Late Jurassic - The Malm Epoch

 Much can be gleaned of the biogeography of this ancient world by studying the distribution of the dinosaurs.  Four localities of importance as regards fossil remains are known: central North America (the Morrison formation of the South-West), East Africa (Tendaguru formation of Mkwoa wa Mtwara, Tanzania), Europe (scattered localities mainly in England, France, Bavaria, and Portugal), and China (especially the Upper Shangshaximiao Formation of Sichuan Province).  Smaller vertebrates and invertebrates are also known in abundance from France and Bavaria.   A comparison of the types of creatures from these localities shows often the same family, and sometimes even the same genus, is represented in different localities, showing that, just as in the preceding Triassic period, the world was a much more biotically uniform place than it is now.  This is due to the fact that all the continents had merged into a single landmass. Yet high sea levels broke this supercontinent into a series of large islands, causing regional differences and faunal as well as floral provinces.

The following faunal (and megafaunal) provinces can be tentatively distinguished:

West Laurasian: - see top of page: Probably everyone is familiar with the great American dinosaurs of the Jurassic: the huge Brontosaurus (more properly called Apatosaurus), its slender cousin Diplodocus, the large carnivores Allosaurus and Ceratosaurus, and the striking plated Stegosaurus.  Alongside these were a number of smaller herbivores and carnivores (Camptosaurus, Dryosaurus, Ornitholestes, etc).  In this megafauna sauropods predominated, and were primarily of the diplodocid and camarasaurid families, with a few haplocanthosaurs (especially early on) and the occasional huge Brachiosaur (in fact the Brachiosaurs were bigger here than anywhere else).  As Dr Bob Bakker has shown, the West Laurasian dinosaurs exhibited a tendency to increase in size, possibly caused by increasingly harsh and arid conditions, so that by the mid Tithonian they were truly gigantic.  Then they suddenly died out, leaving a number of small dinosaurs, mammals, and lower vertebrates to carry on.

West Gondwanaland. West Gondwanaland  (e.g. Africa) had a fauna very similar to that of North America, with many types of dinosaurs in common.  The theropod Ceratosaurus, the diplodocid Barosaurus, and the ornithopod Dryosaurus occur on both continents.  But the Laurasian Camarasaurus is absent, and the mighty Brachiosaurus, which is rare in North America (West Laurasia), is the most common sauropod in the Tendaguru formation (West Gondwana).  However, it appears to be a distinct genus (designated Giraffititan) from the West Laurasian form. The small stegosaur Kentrosaurus closely resembles primitive European and Chinese forms, but is quite unlike the large and advanced American Stegosaurus.  Pterosaurs are known too, these belong to the same genus (Rhamphorhynchus) that inhabited the islands of Europe.  There were also several types of dinosaurs that were Gondwanan endemics as well.  Of significance were Dicraeosaurus (a small short-necked persistently primitive diplodocoid sauropod), Torniera (a medium-sized sauropod, possibly ancestral to the Cretaceous Titanosauridae), and the poorly known ceratosaur Elaphrosaurus.  These three taxa indicate the development of a unique Gondwanaland fauna.

Central Laurasia:  Whereas Africa and North America were large landmasses, Europe during the Jurassic seems to have consisted of a number of large islands separated by shallow sea.  A number of different types of dinosaurs have been found in Portugal.  They include not only most "standard" Jurassic types (brachiosaurids, Stegosaurs, megalosaurs, etc) but also small nodosaurid ankylosaurs [armoured dinosaurs]).  As with the early Jurassic scelidosaurs, this latter group are almost totally indigenous to Europe (which may well be the centre of evolution of these armoured dinosaurs) during the Jurassic period.  But during the following early Cretaceous period they spread throughout the globe.  The first protobirds like Archaeopteryx also inhabited the islands of Europe, no doubt benefiting from the isolation from large predators and tendency to evolutionary novelty that small islands provide.

East Laurasia and East Gondwana:  Animals on the Eastern part of the world-continent remained quite different to those of the west.  Stegosaurs were exceedingly diverse in China, and include both primitive and advanced forms.  They lived alongside basal (ancestral) marginocephalians, persistently primitive euhelopid sauropods and Triassic holdovers like tritylodontid therapsids and short-headed brachyopid labyrinthodonts.  Similar chigutosaurid labyrinthodonts must have lived in East Gondwanaland (Australia) as well, because they are known there from both the Early Jurassic and the middle Cretaceous.  Allosaurids are also known from the Late Jurassic of China and the middle Cretaceous of Australia, indicating that these carnivores were able to wander widely over the Earth's surface (unlike the euhelopid sauropods which are not known outside of China).

notes:

col. (a) gives the standard series of "Ages" or "Stages".  This is the time division beneath "epoch" on the geological/stratigraphic time-scale).  I consider these time-divisions (and the others of the Mesozoic) to be "epochs", because with spans (in the case of the Tithonian and Oxfordian) of around 5 or 6 million years or so each they are closer in duration to epochs than to "ages" (which at least in the Cenozoic are shorter - say in the order of one to three million years).

col (b) gives the sub-divisions (what I would term "ages").  This follows W. B. Harland et. al, 1982, A Geologic Time Scale, p.29, apart from the subdivision of the Kimmeridgian into Early and Late, which is based on casual references in Weishampel et al (The Dinosauria), and comments under "Late Kimmeridgian" which are from Thomas Holtz's post to the Dinosaur mail list.  This indicates that the Tithonian could perhaps be subdivided into four ages, the Early, Middle, earlier Late, and Portlandian (later Late), each lasting around one or two million years or so.

col (c) is a purely extravagant and fanciful correlation of these stages given in terms of millions of years ago, on the basis of combining the dates given by Harland,  Armstrong, Cox, Lorraine, Smith and Smith,  A Geologic Time Scale 1989; and  Gradstein, Agterberg, Ogg, Hardenbol,  van Veen,  Thierry and  Huang "A Triassic, Jurassic and Cretaceous time scale".  It is intended as a very approximate guide and should not be taken too seriously.

col (d) gives marine Dinoflagellate zonation, according to W. B. Harland et al., 1982, A Geologic Time Scale, p.29

col (e) gives marine Ammonite zonation, according to W. B. Harland et al., 1982, A Geologic Time Scale p.29, and British Mesozoic Fossils, pp.15-16

col (f) presents a simplistic representation of the general stratigraphic marine sequence from England, on the basis of  W. B. Harland et al. p. 28, and British Mesozoic Fossils, pp.15-16

col (g) gives some terrestrial strata (many of which are unnamed).   The famous Solnhofen limestone where Archaeopteryx and many other animals have been found preserved is given as only "?Kimmeridgian" by Weishampel et al (The Dinosauria) but "Lower [=Early] Tithonian" by Wellnhofer.  The age of the Lithographic Limestone of Cerin, where many small crocodilians and other small vertebrates have been found, is given as "Late Kimmeridgian" by Weishampel et al.

col (h) gives the zonation of dinosaurs from the Morrison formation, according to Robert Bakker (see e.g. The Dinosaur Heresies, p.400).  The stratigraphic correlation of each of these stages is pure fancy on my part.  The date of the Morrison formation is given as "Kimmeridgian-Tithonian." by Weishampel et al (citing Dodson, Behrensmeyer and Bakker "Taphonomy of the Morrison and Cloverly Formation of the western United States"  in  Ecosystèmes Continentaux du Mésozoïque, Mémoires de la Société Géologique de France,  no.139,),  but Greg Paul gives the age uniformly as Tithonian,  and Peter Wellnhofer has Tithonian likewise for Como Bluff (= Camarasaurus lentus zone) and Dry Mesa (=Apatosaurus ajax zone).

col (i) gives the zonation of the Tendaguru formation, [ref. D. Russell et al].   In that article the authors write: "Marine sandstones within the Tendaguru Beds contain ammonites indicative of an Upper Kimmeridgian-Tithonian age for the two main dinosaur-producing units (Aitken 1961)" [p.169] and "The morphological difference between barosaurs and dicraeosaurs above and below the marine sands suggest that the Trigonia smeei Bed (that separates the Middle and Upper Saurian Beds) was deposited over an interval of the order of one million years." [p.172].  However, Weishampel et al , also quoting Aitken, W.G.,  1961 [Geology and Palaeontology of the Jurassic and Cretaceous of southern Tanganyika, Geological Survey of  Tanganyika, Bulletin 31] give the age of the Tendaguru as simply "Kimmeridgian".  Greg Paul gives it as "Tithonian", and Peter Wellnhofer has only a cautious "Upper Jurassic" for the Upper Saurian Bed.

References

• British Mesozoic Fossils, 2nd Ed., British Museum (Natural History), London, 1964
• Robert Bakker, 1986, The Dinosaur Heresies, p.400 (Longman)
• W. B. Harland, A.V. Cox, P. G. Llewellyn, C.A.G. Pickton, A.G. Smith and R. Walters, 1982, A Geologic Time Scale Cambridge University Press (unfortunately I don't have the later edition on me)
• Gregory  S. Paul, 1988, Predatory Dinosaurs of the World, Simon & Schuster
• D. Russell, P. Béland, & J.S. McIntosh, 1980, "Paleoecology of the dinosaurs of Tendaguru (Tanzania)" in  Ecosystèmes Continentaux du Mésozoïque, Mémoires de la Société Géologique de France,  no.139, pp.169-175
• David B. Weishampel, Peter Dodson, and Halszka Osmólska, eds., 1988, The Dinosauria,
• Peter Wellnhofer, 1991, The Illustrated Encyclopedia of Pterosaurs, Crescent Books, New York, p.107
• Re. Portlandian - Thomas Holtz

The term "Malm epoch" is used by W. B. Harland et al., in A Geologic Time Scale,  to refer to the third division of the Jurassic Period (i.e. the Late Jurassic).  "Malm" was originally part of the three-fold division of Jurassic rocks in Britain, but has since become a more universal term (thanks to the accident of history that modern geology and paleontology started in Britain).

Personally I feel "epoch" (which is usually used to refer to periods like the Eocene, Oligocene etc) is inappropriate for a division on this scale (over 20 million years) and I would prefer "sub-period"