Mesozoic ammonites

--- fossils from Madagascar

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Figure 1. A fine specimen of an ammonite, sawn laterally in two and polished to a fine finish. Both polished interior and natural exterior reveal details of the complex morphology of a fossil cephalopod. This beast may be an example of the Albian ammonite Cleoniceras, or a similar genus. The shell is roughly 18.5 x 15 x 4 cm in maximum dimensions. The internal space in the shell is divided into 50-plus chambers, which grow larger on the outward spiral, from roughly the size of a rice grain to a human thumb. Ammonite shells were made of aragonite. Some of the chambers in the fossil are infilled by fine silty sediment, while others are vuggy cavities, with an open space between walls lined by calcite crystals.

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Figure 2. The exterior of the shell displays the complex patterns of the septae. Sample from Stenelux of Amsterdam, Netherlands.

"Rock of the Month #163, posted for January 2015" ---


such as this fine example are common fossils in many marine strata of Jurassic and Cretaceous age. They provided an early tool of biostratigraphy for the precise subdivision of Jurassic sediments across Europe (e.g., Arkell, 1933). Ammonites, with their buoyancy controlled through a series of walled chambers, were pelagic organisms that could move through the sea near surface, or at a suitable depth. Thus their utility as marker fossils in the widespread marine sediments laid down throughout Mesozoic time. This includes the Jurassic period (circa 202-145 Ma) and the Cretaceous (circa 145-65 Ma). Sediments of both Jurassic and Cretaceous ages are present in the Mahajanga sedimentary basin, in Mahajanga (Mahajunga) province, on the Mozambique Channel (northwestern) coast of Madagascar.

Ammonite biology

Although ammonites are extinct, the modern Nautilus provides clues as to the habits of its relatives. These organisms lived, or live, in the upper levels of the ocean (pelagic), and can swim (nektonic) rather than passively floating (planktonic). The soft body of each animal resides in the outer chamber of the shell. In the case of Nautilus the animal can float through the water with its aperture pointed upwards and forwards, such that the shell is camouflaged from above (with a striped top) and below (white lower surface, to mimic the brightness from above: see Fig. 3). While mobile, ammonites, being common pelagic animals in many seas and many times, nevertheless fell prey to those higher on the marine food chain, such as plesiosaurs (Sato and Tanabe, 1998).

Shells of spiral cephalopods are divided into many chambers by thin walls or septae. The point of connection of these chamber walls may be visible on the outer shell as wavy lines known as sutures. These are a tool for identification and classification. Early forms had simple sutures, whereas during the Jurassic and Cretaceous sutures became increasingly ornate (Black, 1972, pp.73-103), as seen in the example here (Fig. 2).

Jurassic ammonites (and other creatures)

As an example, Perisphinctes and kindred genera are very widespread, across Europe, Africa (Somalia, Kenya, Madagascar), the Middle East (Yemen), Pakistan and northern India (e.g., Spath, 1934; Dainelli, 1943; Turek et al., 1988; Krishna and Pathak, 1991; Krishna et al., 2009), as well as the Jurassic strata of Chile (e.g., Marinovic and Lahsen, 1984). The Kachchh region of Gujarat state in west India is considered the most important Jurassic locality in the marine faunal province of India and east Africa, most notably the Ler-Katrol area, type area of the Katrol Formation (Krishna and Pathak, 1991).

Marine invertebrates are not the only known fossils in the region. Fossil teeth preserved in Bathonian sediments, age dated at some 167 Ma, have been attributed to tiny, ancient mammal species (Flynn et al., 1999). Later mammals are also found in the Cretaceous of Madagascar (Krause et al., 1997). Yet more impressive, this region of Madagascar, when it was part of the supercontinent of Gondwana, was home to much larger reptiles. These included Majungasaurus, a large and little-known carnivore, and a small crocodile, Araripesuchus (Sampson et al., 1997, 1998).

The Tsingy de Bemaraha national park and reserve in Madagascar is a 600 square mile (1,553 km2) area of Jurassic karst limestone landforms. The jagged, spectacular limestone pinnacles are known as tsingy in the Malagasy language (Shea and Alvarez, 2009).

Cretaceous ammonites

Cretaceous genera include Cleoniceras, which appears by simple comparison with published photographs to be the subject of this month's article (Figs. 1-2). These ammonites are of Albian age, circa 112-100 Ma. Some 35 million years later, strata were preserved on Madagascar that help to illustrate the mass extinction event that saw the elimination of dinosaurs and many other life forms from the global biosphere (Sankaran, 1998). What of the cephalopods? Ammonites and belemnites gradually declined in abundance through the late Cretaceous, and vanished at the Cretaceous-Tertiary ("KT") boundary (Sankaran, 1998). The KT boundary is now generally agreed to coincide with the third-largest asteroidal impact event recognized on Earth, in the region of Chicxulub, on the Yucatan peninsula of Mexico. That global chaos notwithstanding, the nautiloids are still with us (e.g., Fig. 3), in the warm seas enjoyed by their ancestors.

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Figure 3. A modern cephalopod shell, Nautilus pompilius, the chambered nautilus, from Koh Jum Island, in Krabi province, on the west coast of southern Thailand. This particular species is a baby in cephalopod terms, the oldest specimens being fossils recovered in early Pleistocene sediments off Luzon, Philippines. This shell is 15 x 11 x 9 cm in maximum dimensions. The interior of the living chamber (the outermost in the spiral) is covered in a white nacreous layer. These shells have a lineage that extends back some 500 million years, to the late Cambrian and Ordovician periods, and ancestors such as the straight-shelled Orthoceras that is common in some Ordovician strata. The nautiloids themselves arose somewhat later, in the late Triassic period, and are one of those successful life forms that have changed little in many millions of years. Click here to read more on the modern Nautilus.

Madagascar and its old neighbours

In paleogeographic terms, Madagascar was part of the supercontinental assemblage of East Gondwanaland, that part of Gondwanaland comprising India, Madagascar, Australia and Antarctica (Master, 1992). To judge the fit of southwest India with Madagascar, see Katz and Premoli (1979). For a pictorial view of old Gondwanaland, it is hard to beat a large compilation, presented as a wall map (DeWit et al., 1988). The ultimate separation of India from Madagascar may have occurred near 93 Ma, which is the age of columnar rhyodacitic volcanic rocks of the St. Mary Islands, off the coast of Karnataka (Subrahmanya, 1994).


Arkell,WJ (1933) The Jurassic System in Great Britain. Clarendon Press, Oxford, 681pp.

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

Dainelli,G (1943) Geologia Dell'Africa Orientale, Vol. I. Il Progresso delle Conoscenze. Reale Accademia d'Italia, Rome, 464pp. (in Italian).

De Wit,M, Jeffery,M, Bergh,H and Nicolaysen,L (1988) Geological map of sectors of Gondwana, reconstructed to their disposition ≈150 Ma. AAPG / University of the Witwatersrand, 2 sheets, 1:10,000,000 scale.

Flynn,JJ, Parrish,JM, Rakotosamimanana,B, Simpson,WF and Wyss,AR (1999) A middle Jurassic mammal from Madagascar. Nature 401, 57-60, 02 September.

Katz,MB and Premoli,C (1979) India and Madagascar in Gondwanaland based on matching Precambrian lineaments. Nature 279, 312-315, 24 May.

Krause,DW, Prasad,GVR, von Koenigswald,W, Sahni,A and Grine,FE (1997) Cosmopolitanism among Gondwanan late Cretaceous mammals. Nature 390, 504-507, 04 December.

Krishna,J and Pathak,DB (1991) Ammonoid biochronology of the upper Jurassic Kimmeridgian stage in Kachchh, India. J.Palaeontological Society of India 36, 1-13.

Krishna,J, Pandey,B and Pathak,DB (2009) Characterization of Dichotomoceras in the Oxfordian of Kachchh. J.Geol.Soc.India 74, 469-479.

Marinovic S,N and Lahsen A,A (1984) Hoja Calama, Region de Antofagasta. Servicio Nacional de Geologia y Mineria, Carta Geologica de Chile, sheet 58, 146pp. report plus 1:250,000 scale map (in Sp.).

Master,S (1992) Early Proterozoic assembly of "Ubendia" in proto-West Gondwana (equatorial and southern Africa and adjacent parts of South America). Abs. 29th International Geological Congress, 272, Kyoto.

Sampson,SD, Krause,DW and Forster,CA (1999) Madagascar's buried treasure. Natural History 106 no.2, 24-27, March.

Sampson,SD, Witmer,LM, Forster,CA, Krause,DW, O'Connor,PM, Dodson,P and Ravoavy,F (1998) Predatory dinosaur remains from Madagascar: implications for the Cretaceous biogeography of Gondwana. Science 280, 1048-1051, 15 May.

Sankaran,AV (1998) K-T fossil patterns dispute abrupt mass extinction. Current Science 74 no.7, 569-572, 10 April.

Sato,T and Tanabe,K (1998) Cretaceous plesiosaurs ate ammonites. Nature 394, 629-630, 13 August.

Shea,N and Alvarez,S (2009) Living on a razor's edge: Madagascar's labyrinth of stone. National Geographic 216 no.5, 86-109, November.

Spath,LF (1934) The Jurassic and Cretaceous ammonites and belemnites of the Attock district. Palaeontologia Indica New Series XX, Memoir 4, Geol.Surv.India.

Subrahmanya,KA (1994) Post-Gondwana tectonics of the Indian Peninsula. Current Science 67 no.7, 527-530, 10 October.

Turek,V, Marek,J and Benes,J (1988) Fossils of the World: a Comprehensive Practical Guide to Collecting and Studying Fossils. Arch Cape Press, New York, 1990 edition, 495pp.

Graham Wilson, 01-15 October 2014.

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