shape to crocodiles, but they were still amphibians. Most of them were carnivorous, attacking prey in the shallows and swampy areas that were abundant during Pennsylvanian time. They reached their peak of diversity in Pennsylvanian and Permian time, especially following the collapse of the Pennsylvanian rainforest ecosystem which we’ll talk about later this month. But the dry climate that developed during the Permian was unfavorable to amphibians, and most groups of labyrinthodonts declined before the end-Permian extinction event. The few survivors made it all the way to the Cretaceous before all labyrinthodonts became extinct.

Labyrinthodonts are the ancestors of modern amphibians, but it’s definitely not clear exactly how the descent worked. There is controversy as to which of the sub-groups of labyrinthodonts gave rise to modern amphibians such as frogs, toads, and salamanders. It’s not even completely agreed whether modern amphibians derive from one lineage of labyrinthodont or two. The fossil record is sparse for these animals during the Permian and Triassic, when the change to modern forms was taking place.

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On this day, July 9, 1812, Mammoth Cave in Kentucky was sold three times for its saltpeter resources. Saltpeter is potassium nitrate and forms in caves by alteration of bat guano. Saltpeter is one of the important components of gunpowder, and the sales of the cave were a response to the start of the War of 1812. The value of the saltpeter is reflected in the three selling prices that day: $116.67, $400, and finally $3,000 late that afternoon.

—Richard I. Gibson

Drawing (public domain) from Wikipedia