The CGS Field School: Field School 11 Itinerary

Karoo rocks cover more than two-thirds of the surface area of South Africa, and therefore we will most certainly examine these rocks in great detail. Beside these rocks containing most of the South Africa's fossil fuel, it also contains abundant vertebrate fossils. We will have a look at some of these fossils during our trip, especially around the Beaufort West region.

Here is an excellent guide written by 2015 field school graduate (with distinction) Tshilidzi Radzuma:

In today’s world, ancient life on earth is represented by million-year-old fossils, these are life forms (both plants and animals) that existed millions of years ago; which died and subsequent favourable conditions; rapid burial of organism, exclusion of oxygen, presence of hard parts such as shells or skeletons and lack of metamorphism, were conducive of preservation of the past life.

There are different ways in which fossils may be preserved; these are;

Petrification, where fossils are replaced by the rock, preserving the structure of the plant or animal in rock form similar to a sculpture.

Impressions marks which are a mark of the shape of the trace such as an imprint of a leaf.

Mummification is the preservation of both the skin and body of an animal. This is common in very arid environments.

Frozen fossils are found in regions where there are persisting freezing temperatures. These fossils may be ice-frozen or buried in ice, think woolly mammoths.

Moulds and casts cover and take the shape of the fossils, if, at a later stage, the fossil structure dissolves, a mould would form, if sediments fill the cavity, then a cast would be formed, which is a copy of the original fossil body.

Fossils are spread throughout Earth’s continents, in this case, South America and Asia; India, Russia, China, as the continents were once conjoined into the Gondwana supercontinent (more here). As that, fossils may be used to correlate geological and biological stratigraphies in today’s continents.

Before lifeforms roamed the land, life on earth was represented by early plant and marine life preserved now preserved in the Bokkeveld Group. At ca. 300 Ma, Southern Africa was located around the South Pole, as time progressed, Gondwana was moving northwards, prompting the melting of icebergs. Melting continued and blocks of rocks loosened at the bottom of the icebergs to deposit the Dwyka Group of the Karoo Supergroup (more here).

This was flowed by the deposition of the Ecca Group where much of plant life flourished in swamps at deltaic regions. The burial of these swamps led to the formation of the coal beds we mine today. The Beaufort is a product of large northward-flowing, meandering rivers in which sand accumulated. At the same time, flood plains periodically deposited mud. The Lower Beaufort Group has records of the earliest terrestrial reptiles. The rocks indicate an evolution in diversity in mammals and birds as climatic conditions favoured life prosperity.

The well-known Karoo fossils have a dedicated 400 m Fossil Trail at the Karoo National Park in Beaufort West. However, the story of these fossils is far-reaching. This is the story of the first reptiles that evolved into the various animals of today. The beauty of the Karoo fossils is the traceable evolution in a single depositional basin, from the early therapsids at Laingsburg and the evolved mammals at the foothills of the Drakensberg.

There are three types of reptiles (Figure 1): Synapsid, Diapsid and Anapsid. These can be differentiated by skull openings on the skull roof for the attachment of the jaw muscles. Synapsids have one opening, Diapsids have two openings and Anapsids have no openings. The Illustration below ties together the evolution of the three types noted above.

Synapids

First of their form are Pelycosaurs which lived between the Carboniferous and Permian Age. They evolved to therapsids which are Biamorsuchians, Dinocephalians, Dicynodonts, Gorgonophians, and Therocephalians and the later evolutionary lifeforms. Therapsids are mammal-like, four-limbed opposite to the other reptiles, a distinction given to the non-mammal therapsids which are specially termed anomodonts.

Therapsids had types that fed on plant (e.g. Dicynodont) and flesh (e.g. Gorgonopsians). The pair of Dicynodont had body size ranging from mongoose to cow and were paired with tusks they used to break up plant material. Flesh-eating Gorgonopsians were paired with sabre-like canine teeth. Dinocephalians ate both plants and flesh and they represented he largest land-living therapsids. The smaller therapsids (Therocephalians and Cynodonts) survived to eloved into mammals their structure developed cuffs and crushing surfaces on molar teeth useful for grinding up food.

Diapsids

The extinction of most of therapsids gave way to the domination of Diapsids. These are great-great-great-(great)-grandparents to most modern-day reptiles such as birds, crocodiles and alligators, and lizards and snakes. Within the Karoo, the Rhynchosaurs, a distant relative to frogs and salamanders, lived around river banks and floodplains and fed on smaller therapsids. Its mask rests in one of the cases in the Karoo National Park (Figure below). These reptilian forms evolved much later to introduce early forms of dinosaurs in the Late Permian and later dinosaurs towards the Jurassic and Cretaceous.

Anapsids

There is little diversity in the evolution of anapsids. Their existence was supported by the vast plat life in the Middle to Late Permian. The early Eunotosaurus (figure below) is seen as an early ancestor of the tortoises we see today based on the broadened body. Moreover, this is not the only nominee to the ancestorship of tortoises.

The Bardysaurus (see here) was a 2.5 m long species which roamed drying muddy floodplains, it is for that reason their bone fossil are found complete. Yet another possible ancestor is the procolophonoid superfamily which is preserved in the lower Beaufort Group. It diversified until it evolved into tortoises.

Other fossils

Not only were moving lifeforms preserved in the Beaufort Group. Plants were also subjected to the process of fossilisation, particularly, petrification as seen in the blocks of wood found in the Karoo National Park Fossil Trail. See figures below.

Extinction

The Permian-Triassic extinction obliterated some of these early therapsids; those that survived the extinction were the Dicynodont, Therocephalians and the later Cynodonts. Gorgonophians, Biarmosuchians and Dinocephalians were unfortunate (Figure 1).

The annihilation of the therapsids provides an opportunity to biologically divide the stratigraphy into zones, Tetrapod Assemblage Zones (AZ) as seen in the Figure5 and 6. The extinction of the Dinocephalian is an interesting one as in provides evidence of extinction in the upper Tapinocephalus AZ ca. 260.26 Ma owing to the Emeishan eruption (Figure 5). This extinction gave way to the flourishing of Dicynodonts and other lifeforms into the Pristerognathus AZ.

The greater extinction, the Permian-Triassic Extinction ca 251 Ma killed most of the therapsids species, anapsids and plant forms. It is termed the Mother of All Extinctions. The scale/severity of this extinction is seen by the survival of only 4 species of tetrapods out of more than 50 species. As one thing replaces another, the earlier forms of dinosaurs (anchosaurs) emerged together with the dicroidium flora.

What to see on the Fossil Trail

The hiking-guide blogspot (http://hiking-guide.blogspot.co.za/2010/04/karoo-national-park-fossil-trail.html) is a glimpse on what to see in the Fossil Trail. However, the story of these living species is bigger and much is still to be uncovered.

*much of the content comes from the book: The Story of Earth and Life: A Southern Africa perspective on a 4.6-bilion-year journey (see below).

South African Supergroups

Friday, November 4, 2016

Field School 11 Itinerary - Karoo Fossils