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    Belemnites

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    Belemnites were an extinct type of cephalopod that lived from the Lower Jurassic to the end of the Cretaceous period. They had internal shells composed of three parts - a cigar-shaped rostrum, a conical phragmocone, and a thin calcareous shield called the pro-ostracum. Belemnites were predatory mollusks that likely used jet propulsion and ink to escape from predators, and their fossils provide insight into Jurassic and Cretaceous ecosystems.

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    Belemnites

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    8811791
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    Belemnites were an extinct type of cephalopod that lived from the Lower Jurassic to the end of the Cretaceous period. They had internal shells composed of three parts - a cigar-shaped rostrum, a conical phragmocone, and a thin calcareous shield called the pro-ostracum. Belemnites were predatory mollusks that likely used jet propulsion and ink to escape from predators, and their fossils provide insight into Jurassic and Cretaceous ecosystems.

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    Belemnites were an extinct type of cephalopod that lived from the Lower Jurassic to the end of the Cretaceous period. They had internal shells composed of three parts - a cigar-shaped rostrum, a conical phragmocone, and a thin calcareous shield called the pro-ostracum. Belemnites were predatory mollusks that likely used jet propulsion and ink to escape from predators, and their fossils provide insight into Jurassic and Cretaceous ecosystems.

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    Download as PDF, TXT or read online from Scribd
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    194 views2 pages

    Belemnites

    Uploaded by

    8811791
    Belemnites were an extinct type of cephalopod that lived from the Lower Jurassic to the end of the Cretaceous period. They had internal shells composed of three parts - a cigar-shaped rostrum, a conical phragmocone, and a thin calcareous shield called the pro-ostracum. Belemnites were predatory mollusks that likely used jet propulsion and ink to escape from predators, and their fossils provide insight into Jurassic and Cretaceous ecosystems.

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    Fossils explained 5: Belemnites

    Belemnites, familiar fossils in some rocks, are sometimes referred to as cigars or bullets. In fact they are extinct cephalopod molluscs related to the modern squid and cuttlefish, and their name is appropriately derived from the Greek word belemnos, meaning dart. The first true belemnites appeared in the Lower Jurassic, and the last died out with the ammonites at the end of the Cretaceous, although problematic belemnite-like forms have been found in Carboniferous and Eocene rocks (Eobelemnites and Buyanoteuthzs)
    m

    1 he belemnite possessed a calcareous internal shell consisting of three parts: the cigar-shaped rostrum (or guard), which is the part most commonly found in fossil form, the phragmocone and the proostracum. The rostrum probably acted as a counterbalance for the main bulk of the living body, and its fossil is constructed of a series of massive, compact, radially-arranged calcite crystals. It is now thought that the rostrum of a living belemnite would have had a more open structure, growing concentrically at a probable rate of one major growth stage per year, suggesting a life span of 3-4 years for some species. Less commonly preserved than the rostrum is the phragmocone, a conical chambered shell, whose apical part fitted snugly into a matching cavity (the alveolus) in the anterior of the rostrum.
    T h e pro-ostracum is the third, very rarely preserved, skeletal element, which consists of a thin calcareous shield that extended anteriorly from the dorsal surface of the phragmocone and covered the soft parts in life. The phragmocone is analogous to the chambered shell o: the nautiloids, and the proostracum represems a remnant of its body chamber. To some extent, the familiar calcareous cuttlebone of the cuttlefish Sepia resembles an opened out belemnite phragmocone and pro-ostracum. Although belemnites all look very similar to the untrained eye, they may be readily separated into taxonomic groups using the pattern of the grooves that adorn their rostra1 surfaces and by their different shapes, sizes and general form. T h e living belemnite probably possessed lateral fins, and lateral lines on the flanks of some rostra have been interpreted as the point of fin attachment. In addition to fins, the belemnite possessed a head with 10 arms, each equipped with a double row of small, but dangerous-looking, hooks. Ink sacs and surface colour patterns, similar to those of modern cephalopods, have also been found preserved within belemnite remains. Belemnites were probably voracious predators, living in the shallow shelf seas where prey, such as other mollusca and crustacea, abounded. No doubt they were able to move in search of food by gently pulsing their arms. When threatened, they probably moved backwards through a cloud of ink using the jet propulsion mechanism (water ejected through a fleshy tube - the hyponome) common to all modern

    A
    Fig. 1. Reconstruction of the Middle Jurassic belemnite Megateuthts giganteus (young individual). The scale bar represents approximately 0.1 m.

    4
    Fig. 2. Two views of the dimitobelid belemnite Dimitobelus diptychus from the Albian (mid-Cretaceous) of South Australia showing deeply incised alveolar grooves (top) and double lateral lines. The dimitobelids were restricted to the Indo-Pacific region in the Cretaceous. Lenrth = 81 mm.

    GEOLOGY TODAY MayJune 1986187

    cephalopods. This defensive system was not always effective, however, as fossil sharks and reptiles such as Ichthyosaurus have been found with stomach contents of hundreds of belemnite hooks - evidence perhaps that only the heads of these unfortunate cephalopods were consumed. M a n y present-day squids congregate in communal spawning grounds in the breeding season, an event often culminating in mass mortality. Similar events may have occurred during the Mesozoic, and possible belemnite spawning grounds are suggested by beds crowded with adult rostra. It is perhaps not suprising that such beds have been termed belemnite battlefields. Differences between sexes seen in the ammonites and the modern cephalopods, such as size, have also been identified in the belemnites. I n early Jurassic times the belemnites, like many ammonites, were fairly cosmopolitan; but by the mid-Cretaceous distinct, geographically separate belemnite groups can be identified in the north (Boreal Realm), the Mediterranean region (Tethyan Realm), and in the Indo-Pacific region (Austral Realm). Possible causes of such separation may have been climatic, such as differences in water temperature, and/or physical, such as differences in water depth. The concept of temperate to cool-temperate

    waters for the Boreal and Austral Realms, with warmer Tethyan waters, has been developed from calculation of the proportion of the oxygen isotopes 0 l 6 and O* (a proportion that varies with water temperature) locked into the calcite of belemnite rostra from these regions, but recently the validity of the technique has been questioned. T h e r e is great potential for future belemnite research, and recent discoveries include rostra preserved with their soft parts (in the Posidonia Shale of the German Lower Jurassic). Perhaps this will help revive interest in a neglected group.

    Suggestions for further reading


    Jeletzky, J.A. 1966. Comparative morphology, phylogeny, and classification of fossil Coleoidea. University of Kansas Palaeontological Contributions, Mollusca, Art. 7. Stevens, G.R. 1965. The Jurassic and Cretaceous Belemnites of New Zealand and a Review of the Jurassic and Cretaceous Belemnites of the IndoPacific Region. New Zealand Geological Survey Palaeontological Bulletin, No. 36. PETER DOYLE British Antarctic Survey, Cambridge

    Museum file 7: Camborne School of Mines Geological Museum


    Address: Trevenson, Pool, Redruth, Cornwall TR15 3SE. Telephone (0209) 714866. Administration: Camborne School of Mines (Registered CharityiAssisted Establishment). Local Education Authority - Cornwall County Council. A teaching museum. Admission: Free. Times of opening: Monday to Friday 9 a.m. to 4.30 p.m. (excluding Bank Holidays). Foundation: Classes were started for the improvement of Cornish miners at various localities in 1859. By 1887 a Mining School was established at Camborne and similar courses were being run at Redruth and Penzance. Following the death of Robert Hunt, the great innovator of mining education in Cornwall, a memorial museum of minerals and geological specimens was erected in Redruth. The three mining schools were amalgamated in 1909 and became the Camborne School of Metalliferous Mining. An extension to the laboratory in 1891 included a mineralogical museum dedicated to one of the major contributors, G.L. Basset. The Basset Memorial Museum became an important feature of the School with its minerals of interest to mining students. A few specimens were purchased but most were given. The greater part of the material, containing over one thousand samples, was donated by J.C. Williams of Caerhays Castle. In 1953 the Robert Hunt Collection was transferred to Camborne when the museum building in Redruth was sold. The School moved from the centre of Camborne in 1975 to new premises at Pool. The museum was rehoused in purposebuilt accommodation with modern display facilities. Foundation of geological collections: 1891 (The Basset Memorial Museum). Foundation of geology department: 1882 (Geology included in School syllabus). Number of specimens in the collections: 15000 approximately. Principal collections: J.C. Williams (1891); Robert Hunt (1953); F.B. Michell(1975); H.A. Thomas (1983); D. Dew (1984); T. Andrews (1985); and many smaller collections. Major strengths: Ores and minerals from UK and abroad; fluorescent and radioactive minerals; teaching displays. Research facilities: Access to the following may be arranged: research microscopes with photographic facilities; specimen photography, also a range of photographic equipment including dark

    SIGEOLOGY TODAY MapJune 1986

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