|Early Cretaceous I||Jurassic||Cretaceous|
Early Cretaceous I (Neocomian)
Early Cretaceous II (Aptian-Albian)
|Aptian-Albian (Early Cretaceous II)
image from American Museum of Natural History
The Cashenranchian dinosaur fauna
|Epoch||Age||ICS Base (Mya)||ICS Duration (My)|
|Late Cretaceous I (High Cretaceous)||Cenomanian||99.6||6.1|
|Early Cretaceous II (Aptian-Albian)||Albian||112||12.4|
|Early Cretaceous I (Neocomian)||Barremian||130||5.0|
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Echinoderms are well-represented in Albian microfossils in the Gault Fm of southern England. A number of excellent examples may be found on the website of the late Jim Craig (now maintained by Fred Clouter) at Echinodermata_Home. Note the unusually high proportion of ophiuroidian remains. These include some fragments of a Nielsenicrinus which must surely have been a monster in life. Echinoids are well represented and essentially modern. See, these examples of the sea urchins Macraster, Heteraster and Hemiaster.
The Asteroidea went through a process of rapid evolution and diversification during the Early and Middle Jurassic. This resulted in some fundamental changes in the organization of the arms associated with the transfer of the ampullae to the interior of the arms. Blake 2000). This change may have been associated with some unusual selective pressure. In any case, starfish are not frequently found in the Neocomian, and it is only in the Aptian-Albian that the Asteroidea again become common fossils. At this point, all of the remaining forms are essentially modern types and fall easily into well-characterized Recent families. Id.
Among the Crinoidea, the Aptian-Albian is especially noteworthy for the Roveacrinidae. These are odd, tiny, stemless, forms. Both nektonic and benthic forms are known, although both were motile to varying degrees. The benthic roveacrinids are characterized by the presence of flanges and spines, and by limited arm mobility. Nektonic roveacrinids have a large dorsal cavity, good arm mobility, and typically have very little ornamentation. They are first known from the Hauterivian (later Neocomian), bloomed in the Aptian-Albian, and flourished in the High Cretaceous before becoming extinct. Farinacci & Manni (2003).
As you might expect, little is known of annelid evolution in the Mesozoic, since annelids rarely leave fossils. One exception are the serpulid tube worms which add a calcareous cement to their tube walls. The worms themselves are never recovered, but the tubes are frequently preserved in shallow marine sediments. In some cases, useful ultrastructural details can be made out. However, recrystalization and weathering make these details unrecoverable in Mesozoic or Paleogene remains. Two extremely common forms are shown in the image. These are better known from slightly younger strata in the High Cretaceous. However, they are common enough in the Aptian-Albian of the North American Interior Sea.
Brachiopods continued their slow and stately progression toward probable oblivion, a march that continues to this day. The Aptian Albian was the high point of a Late Jurassic to End-Cretaceous family of rhynchonellids known as the Cyclotherididae, and is marked by the appearance of the common genera Burrirhynchia and Septatoechia. Other "new" brachiopods include Orbirhynchia.
Image: Burrirhynchia from Associació Amics de la Paleontologia de Morella.
Unlike some other large invertebrate taxa, it is a simple matter to determine what genera of Bryozoans are known from any part of the Cretaceous. See Checklist of Cretaceous Bryozoa, compiled in incredible detail by Prof. Alan Horowitz of Indiana University. It is a little harder to say anything meaningful about them.
The Aptian-Albian was perhaps the high point of the Cyclostomata, which were extremely diverse, with at least 70 known genera. These are mostly tube-like forms, well integrated and often branching. Benton & Harper (1997). Bicrisina, shown in the image, is a typical example. Another cyclostomate bryozoan, Cellulipora, from the same period, had an encrusting growth pattern. These and other encrusting bryozoans were significant reef builders in the Aptian-Albian and High Cretaceous.
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On land, animals seem to have migrated and spread widely, despite provincialism. The presence of spinosaurids and iguanodontians on at least three continents ) indicates the swamp system was very extensive and connected at certain periods of time. It is unlikely that specialized, advanced, spinosaurids might have traveled and migrated very far form their natural habitat.
A number of tetrapod provinces could be suggested:
What is now China and Mongolia were host to a distinctive tetrapod fauna, charcterised by the Sinochelyid turtle Peishanemys, mesosuchian crocodiles, advanced hadrosaur-like iguanodonts (Probactrosaurus, Altirhinus), the abundant small ceratopsian Psittacosaurus, the strange herbivorous Therizinosaurs (Alxasaurus, Nanshiungosaurus), and large (by Mesozoic standards) triconodont mammal Gobiconodon. This is basically a continuation of the earlier central Asian fauna, although with some changes among the big herbivores. Sauropods are now very rare, and the stegosaurs that had earlier been present have also disappeared (although the dating is difficult, so these form may actually have continued into the Aptian); the large herbivore guild now dominated by iguanodonts and therizinosaurs. Large theropods are present but poorly known; "Alashansaurus" maortuensis may have been related to the Tyrannosauroidea. It is not impossible that several lineages of tyrannosauroid-like animals may have developed from earlier forms at this time.
Psittacosaurus illustration by Steve Kirk - Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals, ed.. Barry Cox, © 1988 Marshall Editions
North America's endemic Aptian-Lower Albian dinosaur fauna features specialized allosaurids like Acrocanthosaurus, a diversity of dromaeosaurs, and herbivorous Sauropelta, Tenontosaurus, and Pleurocoelus sauropods. This megafauna differs strikingly from the more famous late Jurassic and late Cretaceous dinosaurs. It has been documented from the Cloverly Formation, Arundel Formation, Trinity Group, and Cedar Mountain formation. The term Cashenranchian has been proposed for it (from the Cashen Ranch, in southern Montana, where this fauna is well developed in the Cloverly Formation ).
Climatically, the Barremian through Albian in the western North America represents a dry savanna, and water-loving Euro-Gondwanan predators like spinosaurids may have been ecologically excluded. Conditions however were wetter to the east. During the Cenomanian the west become much wetter with many extensive swamps.
During the Mid-Cretaceous (Aptian to Cenomanian) a unique fauna of enormous dinosaurs evolved on a large island of what is now northern Africa. We tend to think of the Tyrannosaurus as being the largest land carnivore, but at this time there were no less than three different evolutionary lines of Tyrannosaurus-sized (12 metres and more in length) types of predators in this region alone: spinosaurs, carcharodontosaurids, and deltadromeids. These great predators preyed on the many kinds of herbivores, especially the giant Sauropoda, such as Rebbachisaurus, Brachiosaurus, and Aegyptosaurus. Also inhabiting this environment were large (3 to 6 meter) lungfish and gigantic (10 to 15 metre) Sarcosuchus crocodiles; a genus also known from South America (at the time part of a single biome and landmass)
During the Aptian-Albian, what is now Australia was located very near the South Pole. Although the climate was not as harsh as it is in today's Antarctic, the animals and plants of this environment still had to endure some freezing weather in winter and six months of darkness. Many unique animals inhabited this environment, including large temnospondyls, nocturnal hypsilophodontid and dwarf allosaurid dinosaurs, and the ancestor of the modern duck-billed platypus.