| Sauropods | |
|---|---|
 | |
| Six sauropods (top left to bottom right):Patagotitan,Barosaurus,Giraffatitan,Omeisaurus,Shunosaurus, andAmargasaurus | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | †Sauropodomorpha |
| Clade: | †Anchisauria |
| Clade: | †Sauropoda Marsh, 1878 |
| Subgroups | |
| Synonyms | |
| |
Sauropoda (/sɔːˈrɒpədə/), whose members are known assauropods (/ˈsɔːrəpɒdz/;[1][2] fromsauro- +-pod, 'lizard-footed'), is aclade ofsaurischian ('lizard-hipped')dinosaurs. Sauropods had very long necks, long tails, small heads (relative to the rest of their body), and four thick, pillar-like legs. They are notable for the enormous sizes attained by some species, and the group includes the largest animals to have ever lived on land. Well-knowngenera includeApatosaurus,Argentinosaurus,Alamosaurus,Brachiosaurus,Camarasaurus,Diplodocus, andMamenchisaurus.[3][4]
The oldest known unequivocal sauropod dinosaurs are known from theEarly Jurassic.[5]Isanosaurus andAntetonitrus were originally described asTriassic sauropods,[6][7] but their age, and in the case ofAntetonitrus also its sauropod status, were subsequently questioned.[8][5][9] Sauropod-like sauropodomorph tracks from theFleming Fjord Formation (Greenland) might, however, indicate the occurrence of the group in theLate Triassic.[5] By theLate Jurassic (150 million years ago), sauropods had become widespread (especially thediplodocids andbrachiosaurids). By theLate Cretaceous, one group of sauropods, thetitanosaurs, had replaced all others and had a near-global distribution. However, as with all other non-avian dinosaurs alive at the time, thetitanosaurs died out in theCretaceous–Paleogene extinction event.Fossilised remains of sauropods have been found on every continent, includingAntarctica.[10][11][12][13]
The name Sauropoda was coined byOthniel Charles Marsh in 1878, and is derived fromAncient Greek, meaning "lizard foot".[14] Sauropods are one of the most recognizable groups of dinosaurs, and have become a fixture in popular culture due to their impressive size.
Complete sauropod fossil finds are extremely rare. Many species, especially the largest, are known only from isolated and disarticulated bones. Many near-complete specimens lack heads, tail tips and limbs.
Sauropods wereherbivorous (plant-eating), usually quite long-necked[15]quadrupeds (four-legged), often with spatulate (spatula-shaped: broad at the tip, narrow at the neck) teeth. They had tiny heads, massive bodies, and most had long tails. Their hind legs were thick, straight, and powerful, ending in club-like feet with five toes, though only the inner three (or in some cases four) bore claws. Their forelimbs were rather more slender and typically ended in pillar-like hands built for supporting weight; often only the thumb bore a claw. Many illustrations of sauropods in the flesh miss these facts, inaccurately depicting sauropods with hooves capping the claw-less digits of the feet, or more than three claws or hooves on the hands. Theproximalcaudal vertebrae are extremelydiagnostic for sauropods.[16]
The sauropods' most defining characteristic was their size. Even the dwarf sauropods (perhaps 5 to 6 metres (16 to 20 ft) long) were counted among the largest animals in theirecosystem. Their only real competitors in terms of size are therorquals, such as theblue whale. But, unlike whales, sauropods were primarilyterrestrial animals.
Their body structure did not vary as much as other dinosaurs, perhaps due to size constraints, but they displayed ample variety. Some, like thediplodocids, possessed tremendously long tails, which they may have been able tocrack like awhip as a signal or to deter or injure predators,[17] or to makesonic booms.[18][19]Supersaurus, at 33 to 34 metres (108 to 112 ft) long,[20] was the longest sauropod known from reasonably complete remains, but others, like the old record holder,Diplodocus, were also extremely long. Theholotype (and now lost)vertebra ofAmphicoelias fragillimus (nowMaraapunisaurus) may have come from an animal 58 metres (190 ft) long;[21] its vertebral column would have been substantially longer than that of the blue whale. However, research published in 2015 speculated that the size estimates ofA. fragillimus may have been highly exaggerated.[22] The longest dinosaur known from reasonable fossils material is probablyArgentinosaurus huinculensis with length estimates of 35 to 36 metres (115 to 118 ft) according to the most recent researches.[23][24] However the giantBarosaurus specimen BYU 9024 might have been even larger reaching lengths of 45–48 meters (148–157 ft).[23][24][25]
The longest terrestrial animal alive today, theAfrican elephant, can only reach lengths of 7.3 metres (24 ft).[26]
Others, like thebrachiosaurids, were extremely tall, with high shoulders and extremely long necks. The tallest sauropod was the giantBarosaurus specimen at 22 m (72 ft) tall.[23] By comparison, thegiraffe, the tallest of all living land animals, is only 4.8 to 5.6 metres (16 to 18 ft) tall.
The best evidence indicates that the most massive wereArgentinosaurus (65–80 metric tons[27][23][24]),Mamenchisaurus sinocanadorum (60-80 metric tons[24]), the giantBarosaurus specimen (60-80+ metric tons[23][24][25]) andPatagotitan withPuertasaurus (50-55 metric tons[23][24]). Meanwhile, 'mega-sauropods' such asBruhathkayosaurus has long been scrutinized due to controversial debates on its validity, but recent photos re-surfacing in 2022 have legitimized it,[28] allowing for more updated estimates that range between 110 and 170 tons, rivaling theblue whale in size.[29] The weight ofAmphicoelias fragillimus was estimated at 122.4 metric tons with lengths of up to nearly 60 meters[21] but 2015 research argued that these estimates were based on a diplodocid rather than the more modern rebbachisaurid, suggesting a much shorter length of 35–40 meters with mass between 80 and 120 tons.[22] Additional finds indicate a number of species likely reached or exceeded weights of 40 tons.[30] The largest land animal alive today, thebush elephant, weighs no more than 10.4 metric tons (11.5 short tons).[31]
Among the smallest sauropods were the primitiveOhmdenosaurus (4 metres or 13 feet long), the dwarftitanosaurMagyarosaurus (6 metres or 20 feet long), and the dwarfbrachiosauridEuropasaurus, which was 6.2 meters long as a fully-grown adult.[32] Its small stature was probably the result ofinsular dwarfism occurring in a population of sauropods isolated on an island of the late Jurassic in what is now the Langenberg area of northernGermany.[33][34] Thediplodocoid sauropodBrachytrachelopan was the shortest member of its group because of its unusually short neck. Unlike other sauropods, whose necks could grow to up to four times the length of their backs, the neck ofBrachytrachelopan was shorter than its backbone.
Fossils from perhaps the largest dinosaur ever found were discovered in 2012 in the Neuquén Province of northwest Patagonia, Argentina. It is believed that they are from a titanosaur, which were amongst the largest sauropods.[35][30]
On or shortly before 29 March 2017 a sauropod footprint about 5.6 feet (1.7 meters) long was found atWalmadany in the Kimberley Region of Western Australia.[36] The report said that it was the biggest known yet. In 2020 Molina-Perez and Larramendi estimated the size of the animal at 31 meters (102 feet) and 72 tonnes (79.4 short tons) based on the 1.75 meters (5.7 feet) long footprint.[23]
As massivequadrupeds, sauropods developed specialized "graviportal" (weight-bearing) limbs. The hind feet were broad, and retained three claws in most species.[37] Particularly unusual compared with other animals were the highly modified front feet (manus). The front feet of sauropods were very dissimilar from those of modern large quadrupeds, such aselephants. Rather than splaying out to the sides to create a wide foot as in elephants, themanus bones of sauropods were arranged in fully vertical columns, with extremely reduced finger bones (though it is not clear if the most primitive sauropods, such asVulcanodon andBarapasaurus, had such forefeet).[38] The front feet were so modified ineusauropods that individual digits would not have been visible in life.
The arrangement of the forefoot bone (metacarpal) columns in eusauropods was semi-circular, so sauropod forefoot prints are horseshoe-shaped. Unlike elephants, print evidence shows that sauropods lacked any fleshy padding to back the front feet, making them concave.[38] The only claw visible in most sauropods was the distinctive thumb claw (associated with digit I). Almost all sauropods had such a claw, though what purpose it served is unknown. The claw was largest (as well as tall and laterally flattened) in diplodocids, and very small in brachiosaurids, some of which seem to have lost the claw entirely based on trackway evidence.[39] Titanosaurs may have lost the thumb claw completely (with the exception of early forms, such asJanenschia).
Titanosaurs were most unusual among sauropods, as, across their history as a clade, they lost not just the external claw but also completely lost the digits of the front foot. Advanced titanosaurs had no digits or digit bones, and walked only on horseshoe-shaped "stumps" made up of the columnar metacarpal bones.[40]
Print evidence fromPortugal shows that, in at least some sauropods (probably brachiosaurids), the bottom and sides of the forefoot column was likely covered in small, spiny scales, which left score marks in the prints.[41] In titanosaurs, the ends of the metacarpal bones that contacted the ground were unusually broad and squared-off, and some specimens preserve the remains of soft tissue covering this area, suggesting that the front feet were rimmed with some kind of padding in these species.[40]
Matthew Bonnan[42][43] has shown that sauropod dinosaur long bones grewisometrically: that is, there was little to no change in shape as juvenile sauropods became gigantic adults. Bonnan suggested that this odd scaling pattern (most vertebrates show significant shape changes in long bones associated with increasing weight support) might be related to a stilt-walker principle (suggested by amateur scientist Jim Schmidt) in which the long legs of adult sauropods allowed them to easily cover great distances without changing their overall mechanics.
Along with othersaurischian dinosaurs (such astheropods, including birds), sauropods had a system ofair sacs, evidenced by indentations and hollow cavities in most of theirvertebrae that had been invaded by them.Pneumatic, hollow bones are a characteristic feature of all sauropods.[44] These air spaces reduced the overall weight of the massive necks that the sauropods had, and the air-sac system in general, allowing for a single-direction airflow through stiff lungs, made it possible for the sauropods to get enough oxygen.[45] This adaptation would have advantaged sauropods particularly in the relatively low oxygen conditions of the Jurassic and Early Cretaceous.[46]
The bird-like hollowing of sauropod bones was recognized early in the study of these animals, and, in fact, at least one sauropod specimen found in the 19th century (Ornithopsis) was originally misidentified as apterosaur (a flying reptile) because of this.[47]
Some sauropods hadarmor. There were genera with smallclubs on their tails, a prominent example beingShunosaurus, and severaltitanosaurs, such asSaltasaurus andAmpelosaurus, had small bonyosteoderms covering portions of their bodies.
A study by Michael D'Emic and his colleagues from Stony Brook University found that sauropods evolved high tooth replacement rates to keep up with their large appetites. The study suggested thatNigersaurus, for example, replaced each tooth every 14 days,Camarasaurus replaced each tooth every 62 days, andDiplodocus replaced each tooth once every 35 days.[48] The scientists found qualities of the tooth affected how long it took for a new tooth to grow.Camarasaurus's teeth took longer to grow than those forDiplodocus because they were larger.[49]
It was also noted by D'Emic and his team that the differences between the teeth of the sauropods also indicated a difference in diet.Diplodocus ate plants low to the ground andCamarasaurus browsed leaves from top and middle branches. According to the scientists, the specializing of their diets helped the different herbivorous dinosaurs to coexist.[48][49]
Sauropod necks have been found at over 15 metres (49 ft) in length, a full six times longer than the world record giraffe neck.[45] Enabling this were a number of essential physiological features. The dinosaurs' overall large body size and quadrupedal stance provided a stable base to support the neck, and the head was evolved to be very small and light, losing the ability to orally process food. By reducing their heads to simple harvesting tools that got the plants into the body, the sauropods needed less power to lift their heads, and thus were able to develop necks with less dense muscle and connective tissue. This drastically reduced the overall mass of the neck, enabling further elongation.
Sauropods also had a great number of adaptations in their skeletal structure. Some sauropods had as many as 19cervical vertebrae, whereas almost all mammals are limited to only seven. Additionally, each vertebra was extremely long and had a number of empty spaces in them which would have been filled only with air. An air-sac system connected to the spaces not only lightened the long necks, but effectively increased the airflow through the trachea, helping the creatures to breathe in enough air. By evolving vertebrae consisting of 60% air, the sauropods were able to minimize the amount of dense, heavy bone without sacrificing the ability to take sufficiently large breaths to fuel the entire body with oxygen.[45] According to Kent Stevens, computer-modeled reconstructions of the skeletons made from the vertebrae indicate that sauropod necks were capable of sweeping out large feeding areas without needing to move their bodies, but were unable to be retracted to a position much above the shoulders for exploring the area or reaching higher.[50]
Another proposed function of the sauropods' long necks was essentially a radiator to deal with the extreme amount of heat produced from their large body mass. Considering that the metabolism would have been doing an immense amount of work, it would certainly have generated a large amount of heat as well, and elimination of this excess heat would have been essential for survival.[51]It has also been proposed that the long necks would have cooled the veins and arteries going to the brain, avoiding excessively heated blood from reaching the head. It was in fact found that the increase in metabolic rate resulting from the sauropods' necks was slightly more than compensated for by the extra surface area from which heat could dissipate.[52]
Dental microwear texture analysis (DMTA) performed on a titanosauriform sauropod from the Turonian-aged Tamagawa Formation suggests that the sauropod fed on plant material that was softer than insect exoskeletons or mollusc shells, with the diet likely consisting of ferns and gymnosperms. The DMTA results also suggested that sauropods likely masticated more energetically than present-day lepidosaurs do.[53]
When sauropods were first discovered, their immense size led many scientists to compare them with modern-daywhales. Most studies in the 19th and early 20th centuries concluded that sauropods were too large to have supported their weight on land, and therefore that they must have been mainlyaquatic. Most life restorations of sauropods in art through the first three quarters of the 20th century depicted them fully or partially immersed in water.[54] This early notion was cast in doubt beginning in the 1950s, when a study by Kermack (1951) demonstrated that, if the animal were submerged in several metres of water, the pressure would be enough to fatally collapse the lungs and airway.[55] However, this and other early studies of sauropodecology were flawed in that they ignored a substantial body of evidence that the bodies of sauropods were heavily permeated withair sacs. In 1878, paleontologist E.D. Cope had even referred to these structures as "floats".
Beginning in the 1970s, the effects of sauropod air sacs on their supposed aquatic lifestyle began to be explored. Paleontologists such as Coombs and Bakker used this, as well as evidence fromsedimentology andbiomechanics, to show that sauropods were primarily terrestrial animals. In 2004, D.M. Henderson noted that, due to their extensive system of air sacs, sauropods would have been buoyant and would not have been able to submerge their torsos completely below the surface of the water; in other words, they would float, and would not have been in danger of lung collapse due to water pressure when swimming.[54]
Evidence for swimming in sauropods comes from fossil trackways that have occasionally been found to preserve only the forefeet (manus) impressions. Henderson showed that such trackways can be explained by sauropods with long forelimbs (such asmacronarians) floating in relatively shallow water deep enough to keep the shorter hind legs free of the bottom, and using the front limbs topunt forward.[54] However, due to their body proportions, floating sauropods would also have been very unstable and maladapted for extended periods in the water. This mode of aquaticlocomotion, combined with its instability, led Henderson to refer to sauropods in water as "tipsy punters".[54]
While sauropods could therefore not have been aquatic as historically depicted, there is evidence that they preferred wet and coastal habitats. Sauropod footprints are commonly found following coastlines or crossing floodplains, and sauropod fossils are often found in wet environments or intermingled with fossils of marine organisms.[54] A good example of this would be the massiveJurassic sauropod trackways found inlagoon deposits onScotland'sIsle of Skye.[56] Studies published in 2021 suggest sauropods could not inhabit polar regions. This study suggests they were largely confined to tropical areas and had metabolisms that were very different to those of other dinosaurs, perhaps intermediate between mammals and reptiles.[57] New studies published by Taia Wyenberg-henzler in 2022 suggest that sauropods in North America declined due to undetermined reasons in regards to their niches and distribution during the end of the Jurassic and into the latest Cretaceous. Why this is remains unclear, but some similarities in feeding niches between iguanodontians, hadrosauroids and sauropods have been suggested and may have resulted in some competition. However, this cannot fully explain the full decline in distribution of sauropods, as competitive exclusion would have resulted in a much more rapid decline than what is shown in the fossil record. Moreover, it must be determined as to whether sauropod declines in North America was the result of a change in preferred flora that sauropods ate, climate, or other factors. It is also suggested in this same study that iguanodontians and hadrosauroids took advantage of recently vacated niches left by a decline in sauropod diversity during the late Jurassic and the Cretaceous in North America.[58]
Many lines of fossil evidence, from both bone beds and trackways, indicate that sauropods were gregarious animals that formedherds. However, the makeup of the herds varied between species. Some bone beds, for example a site from theMiddle Jurassic ofArgentina, appear to show herds made up of individuals of various age groups, mixing juveniles and adults. However, a number of other fossil sites and trackways indicate that many sauropod species travelled in herds segregated by age, with juveniles forming herds separate from adults. Such segregated herding strategies have been found in species such asAlamosaurus,Bellusaurus and somediplodocids.[59]
In a review of the evidence for various herd types, Myers and Fiorillo attempted to explain why sauropods appear to have often formed segregated herds. Studies of microscopic tooth wear show that juvenile sauropods had diets that differed from their adult counterparts, so herding together would not have been as productive as herding separately, where individual herd members could forage in a coordinated way. The vast size difference between juveniles and adults may also have played a part in the different feeding and herding strategies.[59]
Since the segregation of juveniles and adults must have taken place soon after hatching, and combined with the fact that sauropod hatchlings were most likelyprecocial, Myers and Fiorillo concluded that species with age-segregated herds would not have exhibited much parental care.[59] On the other hand, scientists who have studied age-mixed sauropod herds suggested that these species may have cared for their young for an extended period before reaching adulthood.[60] A 2014 study suggested that the time from laying the egg to the time of the hatching was likely to have been between 65 and 82 days.[61] Exactly how segregated versus age-mixed herding varied across different groups of sauropods is unknown. Further examples of gregarious behavior will need to be discovered from more sauropod species to detect possible distribution patterns.[59]
Multiple nesting sites discovered in Argentina and India contain 30-400 clutches of fossilized eggs that were found preserved, providing evidence of sauropod maternal care. Researchers suggest that sauropods might have settled in nesting grounds close to volcanic activity for geothermal incubation, in which the mothers keep their eggs warm. This behavior is similar to modern birds and reptiles who follow the same method.[62][63][64]
Since early in the history of their study, scientists, such asOsborn, have speculated that sauropods could rear up on their hind legs, using the tail as the third 'leg' of a tripod.[65] A skeletal mount depicting thediplodocidBarosaurus lentus rearing up on its hind legs at theAmerican Museum of Natural History is one illustration of this hypothesis. In a 2005 paper, Rothschild and Molnar reasoned that if sauropods had adopted a bipedal posture at times, there would be evidence of stress fractures in the forelimb 'hands'. However, none were found after they examined a large number of sauropod skeletons.[66]
Heinrich Mallison (in 2009) was the first to study the physical potential for various sauropods to rear into a tripodal stance. Mallison found that some characters previously linked to rearing adaptations were actually unrelated (such as the wide-set hip bones oftitanosaurs) or would have hindered rearing. For example, titanosaurs had an unusually flexible backbone, which would have decreased stability in a tripodal posture and would have put more strain on the muscles. Likewise, it is unlikely that brachiosaurids could rear up onto the hind legs, as their center of gravity was much farther forward than other sauropods, which would cause such a stance to be unstable.[67]
Diplodocids, on the other hand, appear to have been well adapted for rearing up into a tripodal stance. Diplodocids had a center of mass directly over the hips, giving them greater balance on two legs. Diplodocids also had the most mobile necks of sauropods, a well-muscled pelvic girdle, and tail vertebrae with a specialised shape that would allow the tail to bear weight at the point it touched the ground. Mallison concluded that diplodocids were better adapted to rearing thanelephants, which do so occasionally in the wild. He also argues that stress fractures in the wild do not occur from everyday behaviour,[67] such as feeding-related activities (contra Rothschild and Molnar).[66]
There is little agreement over how sauropods held their heads and necks, and the postures they could achieve in life.
Whether sauropods' long necks could be used for browsing high trees has been questioned based on calculations suggesting that just pumping blood up to the head in such a posture[68] for long would have used some half of its energy intake.[69] Further, to move blood to such a height—dismissing posited auxiliary hearts in the neck[70]—would require a heart 15 times as large as of a similar-sized whale.[71]
The above have been used to argue that the long neck must instead have been held more or less horizontally, presumed to enable feeding on plants over a wide area with less need to move about, yielding a large energy saving for such a large animal. Reconstructions of the necks ofDiplodocus andApatosaurus have therefore often portrayed them in near-horizontal, so-called "neutral, undeflected posture".[72]
However, research on living animals demonstrates that almost all extanttetrapods hold the base of their necks sharply flexed when alert, showing that any inference from bones about habitual "neutral postures"[72] is deeply unreliable.[73][74] Meanwhile, computer modeling ofostrich necks has raised doubts over the flexibility needed for stationary grazing.[75][76][77]
Sauropodtrackways and otherfossil footprints (known as "ichnites") are known from abundant evidence present on most continents. Ichnites have helped support other biological hypotheses about sauropods, including general fore and hind foot anatomy (seeLimbs and feet above). Generally, prints from the forefeet are much smaller than the hind feet, and often crescent-shaped. Occasionally ichnites preserve traces of the claws, and help confirm which sauropod groups lost claws or even digits on their forefeet.[78]
Sauropod tracks from theVillar del Arzobispo Formation of earlyBerriasian age in Spain support the gregarious behaviour of the group. The tracks are possibly more similar toSauropodichnus giganteus than any other ichnogenera, although they have been suggested to be from a basal titanosauriform. The tracks are wide-gauge, and the grouping as close toSauropodichnus is also supported by the manus-to-pes distance, the morphology of the manus being kidney bean-shaped, and the morphology of the pes being subtriangular. It cannot be identified whether the footprints of the herd were caused by juveniles or adults, because of the lack of previous trackway individual age identification.[79]
Generally, sauropod trackways are divided into three categories based on the distance between opposite limbs: narrow gauge, medium gauge, and wide gauge. The gauge of the trackway can help determine how wide-set the limbs of various sauropods were and how this may have impacted the way they walked.[78] A 2004 study by Day and colleagues found that a general pattern could be found among groups of advanced sauropods, with each sauropod family being characterised by certain trackway gauges. They found that most sauropods other thantitanosaurs had narrow-gauge limbs, with strong impressions of the large thumb claw on the forefeet. Medium gauge trackways with claw impressions on the forefeet probably belong to brachiosaurids and other primitivetitanosauriformes, which were evolving wider-set limbs but retained their claws. Primitive true titanosaurs also retained their forefoot claw but had evolved fully wide gauge limbs. Wide gauge limbs were retained by advanced titanosaurs, trackways from which show a wide gauge and lack of any claws or digits on the forefeet.[80]
Occasionally, only trackways from the forefeet are found. Falkinghamet al.[81] used computer modelling to show that this could be due to the properties of the substrate. These need to be just right to preserve tracks.[82] Differences in hind limb and fore limb surface area, and therefore contact pressure with the substrate, may sometimes lead to only the forefeet trackways being preserved.
In a study published in PLoS ONE on October 30, 2013, byBill Sellers,Rodolfo Coria,Lee Margettset al.,Argentinosaurus was digitally reconstructed to test its locomotion for the first time. Before the study, the most common way of estimating speed was through studying bonehistology andichnology. Commonly, studies about sauropod bone histology and speed focus on the postcranial skeleton, which holds many unique features, such as an enlarged process on theulna, a wide lobe on theilia, an inward-slanting top third of thefemur, and an extremely ovoid femur shaft. Those features are useful when attempting to explain trackway patterns of graviportal animals. When studying ichnology to calculate sauropod speed, there are a few problems, such as only providing estimates for certain gaits because ofpreservation bias, and being subject to many more accuracy problems.[83]
To estimate the gait and speed ofArgentinosaurus, the study performed a musculoskeletal analysis. The only previous musculoskeletal analyses were conducted onhominoids,terror birds, and otherdinosaurs. Before they could conduct the analysis, the team had to create a digital skeleton of the animal in question, show where there would be muscle layering, locate the muscles and joints, and finally find the muscle properties before finding the gait and speed. The results of the biomechanics study revealed thatArgentinosaurus was mechanically competent at a top speed of 2 m/s (5 mph) given the great weight of the animal and the strain that its joints were capable of bearing.[84] The results further revealed that much larger terrestrial vertebrates might be possible, but would require significant body remodeling and possible sufficient behavioral change to prevent joint collapse.[83]
Sauropods were gigantic descendants of surprisingly small ancestors. Basal dinosauriformes, such asPseudolagosuchus andMarasuchus from the Middle Triassic of Argentina, weighed approximately 1 kg (2.2 lb) or less. These evolved into saurischia, which saw a rapid increase ofbauplan size, although more primitive members likeEoraptor,Panphagia,Pantydraco,Saturnalia andGuaibasaurus still retained a moderate size, possibly under 10 kg (22 lb). Even with these small, primitive forms, there is a notable size increase among sauropodomorphs, although scanty remains of this period make interpretation conjectural. There is one definite example of a small derived sauropodomorph:Anchisaurus, under 50 kg (110 lb), even though it is closer to the sauropods thanPlateosaurus andRiojasaurus, which were upwards of 1 t (0.98 long tons; 1.1 short tons) in weight.[51]
Evolving from sauropodomorphs, the sauropods were huge. Their giant size probably resulted from an increased growth rate made possible bytachymetabolic endothermy, a trait which evolved in sauropodomorphs. Once branched into sauropods, sauropodomorphs continued steadily to grow larger, with smaller sauropods, like the Early JurassicBarapasaurus andKotasaurus, evolving into even larger forms like the Middle JurassicMamenchisaurus andPatagosaurus. Responding to the growth of sauropods, their theropod predators grew also, as shown by anAllosaurus-sizedcoelophysoid fromGermany.[51]
Neosauropoda is quite plausibly the clade of dinosaurs with the largest body sizes ever to have existed. The few exceptions of smaller size are hypothesized to be caused byisland dwarfism, or otherecological pressures, although there is a trend in someTitanosauria towards a smaller size. Thetitanosaurs, however, were some of the largest sauropods ever. Other than titanosaurs, diplodocoids also reached truly gigantic sizes. Meanwhile, a clade of diplodocoids, calledDicraeosauridae, are identified by a small to medium[clarification needed] body size. No sauropods were very small, however, for even "dwarf" sauropods are larger than 500 kg (1,100 lb), a size reached by only about 10% of all mammalian species.[51]
Although in general, sauropods were large, a gigantic size (40 t (39 long tons; 44 short tons) or more) was reached independently at multiple times in their evolution. Many gigantic forms existed in the Late Jurassic (specificallyKimmeridgian), such as theturiasaurTuriasaurus, the mamenchisauridsMamenchisaurus andXinjiangtitan, the diplodocoidsMaraapunisaurus,Diplodocus,Apatosaurus,Supersaurus andBarosaurus, the camarasauridCamarasaurus, and the brachiosauridsBrachiosaurus andGiraffatitan. Through the Early to Late Cretaceous, the giantsBorealosaurus,Sauroposeidon,Paralititan,Argentinosaurus,Puertasaurus,Antarctosaurus,Dreadnoughtus,Notocolossus,Futalognkosaurus,Patagotitan andAlamosaurus lived, with all possibly being titanosaurs. One sparsely known possible giant isHuanghetitan ruyangensis, only known from 3 m (9.8 ft) long ribs. These giant species lived in the Late Jurassic to the Late Cretaceous, appearing independently over a time span of 85 million years.[51]
Two well-known island dwarf species of sauropods are the CretaceousMagyarosaurus (at one point its identity as a dwarf was challenged) and the JurassicEuropasaurus, both from Europe. Even though these sauropods are small, the only way to prove they are true dwarfs is through a study of their bone histology. A study by Martin Sander and colleagues in 2006 examined eleven individuals ofEuropasaurus holgeri using bone histology and demonstrated that the small island species evolved through a decrease in the growth rate of long bones as compared to rates of growth in ancestral species on the mainland.[85] Two other possible dwarfs areRapetosaurus, which existed on the island ofMadagascar, an isolated island in the Cretaceous, andAmpelosaurus, a titanosaur that lived on the Iberian peninsula of southern Spain and France.Amanzia from Switzerland might also be a dwarf, but this has yet to be proven.[51] One of the most extreme cases of island dwarfism is found inEuropasaurus, a relative of the much largerCamarasaurus andBrachiosaurus: it was only about 6.2 m (20 ft) long, an identifying trait of the species. As for all dwarf species, their reduced growth rate led to their small size.[32][51] Another taxon of tiny sauropods, thesaltasaurid titanosaurIbirania, 5.7 metres (19 feet) long, lived a non-insular context in Upper Creaceous Brazil, and is an example ofnanism resultant from other ecological pressures.[86]
Sauropods are rarely known for preserved injuries or signs of illnesses, but more recent discoveries show they could suffer from such pathologies. A diplodocid specimen from theMorrison Formation referred to as "Dolly" was described in 2022 with evidence of a severe respiratory infection.[87][88] Sauropod ribs fromYunyang County,Chongqing, in southwest China show evidence of rib breakage by way of traumatic fracture, bone infection, andosteosclerosis.[89] A sauropod tibia exhibiting initial fracture has been described from the Middle Jurassic of Yunyang County in southwestern China.[90]
Ibirania, a nanoidtitanosaur fossil fromBrazil, suggests that individuals of various genera were susceptible to diseases such asosteomyelitis and parasite infestations. The specimen hails from the late cretaceousSão José do Rio Preto Formation,Bauru Basin, and was described in the journalCretaceous Research byAureliano et al. (2021).[91] Examination of the titanosaur's bones revealed what appear to be parasitic blood worms similar to the prehistoricPaleoleishmania but are 10-100 times larger, that seemed to have caused the osteomyelitis. The fossil is the first known instance of an aggressive case of osteomyelitis being caused by blood worms in an extinct animal.[92][93][94]
The first scraps of fossil remains now recognized as sauropods all came fromEngland and were originally interpreted in a variety of different ways. Their relationship to other dinosaurs was not recognized until well after their initial discovery.
The first sauropod fossil to be scientifically described was a single tooth known by the non-Linnaean descriptorRutellum implicatum.[95] This fossil was described byEdward Lhuyd in 1699, but was not recognized as a giant prehistoric reptile at the time.[96] Dinosaurs would not be recognized as a group until over a century later.
Richard Owen published the first modern scientific descriptions of sauropods in 1841, in a book and a paper namingCardiodon andCetiosaurus.Cardiodon was known only from two unusual, heart-shaped teeth (from which it got its name), which could not be identified beyond the fact that they came from a previously unknown largereptile.Cetiosaurus was known from slightly better, but still scrappy remains. Owen thought at the time thatCetiosaurus was a giant marine reptile related to moderncrocodiles, hence its name, which means "whale lizard". A year later, when Owen coined the nameDinosauria, he did not includeCetiosaurus andCardiodon in that group.[97]
In 1850,Gideon Mantell recognized the dinosaurian nature of several bones assigned toCetiosaurus by Owen. Mantell noticed that the leg bones contained amedullary cavity, a characteristic of land animals. He assigned these specimens to the newgenusPelorosaurus, and grouped it together with the dinosaurs. However, Mantell still did not recognize the relationship toCetiosaurus.[47]
The next sauropod find to be described and misidentified as something other than a dinosaur were a set of hip vertebrae described byHarry Seeley in 1870. Seeley found that the vertebrae were very lightly constructed for their size and contained openings forair sacs (pneumatization). Such air sacs were at the time known only inbirds andpterosaurs, and Seeley considered the vertebrae to come from a pterosaur. He named the new genusOrnithopsis, or "bird face" because of this.[47]
When more complete specimens ofCetiosaurus were described by Phillips in 1871, he finally recognized the animal as a dinosaur related toPelorosaurus.[98] However, it was not until the description of new, nearly complete sauropod skeletons from theUnited States (representingApatosaurus andCamarasaurus) later that year that a complete picture of sauropods emerged. An approximate reconstruction of a complete sauropod skeleton was produced by artist John A. Ryder, hired by paleontologist E.D. Cope, based on the remains ofCamarasaurus, though many features were still inaccurate or incomplete according to later finds and biomechanical studies.[99] Also in 1877,Richard Lydekker named another relative ofCetiosaurus,Titanosaurus, based on an isolated vertebra.[47]
In 1878, the most complete sauropod yet was found and described byOthniel Charles Marsh, who named itDiplodocus. With this find, Marsh also created a new group to containDiplodocus,Cetiosaurus, and their increasing roster of relatives to differentiate them from the other major groups of dinosaurs. Marsh named this group Sauropoda, or "lizard feet".[47]
The firstphylogenetic definition of Sauropoda was published in 1997 by Salgado and colleagues. They defined the clade as anode-based taxon, containing "themost recent common ancestor ofVulcanodon karibaensis and Eusauropoda and all of its descendants".[100] Later, severalstem-based definitions were proposed, including one by Yates (2007), who defined Sauropoda as "the most inclusive clade that includesSaltasaurus loricatus but notMelanorosaurus readi".[101][102]
Proponents of this definition also use the clade nameGravisauria, defined as the most recent ancestor ofTazoudasaurus naimi andSaltasaurus loricatus and all of its descendants[103] for the clade equivalent to Sauropoda as defined by Salgadoet al.[104] Theclade Gravisauria was appointed by theFrenchpaleontologist Ronan Allain andMoroccan paleontologist Najat Aquesbi in 2008 when acladistic analysis of the dinosaur found by Allain,Tazoudasaurus, as the outcome was that thefamilyVulcanodontidae. The group includesTazoudasaurus andVulcanodon, and the sister taxon Eusauropoda, but also certainspecies such asAntetonitrus,Gongxianosaurus andIsanosaurus that do not belong in Vulcanodontidae but to an even more basic position occupied in Sauropoda. It made sense to have Sauropoda compared to this, more derived group that included Vulcanodontidae and Eusauropoda in a definition: defined as the group formed by the last common ancestor ofTazoudasaurus andSaltasaurus (Bonaparte and Powell, 1980) and all its descendants. Aquesbi mentioned two synapomorphies, shared derived characteristics of Gravisauria: thevertebrae are wider side to side than front to rear and possession of asymmetrical condyles femoris at the bottom of thefemur. Those were previously not thought to be Eusauropoda synapomorphies but Allian found these properties also onTazoudasaurus.[105]
Gravisauria split off in theEarly Jurassic, around thePliensbachian andToarcian, 183 million years ago, and Aquesbi thought that this was part of a much larger revolution in thefauna, which includes the disappearance ofProsauropoda,Coelophysoidea andbasalThyreophora, which they attributed to a worldwidemass extinction.[105]
The phylogenetic relationships of the sauropods have largely stabilised in recent years, though there are still some uncertainties, such as the placement ofEuhelopus,Haplocanthosaurus,Jobaria andNemegtosauridae.
Cladogram after an analysis presented by Sander and colleagues in 2011.[51]
| †Sauropoda | |
The necks of the sauropod dinosaurs were by far the longest of any animal...
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