September 3. Cynodonts

Triassic Dicynodont from Poland. Drawing by Dmitry Bogdanov.

A few days ago, talking about the end Permian extinction, I said something that’s wrong. I said the therapsids became extinct. That’s not true, since the therapsids include modern mammals. I should have made it clear that it was the early therapsids, dimetrodons and others, that did not survive into the Triassic. Other groups were decimated, but they did survive.

One surviving group was the cynodonts, whose name means “dog-teeth.” Cynodonts are generally seen as the early therapsids that ultimately gave rise to true mammals later in the Triassic, but as the Triassic began, only a few varieties existed. In addition to differentiated teeth, cynodonts had skull and jaw structures similar to modern mammals. It’s speculated that cynodonts were warm-blooded and laid eggs, like modern platypuses.

The recovery from the Permian extinction was slow, but cynodonts did diversify during the Triassic, increasingly through the period. The variety called dicynodonts, meaning “two dog teeth” for the pair of large tusks that they had, were perhaps the most diverse of the pre-mammalian therapsids. They started in late Permian time but were pretty common during the Triassic. Dicynodont species ranged from the size of a rat to the size of an ox, and despite the tusks they were herbivores. They probably used the tusks for digging, like modern boars. Dicynodonts were probably the most widespread large herbivores on land during the early Triassic, expanding from two families that survived the extinction to 14 or 15 families by mid-Triassic time.

The Triassic’s changing climates and arid conditions in many locations were probably both stimulating in terms of evolutionary pressure and challenging in terms of survival. The dicynodonts were essentially extinct by the late Triassic, but there is a possibility that some survived until the Cretaceous, based on fragmentary fossils from Queensland, Australia. But by the end of the Triassic, most of the terrestrial herbivores were dinosaurs, not pre-mammalian therapsids like the cynodonts and dicynodonts.

—Richard I. Gibson

Image by Dmitry Bogdanov under GFDL.

September 2. The Triassic

The first period of the Mesozoic Era is called the Triassic. Its name comes from the distinctive three-fold division of these rocks in central Germany. Red beds in the lower part of the geologic section were overlain by marine limestone, with terrestrial sandstones on top of that. The period was named by geologist Friedrich von Alberti in 1834, and while the Triassic in much of the rest of the world lacks the distinctive triple division, the name is used everywhere.   

The subdivisions of Triassic time are simple at the first level, with three epochs – early, middle, and late, but those epochs are not the same length – they represent about 4, 12, and 34 million years, respectively, which adds up to a total length of about 50 million years. The ages, the subdivisions of those epochs, are based on the appearance and disappearance of various faunal packages, and they vary around the world, but the general internationally agreed ages total seven in the whole Triassic. Five of the seven ages were defined and named for localities in the Austrian and Italian Alps, and the other two come from sites in Russia.

The Triassic began with the end of the Permian at 251 million years ago, with the great extinction event. The Siberian basalt flows that started toward the end of the Permian continued into the Triassic, and the period ended at about 201 million years ago with yet another mass extinction event – not as severe as the Permian-Triassic devastation, but bad enough. We’ll talk about it at the end of this month.

—Richard I. Gibson

September 1. The Mesozoic begins

The three eras of geologic time after the Precambrian are sometimes called the Phanerozoic Eon – that name means “visible life.” The Paleozoic, which ended yesterday, means “ancient life,’ Mesozoic, beginning today, means “middle life,” and the Cenozoic Era, which we’ll cover in December, means “recent life.” This is a good time to repeat my disclaimer that we are NOT going through earth history at a proper scale. If we were, we’d be in the Precambrian until mid-November, and it would be a pretty boring commentary. So I have arbitrarily assigned January to all of the Precambrian, February through August to the periods of the Paleozoic, September, October, and November to the Mesozoic, and December represents the Cenozoic Era. 

So today we begin the Mesozoic, sometimes called the Age of Reptiles or the Age of Dinosaurs, but lots of other living things diversified during this time and we’ll try to touch on many of them in the coming three months.

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On September 1, 1923, at two minutes to noon, a massive earthquake shook Tokyo and Yokohama, Japan. Its magnitude is given as 7.9 on the moment magnitude scale, and the estimated death toll was at least 143,000, making it one of the deadliest quakes in history. A strong typhoon hit the area at about the same time – and it has been suggested that the heavy storm surge from the typhoon might have put enough pressure on an already highly stressed fault that it could have triggered the quake – but in any event, the fault must have been ready to break already. Japan sits at the complex boundaries between four tectonic plates – the Pacific, Philippine, Eurasian, and a branch of the North American Plate called the Okhotsk Plate. At Tokyo, the northern point of the oceanic Philippine plate is subducting beneath both Eurasia and this extension of North America – a real mess, tectonically, so it’s not much of a surprise that the area is prone to great earthquakes.

—Richard I. Gibson