AskDefine | Define titanosaur

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Dictionary Definition

titanosaur n : amphibious quadrupedal herbivorous dinosaur with a long thin neck and whiplike tail; of the Cretaceous mostly in the southern hemisphere [syn: titanosaurian]

Extensive Definition

Titanosaurs (members of the groups Titanosauria and/or Titanosauroidea) were a diverse group of sauropod dinosaurs, which included Saltasaurus and Isisaurus. It includes some of the heaviest creatures ever to walk the earth, such as Argentinosaurus and Paralititan — which might have weighed up to 100 tonnes (110 short tons) or, perhaps, even double that, if some poorly-described data are to be believed (see Bruhathkayosaurus). They were named after the mythological Titans, the early deities of Ancient Greece, who preceded the Twelve Olympians.

Description

Titanosaurs had small heads, even when compared with other sauropods. The head was also wide, similar to the heads of Camarasaurus and Brachiosaurus but more elongated. Their nostrils were large ('macronarian') and they all had crests formed by these nasal bones. Their teeth were either somewhat spatulate (spoon-like) or like pegs or pencils, but were always very small.
Their necks were relatively short, for sauropods, and their tails were whip-like, but not as long as a diplodocid. While the pelvis (hip area) was slimmer than some sauropods, the pectoral (chest area) was much wider, giving them a uniquely 'wide-gauged' stance. As a result, the fossilised trackways of titanosaurs are distinctly broader than other sauropods. Their forelimbs were also stocky but their rear limbs were longer. Their vertebrae (back bones) were solid (not hollowed-out), which may be a throwback to more primitive saurischians. Their spinal column was more flexible, so they were probably more agile than their cousins and better at rearing up.
From skin impressions found with the fossils, it has been determined that their skin was armored with a small mosaic of small, bead-like scales around a larger scale. One species has even been discovered with bony plates, like the Ankylosaurus.
While they were all huge, many were fairly average in size compared with the other giant dinosaurs. There were even some island-dwelling dwarf species, probably the result of allopatric speciation and insular dwarfism.

Range

The titanosaurs were the last great group of sauropods before the Cretaceous-Tertiary extinction event, about 90–65 million years ago and were the dominant herbivores of their time. The fossil evidence suggests they replaced the other sauropods, like the diplodocids and the brachiosaurids, which died out between the late Jurassic and the mid-Cretaceous Periods.
They were widespread, especially in the southern continents (then part of the supercontinent of Gondwana) and even in Australia, where sauropod remains in Queensland have been determined to be titanosaurid. New remains from an outback town in Queensland, from rocks around 96 million years old, show that Australia too had large titanosaurs, around 25 meters long (82 feet). Four well preserved skeletons of a titanosaur species were found in Italy, a discovery first reported on May 2, 2006. http://www.sciencedaily.com/upi/index.php?feed=Science&article=UPI-1-20060502-14331100-bc-italy-dinosaur.xml Only Antarctica has yielded no titanosaur remains.

Paleobiology

Diet

Fossilized dung associated with late Cretaceous titanosaurids has revealed phytoliths, silicified plant fragments, that offer clues to a broad, unselective plant diet. Besides the plant remains that might have been expected, such as cycads and conifers, discoveries published in 2005 http://www.sciencemag.org/cgi/content/abstract/310/5751/1177 revealed an unexpectedly wide range of monocotyledons, including palms and grasses (Poaceae), including ancestors of rice and bamboo, which has given rise to speculation that herbivorous dinosaurs and grasses co-evolved.

Nesting

A large titanosaurid nesting ground was discovered in Auca Mahuevot, in Patagonia, Argentina and another colony has reportedly been discovered in Spain. The small eggs, about 11–12 cm (4–5 in.) in diameter, contained fossilised embryos, complete with skin impressions (though there was no indication of feathers or dermal spines). Apparently several hundred female saltasaurs dug holes, laid their eggs and then buried them under dirt and vegetation. This gives evidence of herd behavior, which, along with their armor, may have been a defensive behavior against large predators like the Abelisaurus.

Systematics

For such a widespread and successful group (they represent roughly a third of the total sauropod diversity known to date), the fossil record of titanosaurs is poor. Only recently have skulls or relatively complete skeletons (see Rapetosaurus) of any of the roughly 50 species of titanosaur been discovered. Many are poorly known, and much of the material may either be deemed invalid or be reclassified as understanding of the clade grows.
The family Titanosauridae was named after and anchored on the poorly known genus Titanosaurus, which was coined by Richard Lydekker in 1877, on the basis of a partial femur and two incomplete caudal vertebrae. Fourteen species have since been referred to Titanosaurus, which distribute the genus across Argentina, Europe, Madagascar, India and Laos and throughout 60 million years of the Cretaceous Period. Despite its centrality to titanosaur systematics and biogeography, a re-evaluation of all Titanosaurus species recognises only five as diagnostic. The type species T. indicus is invalid, because it is based on 'obsolescent' characters - once diagnostic features that have gained a broader taxonomic distribution over time. Consequently, use of the genus Titanosaurus has largely been abandoned. The most well known Titanosaurus specimens have since been re-assigned to other genera, including Isisaurus. However, if Titanosaurus indicus is redescribed in the future, on the basis of new finds, Titanosaurus itself becomes provisionally valid.
Some paleontologists (such as Sereno, 2005 http://www.taxonsearch.org/dev/taxon_edit.php?Action=View&tax_id=316) have contended that Titanosaurus is too poorly known to use as a basis for classification, family names for which it is the type genus (e.g. Titanosaurinae, Titanosauridae, Titanosauroidea) should not have other genera referred to them. Weishampel et al., in the second edition of The Dinosauria, also did not use the family Titanosauridae, and instead used several smaller titanosaur families such as Saltosauridae and Nemegtosauridae.

Taxonomy

Family-level taxonomy follows the definitions proposed by Paul Sereno in 2005, and referrals of genera to families in their individual descriptions.
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