September 14. The first frog

Update:  A week ago, on September 8, when I was talking about the initial breaks that were starting the dismemberment of Pangaea, I mentioned the possibility that mantle plumes, rising heat and magma that could produce hotspots like Yellowstone and Iceland, the possibility that mantle plumes might contribute to initiating continental breakup. There’s a link on the Sept. 8 episode to a 2014 article supporting that idea, which I said was controversial. Just last week, a new paper by researchers at CalTech came out suggesting just the opposite – that mantle plumes, at least as narrow features a few hundred kilometers across, like hotspots, may not exist at all. This idea is that heat and magma do rise, but not as narrow jets or plumes. Rather, broad upwellings of heat are pulled upward by cooling and subduction near the surface, more or less in the standard model of heat convection. Magma pools beneath the crust or within it, constrained by solid rocks. It erupts to the surface in volcanoes – both above subduction zones and above hotspots – through cracks and weak zones created by tectonic processes. This work was based on seismological studies that fail to find evidence of the narrow mantle plumes the older hypothesis requires. Here's a link to a summary of the new paper.  

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Triadobatrachus fossil at
Paris Natural History Museum
photo by Ghedoghedo used under
Creative Commons license

Now back to the Triassic and a short episode on the first frog. It’s September 14. Despite the rigorous conditions in the early Triassic just after the devastating extinction at the end of the Permian, life found ways to survive. The first known frog dates to the early Triassic of Madagascar. This critter, named Triadobatrachus, meaning “triple frog,” wasn’t exactly a true frog in the modern sense. It had many more vertebrae – 14 compared to 4 to nine in modern frogs – and a short tail, lacking in modern frogs. In fact modern frogs and toads comprise the order Anura, which means “tailless.” The first true frogs in the modern sense did not evolve for 40 million years, in the early Jurassic.

The Madagascar ancestral frog was found in sedimentary rocks that indicate it lived in a near-shore environment, which you could expect for an amphibian. There is only one specimen of this animal, but it’s an excellently preserved fossil, with the skeleton still fully articulated. The animal was about 4 inches long, and its leg bones suggest that it probably could not hop like modern frogs, but this is not certain. A 2012 interpretation of the bone structure concluded that it wasn’t capable of long jumps, but might have been able to do short hops.

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We have four geological birthdays today. Friedrich Wilhelm Heinrich Alexander von Humboldt was born September 14, 1769, in Berlin. He was a naturalist and explorer, probably best noted for botanical studies, but he also recognized the change in the strength of the earth’s magnetic field from the poles to the equator, and his studies of volcanoes contributed significantly to the opposition to the Neptunist view that igneous rocks were formed from water.  Victor Hugo Benioff was born this day in 1899, in Los Angeles. He worked as a seismologist at Cal Tech, characterizing the locations of deep earthquakes. He recognized that there is an inclined array of earthquakes along a subduction zone, strong support for the mechanisms of plate tectonics. A subduction zone is called a Wadati–Benioff zone, honoring him and Kiyoo Wadati, the Japanese seismologist who also independently discovered the seismic activity on subduction zones. Robert S. Dietz was born this day in 1914, in Westfield, New Jersey. His work in geophysics and oceanography pioneered the concept of sea-floor spreading, the basis for plate tectonics. And today is also the birthday of geologist Patricia Dickerson, in San Jose, California. She has worked extensively on the geology and tectonics of West Texas and the Big Bend area, as well as central South America.

—Richard I. Gibson

Triadobatrachus fossil at Paris Natural History Museum photo by Ghedoghedo used under Creative Commons license

Did Triadobatrachus jump?