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

Ammonites

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.