"Rock of the Month #201, posted for March 2018" ---

Mesosaurus

does not command as much attention as its larger, distant relatives. It is an early aquatic reptile, and the specimen shown here is comparable in size to a medium dog breed, at best, not to a bus! Black (1972) shows an example from the early Permian, in the "Karroo Beds" of South Africa. Another museum specimen, from the Permian of Brazil, appears in the explanatory catalogue of the excellent Museo Geominero in Madrid, Spain (Anon, 2014). See also Stinchcombe (2008, p.176).

Mesosaurus is a small, early reptile genus from the early Permian of both southern Africa (Namibia, South Africa) and South America (southeast Brazil and Uruguay). The animal was very streamlined, up to about 1 metre in length, and is thought to have eaten crustaceans and other small prey. Well-adapted to life in the water, with webbed feet, Mesosaurus has been attributed to both saline coastal and - especially - freshwater environments (though it was earlier considered a marine, sea-going creature). For a more detailed discussion of paleobiological inference, see, e.g., Wikipedia. Mesosaurus was first described in 1864-1866. A slightly smaller mesosaur, Stereosternum was recognized in a museum collection in Brazil in 1886. A third genus, Brazilosaurus, was described in 1966. Romer (1959, pp.155-157) gives a good introduction to this small, toothy animal with its long, formidable jaws, in its paleogeographic context.

A note on the Karoo Strata

The Karoo sedimentary basin is an important remnant of Gondwanaland. Known to host uranium and molybdenum occurrences, the Karoo strata include sandstones, and are of terrestrial (fluvial, lacustrine, deltaic) facies. The Karoo sediments contain picritic to basaltic lavas and dykes, including basalt flows and diabase dykes, and lateral equivalents are preserved in South America and in Dronning Maud Land, Antarctica. Many of the dykes are substantially younger than their host rocks, circa 180 Ma (middle Jurassic). The Karoo is thickest (7 km) at the Cape in South Africa (Yemane, 1987), but is found far to the north and east, across countries from Botswana to Malawi, Tanzania and (Mohr, 1963) Ethiopia. As seen in Tanzania, the Karoo sequences are unmetamorphosed, terrigenous, mainly siliciclastic rocks, with little or no marine influence (Nilsen et al., 2001). More recently, evidence of extinctions has been documented in these rocks, across the Permian-Triassic boundary (Retallack et al., 2003).

Paleogeography and Continental Drift

The distribution of Mesosaurus fossils has paleogeographic significance, in that these terrains in southern Africa and South America were united, prior to the evolution of the South Atlantic ocean, which separated them in the Mesozoic. This rifting event, an example of continental drift, is enshrined in the history of science due to the pioneering work of Alfred Wegener (1880-1930).

Wegener compiled observations to support the hypothesis that southern Africa and south America had once been joined together. The fit of the coastlines (either at sea level or deeper down, on the continental shelf) provided the first line of evidence.

Wegener's ideas did not find early acceptance. Several decades passed before geophysical evidence, particularly the recognition of magnetic reversals preserved in seafloor basalts, backed up his hypothesis. Wegener's conclusions were based on geological comparisons, including the distribution of Mesosaurus fossils. The supercontinent of Gondwanaland comprised South America and Africa, India, Antarctica and Australia. Geological evidence in support of the supercontinent, and especially of the former neighbourliness of Brazil and South Africa, comes from multiple lines, such as fossil plants, ancient glaciations, lava flows of the Karoo and Parana provinces, paleomagnetism, and Mesosaurus (Holmes, 1965, pp.1216-1229; A.G. Smith in Gass et al., 1971, pp.212-232; Tarling and Tarling, 1971; Hallam, 1973, p.16, and 1983; Wood, 1985). The importance of Mesosaurus rests on its inferred freshwater, rather than sea-going mode of life: it could not cross a large ocean, therefore the simplest explanation of its distribution involves a single range across parts of two continents, formerly united and now far apart.

Colbert (1975) provided an approachable review of paleogeographic reconstructions. His maps show how, in the Mesozoic, distributions of plant fossils and vertebrates, such as Mesosaurus, Cynognathus and the fern-like Permian plant Glossopteris offer a consistent explanation for the breakup between South America and Africa.

A nice article on Wegener and continental drift appears in Tarbuck et al. (2015, pp.12-16), complete with a range map of Mesosaurus fossils. As Alfred Romer noted (Romer, 1959, p.157): "Mesosaurus gives one vote, at any rate, for Gondwanaland".

References (n=17)

Anon (2014) Geominero Museum: minerals, fossils and rocks. Museo Geominero, Instituto Geologico y Minero de Espana, Rios Rosas 23, Madrid, 16pp., English edition.

Black,RM (1972) The Elements of Palaeontology. Cambridge University Press, 339pp.

Colbert,EH (1975) Life on wandering continents. In "Rediscovery of the Earth" (Motz,L editor), Van Nostrand Reinhold, 279pp., 251-262.

CUGB Museum (2016) The CUGB Museum, Yifu Building, China University of Geosciences - Beijing. CUGB Museum, Xueyuan Road, Haidian district, north Beijing (GCW notes from visits in March and April, 2016).

Gass,IG, Smith,PJ and Wilson,RCL (editors) (1971) Understanding the Earth. Open University Press / Artemis Press, 355pp.

Hallam,A (1973) A Revolution in the Earth Sciences: from Continental Drift to Plate Tectonics. Oxford University Press, 127pp.

Hallam,A (1983) Great Geological Controversies. Oxford University Press, 182pp.

Holmes,A (1965) Principles of Physical Geology. Thomas Nelson and Sons Ltd, London, 2nd edition, 1288pp.

Mohr,PA (1963) Occurrence of Karroo system sediments in Ethiopia. Nature 199, 1086, 14 September.

Nilsen,O, Hagen,E and Dypvik,H (2001) Sediment provenance and Karoo rift basin evolution in the Kilombero rift valley, Tanzania. S.Afr.J.Geol. 104, 137-150.

Retallack,GJ, Smith,RMH, Ward,PD (2003) Vertebrate extinction across Permian-Triassic boundary in Karoo Basin, South Africa. Bull.Geol.Soc.Amer. 115, 1133-1152.

Romer,AS (1959) The Vertebrate Story. University of Chicago Press, 430pp.

Stinchcombe,B (2008) Paleozoic Fossils. Schiffer Publishing Ltd, Atglen, PA, 180pp.

Tarbuck,EJ, Lutgens,FK, Tsujita,CJ and Hicock,SR (2015) Earth: an Introduction to Physical Geology. Pearson Canada, Inc., 4th Canadian edition, 533pp.

Tarling,DH and Tarling,MP (1971) Continental Drift, a Study of the Earth's Moving Surface. Penguin Books Ltd, 142pp.

Wood,RM (1985) The Dark Side of the Earth. Allen and Unwin, London, 246pp.

Yemane,K (1987) Depositional environment of Lower Beaufort (Upper Permian) Karoo deposit of northern Malawi and similar sequences in southern Africa. In `Current Research in African Earth Sciences' (Matheis,G and Schandlmeier,H editors), A.A.Balkema, Rotterdam, 486pp., 231-234.