Titanosaurs (or titanosaurians; members of the group Titanosauria) were a diverse group of sauropod dinosaurs, including genera from every continent. The titanosaurs were the last surviving group of long-necked sauropods, with taxa still thriving at the time of the extinction event at the end of the Cretaceous. This group includes some of the largest land animals known to have ever existed, such as Patagotitan, estimated at long with a mass of , and the comparably sized Argentinosaurus and Puertasaurus from the same region.
The group's name alludes to the mythological Titans of ancient Greek mythology, via the type genus (now considered a nomen dubium) Titanosaurus. Together with the brachiosaurids and relatives, titanosaurs make up the larger sauropod clade Titanosauriformes. Titanosaurs have long been a poorly known group, and the relationships among titanosaur species are still not well understood.
Fossil record
Due to the near-global distribution of titanosaurs during the Cretaceous, titanosaur fossils have been found on every continent, including Antarctica. However, titanosaurs have the least complete fossil record of any major sauropodomorph group. No complete titanosaur skeletons are known, and many species are only known from a few bones. Titanosaur skulls are especially rare. Though fragmentary cranial remains are known for several titanosaur genera, nearly complete skulls have been described for only four: Nemegtosaurus, Rapetosaurus, Sarmientosaurus, and Tapuiasaurus. As is the case in most other sauropod groups, there are few titanosaur specimens with complete necks preserving all of the cervical vertebrae in sequence.
Only three complete titanosaur necks are known: the holotype of Futalognkosaurus and two undescribed specimens from Argentina. A fourth specimen, of an unidentified titanosaur from Brazil, preserves a nearly complete neck, with only the atlas, the tiny vertebra forming the joint between the skull and neck, missing. Only five titanosaur specimens preserve complete, articulated hind feet. This incompleteness is especially significant for giant titanosaurs, which are generally known from disarticulated and fragmentary remains.
Titanosaurs are one of the few groups of dinosaurs for which fossil eggs are known. The fossil site of Auca Mahuevo preserves a titanosaur nesting ground. Some titanosaur eggs have been found containing fossil embryos, which even preserve fossil skin. These fossil embryos are among the few titanosaur specimens to preserve complete skulls.
Description Titanosauria have the largest range of body size of any sauropod clade, and includes both the largest known sauropods and some of the smallest. One of the largest titanosaurs, Patagotitan, had a body mass estimated to be , whereas one of the smallest, Magyarosaurus, had a body mass of approximately . Even relatively closely related titanosaurs could have very different body sizes, as the small rinconsaurs were closely related to the gigantic lognkosaurs. Fossils from perhaps the largest dinosaur ever found were discovered in 2021 in the Neuquén Province of northwest Patagonia, Argentina. It is believed that they are from a titanosaur. Some of the smallest titanosaurs, such as Magyarosaurus, inhabited Europe, which was largely made up of islands during the Cretaceous, and were likely island dwarfs. Another taxon of tiny titanosaurs, Ibirania, lived a non-insular context in Upper Cretaceous Brazil, and is an example of nanism resultant from other ecological pressures.
Head and neck
The heads of titanosaurs are poorly known. However, several different cranial morphologies are apparent. In some species, such as Sarmientosaurus, the head resembled that of brachiosaurids. In others, such as Rapetosaurus and Nemegtosaurus, the head resembled that of diplodocids. In some titanosaurs, the skull was especially diplodocid-like due to square-shaped jaws; the titanosaur Antarctosaurus is especially similar to the rebbachisaurid Nigersaurus. Titanosaurs had small heads, even when compared with other sauropods. The head was also wide, similar to the heads of Camarasaurus and Brachiosaurus, though somewhat more elongated. Titanosaurian nostrils were large ("macronarian") and all had crests formed by the nasal bones. Their teeth were either somewhat spatulate (spoon-like) or like pegs or pencils, but were always very small.
Titanosaur necks were of average length for sauropods, and their tails were whip-like though not as long as in the diplodocids. While the pelvis was slimmer than some sauropods, the pectoral (chest) area was much wider, giving them a uniquely "wide-legged" stance. As a result, the fossilized trackways of titanosaurs are distinctly broader than those of other sauropods. Their forelimbs were also stocky, and often longer than their hind limbs. Unlike other sauropods, some titanosaurs had no digits, walking only on horseshoe-shaped "stumps" made up of the columnar metacarpal bones.Apesteguía, S. (2005). "Evolution of the titanosaur metacarpus". Pp. 321–345 in Tidwell, V. and Carpenter, K. (eds.) Thunder-Lizards: The Sauropodomorph Dinosaurs. Indianapolis: Indiana University Press. Their vertebrae (back bones) were solid (not hollowed-out), which may be a reversal to more basal saurischian characteristics. Their spinal column was relatively flexible, likely making them more agile than other sauropods, though at the expense of rearing on their hind legs compared to the Diplodocoids. One of the most characteristic features shared by most titanosaurs were their procoelous caudal vertebrae, with ball-and-socket articulations between the vertebral centra.
Torso and limbs
The dorsal vertebrae of titanosaurs show multiple derived features among sauropods. Similarly to the Rebbachisauridae, titanosaurs lost the hyposphene-hypantrum articulations, a set of surfaces between vertebrae that prevent additional rotation of the bones. Andesaurus, one of the most basal titanosaurs, shows a normal hyposphene. The same area is reduced in Argentinosaurus to only two ridges, and is fully absent in taxa like Opisthocoelicaudia and Saltasaurus. Both Argentinosaurus and Epachthosaurus bear similar intermediate "hyposphenal ridges", which suggests they represent a more primitive form of dorsal vertebrae.
Sauropod hands already are highly derived from other dinosaurs, being reduced into columnar metacarpals and blocky phalanges with fewer claws. However, titanosaurs evolved the manus even further, completely losing the phalanges and heavily modifying the metacarpals. Argyrosaurus is the only titanosaur with preserved carpals. Other taxa like Epachthosaurus show a reduction of phalanges to one or two bones. Opisthocoelicaudia shows even more reduction of the hand than other titanosaurs, with both carpals and phalanges completely absent. However, Diamantinasaurus, while lacking carpals, preserves a manual formula of , including a thumb claw and phalanges on all other digits. This, coupled with the preservation of a single phalanx on digit IV of Epachthosaurus and potentially Opisthocoelicaudia (further study is necessary), show that preservation biases may be responsible for the lack of hand phalanges in these taxa. This suggests that Alamosaurus, Neuquensaurus, Saltasaurus and Rapetosaurus - all known from imperfect or disarticulated remains previously associated with a lack of phalanges - may have had phalanges but lost them after death.
Titanosaurs have a poor fossil record of their pedes (feet), only being complete in five definitive titanosaurs. Among these, Notocolossus is the largest, and also has the most specialized pes: like all titanosaurs, its pes is composed of short, thick metatarsals of approximately the same lengths; however, metatarsals I and V are notably more robust than in other taxa.
Integument
From skin impressions found with fossils, it has been determined that the skin of many titanosaurs was armored with a small mosaic of small, bead-like scales surrounding larger scales. While most titanosaurs were very large animals, many were fairly average in size compared to other giant dinosaurs. Some island-dwelling dwarf titanosaurs, such as Magyarosaurus, were probably the result of allopatric speciation and insular dwarfism.
Some titanosaurs had osteoderms. Osteoderms were first confirmed in the genus Saltasaurus but are now known to have been present in a variety of titanosaurs within the clade Lithostrotia. The exact arrangement of osteoderms on the body of a titanosaur is not known, but some paleontologists consider it likely that the osteoderms were arranged in two parallel rows on the animal's back, an arrangement similar to the plates of stegosaurs. Several other arrangements have been proposed, such as a single row along the midline, and it is possible that different species had different arrangements. The osteoderms were certainly far more sparse than those of ankylosaurs, and did not completely cover the back in scutes. Because of their sparse arrangement, it was unlikely that they served a significant role in defense. However, they may have played an important role in nutrient storage for titanosaurs living in highly seasonal climates and for female titanosaurs laying eggs. Osteoderms were present on both large and small species, so they were not solely used by smaller species as protection against predators. New evidence published in 2021 suggests there were indeed some defensive purposes in titanosaur osteoderms; simulated bite marks from both baurusuchid crocodylomorphs and abelisaurids on titanosaurid osteoderms suggest they could be useful for protecting the animals as well as functioning in mineral storage.
Classification
Titanosaurs are classified as sauropod dinosaurs. This highly diverse group forms the dominant clade of Cretaceous sauropods. Within Sauropoda, titanosaurs were once classified as close relatives of Diplodocidae due to their shared characteristic of narrow teeth, but this is now known to be the result of convergent evolution. Titanosaurs are now known to be most closely related to euhelopodids and brachiosaurids; together they form a clade named Titanosauriformes.
For much of the 20th century, most known species of titanosaurs were classified in the family Titanosauridae, which is no longer in widespread use. Titanosauria was first proposed in 1993 as a taxon to encompass titanosaurids and their close relatives. It has been phylogenetically defined as the clade composed of the most recent common ancestor of Saltasaurus and Andesaurus and all of its descendants. The relationships of species within Titanosauria remain largely unresolved, and it is considered one of the most poorly-understood areas of dinosaur classification. One of the few areas of agreement is that the majority of titanosaurs except Andesaurus and some other basal species form a clade called Lithostrotia, which some researchers consider equivalent to the deprecated Titanosauridae. Lithostrotians include titanosaurs such as Alamosaurus, Isisaurus, Malawisaurus, Rapetosaurus, and Saltasaurus.
Early history
Titanosaurus indicus was first named by British paleontologist Richard Lydekker in 1877, as a new taxon of dinosaur based on two caudals and a femur collected on different occasions at the same location in India. While it was later given a position as a sauropod within Cetiosauridae by Lydekker in 1888, he named the new sauropod family Titanosauridae for the genus in 1893, which included only Titanosaurus and Argyrosaurus, united by caudals, presacrals, a lack of pleurocoels and open chevrons. Following this, Austro-Hungarian paleontologist Franz Nopcsa reviewed reptile genera in 1928, and provided a short classification of Sauropoda, where he placed the Titanosaurinae (a reranking of Lydekker's Titanosauridae) in Morosauridae, and included the genera Titanosaurus, Hypselosaurus and Macrurosaurus because they all had strongly procoelous caudals.