August 13. Permian salt

I’ve indicated a couple times that the Earth’s climate was changing pretty dramatically from the Carboniferous into the Permian. This was partly a result of (or a cause of) the end of the Carboniferous rainforests, and partly a result of increased glaciation in the southern hemisphere. It was also to at least some extent a consequence of the land masses combining into the supercontinent of Pangaea – vast areas were distant from shores, and depending on wind circulation patterns, vast areas were also in the rain shadows of some pretty large mountain ranges. By the middle part of the Permian, arid conditions were common, and despite the glaciers in the southern hemisphere, the tropics were pretty hot.

One result of the hot arid climate was extensive deposits of salt. In what is now Kansas and Oklahoma, more than 500 feet of Permian salt lies in the subsurface. Salt mines around Hutchinson, Kansas, led to its nickname, The Salt City. The salt in Kansas and Oklahoma, while more than 500 feet thick in total, is really a package of individual beds that are a few to many tens of feet thick, separated by zones of mudstone and shale. It appears that the area was a shallow, evaporative sea, probably an arm of the restricted basins of West Texas and New Mexico that we’ll talk about later this month.

To an extent, the shallow salt sea in Kansas during the Permian represents the last bit of sea, left over from the seaway that extended through what is now the Great Plains for much of the Paleozoic Era.

Zechstein salt in northern Europe

In northern Europe, a similar basin formed in which the Zechstein salt was deposited. The restricted basin extended from what is now Poland, west across northern Germany, the Netherlands, and beneath extensive areas of today’s North Sea. It formed fairly late in Permian time, about 270 to 260 million years ago. Because it was later in the period, it’s possible that it began at least in part as a result of deglaciation in the southern hemisphere – melting ice flooded shallow areas in the tropics, where evaporation exceeded the rate of rise so salts precipitated out of the water. The Zechstein salt serves as the impermeable seal for some significant oil accumulations in older Permian rocks in the North Sea area. In some places, the buried salt has risen buoyantly to form salt domes that help trap oil and natural gas as well.

Permian salt is found all over the world in the former tropical areas, including northern South America, central Russia, and smaller basins in Australia and Eurasia. Most of southern South America, Africa and Antarctica appear to have been well within or near the Antarctic Circle. 

—Richard I. Gibson

Zechstein salt

Map by San Jose and Drdoht at the German language Wikipedia used under GFDL 

August 12. Permian amphibians

Although amphibians suffered in the collapse of the Carboniferous rainforest ecosystem just before the end of the Carboniferous, they did adapt enough to survive into the Permian.  Since the primary information about fossil vertebrates comes from their bones, it can be difficult to separate the early amphibians, reptiles, and mammals from each other based on fossil evidence, but with more and more discoveries, the branches are becoming clearer. Some groups that were formerly considered to be reptiles are now thought of as amphibians. 

Eryops, a terrestrial-adapted temnospondyl amphibian about 5 feet long.

One such group, the Temnospondyls, have been found in enough diversity that we consider them today to be amphibians. These guys were diverse, often relatively large animals pretty much like crocodiles in body form, but they lived in environments ranging from fully aquatic to fully terrestrial. Temnospondyls got their start in the late Carboniferous and during the Permian became fully adapted to life on land. As things became drier during the course of the Permian, the terrestrial varieties seem to have declined, but aquatic types thrived, and in fact the largest amphibian known is from the Permian. It’s called Prionosuchus and its fossils were found in Brazil, where it lived in tropical lagoons and river systems. It was as much as 9 meters or 30 feet long, and looked much like a crocodile. That particular genus was extinct by the end of the Permian, but Temnospondyls generally survived the Permo-Triassic extinction and continued into the Cretaceous. Evolutionary branches of the temnospondyls probably gave rise to modern toads and frogs sometime during the Triassic, but that history isn’t certain.

There are quite a few amphibians known from the Permian, but most are from the early Permian. A lot of them were little lizard-like critters, and some were almost snake like. The Aïstopods – a name meaning “no visible feet” – had a couple hundred vertebrae but some were only a few inches long. Some aquatic Permian amphibians had wide heads shaped like boomerangs that might have helped them swim, though to be honest it seems to me that we don’t really understand these head ornamentations. Microsaurs, whose name means “small lizards,” were around by Permian time, and they may be the ancestors of modern salamanders and newts. There’s really quite a large diversity of amphibians from the Permian, but many of them are known only from one specimen – so figuring out their interrelationships can be pretty challenging.

By mid- to late Permian, conditions were no longer favorable for amphibians, or perhaps conditions were just not favorable for preservation of their fossils. Or some combination of both. Amphibians obviously have survived to the present, so they must have found some niches where they were able to thrive, or at least hang on, for millions of years.

—Richard I. Gibson

Eryops, a terrestrial-adapted temnospondyl about 5 feet long. Photo by Daderot (public domain) 

August 11. Permian fusulinids

Permian fusulinid from Iowa about 12 mm long (Mark Wilson photo)

Fusulinids, one-celled animals that made shells, had proliferated in the Carboniferous and they continued to expand during the Permian. These critters were really large compared to most single-celled organisms, and some Permian types were more than a centimeter long. Their cross-sections reveal complex internal compartments where the animal lived, and some had coiled spiral chambers. Their shells were calcite, not silica like diatoms and radiolarians, and sometimes they were so abundant that they are the principal component of limestones. 

Fusulinids were an order within the phylum of foraminifera, amoeba-like single-celled organisms that made shells with tiny holes in them. Most of them probably lived within the mud on the sea floor, but some probably were floaters. Either way, they had thin filament-like pseudopods that they extended from the shell to catch food. 

Although there are many living kinds of foraminifera, fusulinids were among the many forms of life that became extinct at the end of the Permian in the great mass extinction event then. The had a run of about 190 million years though – pretty successful for a little animal.

* * *

Today’s geological birthday is Robert Thomas Hill, born August 11, 1858, in Nashville, Tennessee. Hill was a pioneer in the study of Texas geology, and he taught some of the first geology courses at the University of Texas.

—Richard I. Gibson

Permian fusulinids 

Photo by Mark Wilson (public domain) Permian fusulinid from Iowa about 12 mm long