The Prosauropods

a spotlight on some of the largest prosauropods

The members of Prosauropoda were, morphologically at least, the ‘beta versions’ of the more popular gigantic sauropods. Prosauropods were one of the first groups of dinosaurs to evolve, if not the first (a recent find in Madagascar may give the theropod Eoraptor a run for its money). They sprang up in the Upper Triassic around 230 mya and rose to dominance in the Lower Jurassic before being outdone by the emerging sauropods. Their reign lasted on the order of fifty million years (or, to put it another way, they dominated the first quarter of non-avian dinosaur history), and their remains have been found on every continent except Australia.

~  The Prosauropods: A Brief History  ~

The Upper Triassic saw the splitting of the supercontinent Pangaea; as it ripped apart at its seams, northern and southern Pangaea separated, resulting in two new supercontinents. The northern continent is known as Laurasia, while the southern continent is called Gondwana. Though Laurasian bone-beds in Europe have produced the greatest number of prosauropod fossils, it seems that the prosauropod lineage originated far to the south in Gondwana (modern day Africa), since it is from there that the earliest prosauropod specimens have been discovered. Emerging from Gondwana, prosauropods radiated throughout the world, evolving at a rapid pace and with lightning diversification. It’s likely that their success was prodded on by the concurrent domination of the gymnosperms. 

a Plateosaurus facing off against a pair of Liliensternus

Spore-bearing plants are well-suited for aquatic environments but fail in drier territories; gymnosperms (or seed-bearing plants) enclose their seeds in protective coverings, like pinecones, that enable them to survive harsher climates. Seed-bearing plants spread like kudzu during the Triassic Period, due in part to both the formation of Pangaea in the Permian (the ‘closing’ of Pangaea resulted in drier, more arid landscapes in the interior, which were more suitable for seed-bearing plants than for spore-bearers) and the Permian-Triassic extinction event. The latter scarred the planet’s marine and terrestrial ecosystems, and many of the swamplands of the Permian dried up; spore-bearing plants were put at a disadvantage in this harsh, post-apocalyptic, desert-stricken landscape; seed-bearers, however, flourished – and they had a tag-along. Gymnosperms frequently kept their ‘soft parts’ high off the ground to preserve them from the low-grazing dicynodonts (tusked mammal-like herbivores) and rhynchosaurs (reptilian herbivores) that thrived in the Triassic. Prosauropods, however, were well-equipped for feeding on gymnosperms: their long necks, lightweight heads, and counter-balancing tails enabled them to rear back on their hind legs or raise their head to reach the gymnosperms’ foliage. 

As prosauropods and gymnosperms engaged in an evolutionary tug-of-war, prosauropods became more advanced and gymnosperms diversified. The gymnosperms stoked the prosauropod fire, and the prosauropods returned the favor. This Triassic arms race worked well for the prosauropods: they boomed in size, attaining lengths of up to thirty feet and clocking in at as much as two tons. One such beast, Plateosaurus, became one of the dominant dinosaurs of Upper Triassic Europe, roaming the conifer forests in massive herds. The Triassic-Jurassic extinction event killed off many of the Triassic heavyweights, such as Riojasaurus, but a good number of prosauropods survived into the Jurassic. While the sauropods (virtual ‘second cousins’ to the prosauropods) struggled in the Lower Jurassic, the prosauropod survivors reigned: mega-herbivores such as Massospondylus and Yunnanosaurus were the herbivorous kings of the early Jurassic ecosystems. 

an artist's rendition of Massospondylus

Their dominion didn’t last long. Regardless of their origins, sauropods bloomed in the Jurassic. They gained steam at the opening of the period and overtook the prosauropods on the cusp of the mid-Jurassic. Sauropod evolution didn’t relent, and the prosauropods were relegated to the sidelines as the sauropods entered their Jurassic heyday. Sauropods evolved longer necks and more room for digestive organs, advancing beyond the restrictions of common-sense physics, and gaining ground (and height) against contemporary Jurassic carnivores. A key asset in the sauropod sweep was their reproductive abilities against those of the prosauropods: while prosauropods laid small clutches of eggs (around ten eggs per nest), sauropods laid up to four layers of 15-34 eggs, or anywhere between 60 and 136 eggs per nest. Sauropod hatchlings vastly outnumbered those of the prosauropods, so sauropods could chart a steady graph of increasing world dominance. 

The ‘sauropod hell-storm’ led to a decline in the prosauropods that was made worse by the diversification of their archenemies, the theropods. By the late Jurassic, massive pack-hunting predators such as the twenty-foot-long Ceratosaurus and thirty-foot-long Allosaurus had emerged, and prosauropods would’ve made easier killings than the larger sauropods. While prosauropods could protect themselves with giant thumb claws and razor-sharp clawed feet, most probably relied on the protection offered by the herd. Because prosauropods were at a reproductive disadvantage against the sauropods (and because eggs, back then as now, made for nutritious breakfasts), prosauropods found themselves increasingly outnumbered by the sauropods. Stunted by the sauropods’ exponential population growth, and put on the run by the rise of larger carnivores, the prosauropods never recovered. Though it’s generally agreed that prosauropods met their end in the late Jurassic, some scientists have argued that they subtly continued into the Cretaceous, where they took on new forms in the aberrant theropods known as the therizinosaurs (more on this in a moment). Even if such a theory rung true, they nevertheless failed to regain their former glory as the dominant herbivores of the Mesozoic. 

~  Morphology and Behavior  ~

Prosauropods measured smaller than their later cousins, the sauropods: most were medium-sized creatures averaging around thirteen to twenty feet in length (half of that being in the tail), though some stretched over thirty feet and clocked in around two to three tons. They had small, lightweight heads perched atop a long neck consisting of around ten elongated vertebrae. Prosauropods had five-fingered hands with hoof-like structures that made them adept at grabbing, and the ‘thumb’ on each hand had evolved into a half-moon-shaped claw like those of their sauropod cousins. They probably used the thumb claw to cut into the soft parts of woody plants or to manipulate tree limbs towards their mouths, but when in a tight spot they may have used it as a defensive weapon: coupled with their clawed feet, these animals were equipped to put up a decent fight. 

note the thumb claw in this artist's portrayal of a prosauropod

The largest prosauropods were quite slow, reaching top speeds of three miles per hour (the average walking pace in Central Park), and they appear to have been restricted to a fully quadrupedal gait. However, because prosauropod forelimbs were generally half the length of the hind-limbs, many prosauropods would’ve been able to rear up on their hind legs to feast on high-level plants, using their tail as a weight-balancing tripod. The subject of prosauropod gait remains a passionate issue among prosauropod enthusiasts. Some argue that the ‘ancestral’ prosauropod was smaller and bipedal unlike its off-branching descendents; however, the fossil record as we have it implies that prosauropods started off big, remained big, and spawned smaller prosauropod forms along their trajectory into the Jurassic. Nevertheless, the traditional argument goes that prosauropods started off as bipeds but found themselves buckled down under the weight of their success. Having tapped into the high-rise gymnosperm niche, prosauropods needed larger stomachs and longer intestines to facilitate their greed. As they evolved, those with more room for digestive organs did better than their counterparts, and over time prosauropod bellies swelled to the point of literally dragging them down into a quadrupedal stance. Others argue that bipedal locomotion wasn’t an inherited trait but one embraced farther down the line, as is shown by the current state of the fossil record. Recent research suggests that prosauropods began life on all fours, and many scientists theorize that prosauropods shifted from quadrupedal to bipedal locomotion (if possible with their weight and size) as they developed from hatchlings to adults. 

a rendition of a Massospondylus nesting site

Gregarious (or social) behavior in prosauropods is attested by several factors: track-ways, bone-beds, sexual dimorphism, and nesting sites. Plateosaurus bone-beds in Germany and Switzerland have offered up entire communities with specimens from all ages, though some detractors argue that the bone-beds don’t represent decimated herds but natural ‘burying grounds’ that gathered – by the force of a river, perhaps – the bodies of isolated individual for a mass entombing. Nevertheless, the bone-bed specimens reveal sexual dimorphism, in that there are morphological differences between male and female skeletons. Sexual dimorphism is a good indicator of gregariousness within a species. Furthermore, nesting sites for both Mussaurus of Argentina and Massospondylus of South Africa indicate gregarious behavior. The clutches in each nest are small, around ten eggs, and the hatchlings are tiny (adult prosauropods are roughly 500-1000 times larger than the hatchlings). With such low numbers of eggs, it’s likely that the hatchlings received some level of paternal care. More recent studies of these nesting sites have provided a treasure trove of clues implying that some prosauropods, at least, were prone to being decent parents. 

The subject of the ‘Prosauropod Cookbook’ has been controversial. While most scientists currently believe prosauropods were herbivorous, some hold out hope that they may have been opportunistic omnivores. Some of the earliest known prosauropods had characteristics found in carnivorous dinosaurs, so much so that some of the first prosauropod studies dubbed them meat-eaters. Evidence for an herbivorous lifestyle, however, is compelling. Their leaf- or spoon-shaped teeth, albeit weak and fairly useless, resemble those of other herbivorous dinosaurs. Muscular legs enabled them to stand on their hind feet to reach tall vegetation, such as conifers; this is something you wouldn’t expect in a carnivore. While carnivores generally have larger, stronger skulls for bringing down prey, prosauropods had markedly smaller and more lightweight heads. As we’ve seen, many prosauropods probably moved in vast grazing herds (or at the least in family units), and such gregariousness is a behavior common in herbivorous animals to protect against predators. Prosauropods had a broad field of vision, which would be useful if you’re trying to spot predators but useless for hunting (hunting requires a narrow field of vision that provides depth perception). Though the evidence tilts in favor of an herbivorous lifestyle, some paleontologists point to an interesting development in larger prosauropods: better olfactory abilities, as testified by wide nasal cavities. A keen sense of smell is key to a carnivorous lifestyle, so maybe the larger prosauropods were omnivorous; though lacking the mechanics to be efficient hunters, they may have enjoyed a rotting carcass every now and again. Some scientists have argued that these large nostrils housed glands that helped rid their bodies of excessive potassium ions (found in Triassic plant food), making the larger prosauropods more efficient herbivores; others have argued that the nasal cavities housed scent glands that were used to excrete a scent for marking territories or releasing mating pheromones. But the most likely explanation is that these larger prosauropods simply had advanced olfactory structures enhancing their sense of smell. 

a prosauropod showcasing the (hypothetical albeit likely) bipedal stance

Even if prosauropods were omnivorous, their remains indicate they spent most of the time eating plants. While their leaf- or spoon-shaped teeth weren’t ideal for chewing the fibrous plant material of the Triassic, the presence of a cartilaginous beak in many prosauropods may indicate a nutcracker-like eating process: using their beaks to scissor-off tough plant material, they swallowed the food mostly whole and allowed gastric mills to do the chewing for them. The crowns of many prosauropod teeth lack the wear-and-tear expected from chewing, and a gastric mill would explain why this is the case. Indeed, a gastric mill would almost make the teeth redundant: the stones embedded in their stomach walls (called gastroliths) ground up the tough plant material for them. This theory of prosauropod digestion is supported by gastroliths associated with prosauropod skeletons. Even with gastric mills, however, this system was inefficient, and it may be a factor in why prosauropods were superseded by the sauropods. 

~  Taxonomy  ~

The name ‘Prosauropoda’ means ‘before the sauropods,’ and though the name implies that they were the direct ancestors (or grandparents) of the sauropods, the name may be a misnomer. As mentioned earlier, paleontologists originally believed that prosauropods were the direct ancestors of the better-known sauropods. Because prosauropods got a jump-start of tens of millions of years on the sauropods, and because the two groups shared morphological characteristics (e.g. long necks and tails, small heads, muscular and pillar-like legs, and thumb claws), this seemed like a no-brainer. This predominant view lost steam in the twentieth century, and scientists came to see prosauropods as an independent lineage with only loose familial attachments to the sauropods. The cladogram below represents Prosauropoda as a monophyletic clade (in other words, the prosauropods belong to their own distinct lineage from that of the sauropods).

Prosauropods tend to be divided into four families (the Plateosaurs, the Massospondyls, the Melanorosaurs, and the Anchisaurs), though other families have risen and fallen (and will continue to do so!) in the constant flux that is the clade itself. While the cladogram above depicts a ‘cut-and-dry’ family tree, this is more for aesthetics than anything else: many scientists doubt some of the families are even related to one another, and many scientists believe that prosauropods sprang up not from one single ancestor, branching out as time progressed, but that the various families represent, in all their varying glories, distinct and individual lineages from a variety of common ancestors. If this is the case, then the morphological similarities between various prosauropod lineages aren’t due to genetically-inherited traits derived from a shared common ancestor but, rather, are episodes of ‘convergent evolution,’ wherein different lineages employ similar evolutionary adaptations (such as long necks or thumb claws) to deal with shared evolutionary pressures.

The most widely-known family of prosauropod dinosaurs are the heavy, large, and stout-limbed Plateosaurs; the Massospondyls were likewise large, and they appear to have shared a bed with their namesake, Massospondylus. The two other families – the Melanorosaurs and the Anchisaurs – deserve more mention, since their peculiarities have bolstered the belief that maybe, just maybe, prosauropods and sauropods have more in common than we’ve recently thought. The cladogram below represents the New Movement that puts sauropods within the prosauropod lineages:

Many scientists have speculated that the Melanorosaurs—the biggest and baddest of the prosauropods—had close family ties with sauropods. Scientists have studied and restudied the Melanorosaurs in the hope of finding the nail that will close the lid on prosauropod and sauropod relations; the problem is that while Melanorosaurs share a high number of morphological attributes with sauropods, their specializations make ancestry unlikely. It’s more likely that Melanorosaurs are ‘sisters’ to the sauropods. Adam Yates of the Museum of Central Australia sidelined the Melanorosaurs and focused on the Anchisaurs, advocating that Anchisaurs gave birth not only to the Melanorosaurs but to sauropods as well.

Anchisaurus: the current 'Hot Topic' in Prosauropod Studies

The Anchisaurs are fairly small prosauropods (less than ten feet long) with lightly-built bodies and slim limbs. Their blunt, cylindrical teeth had file-like serrations along the edges, reminiscent of the teeth of the herbivorous land iguanas of South America.  A quadrupedal gate is assumed because their forelimbs were only slightly smaller than their hind-limbs. Though they probably spent most of their grazing at ground-level, well-developed ankle joints indicate that they could rear up on their hind legs; their tails would act as counterbalances, enabling them to reach higher vegetation. This family of dinosaurs have been at the center of numerous debates. Some paleontologists argue that the bird-hipped dinosaurs evolved from Anchisaurs; to say ornithischian origins is murky would be an understatement, but it’s unlikely that Anchisaurs had a role in their genesis. It was once assumed that Anchisaurs were the more primitive prosauropods, eventually blossoming into the larger Plateosaurs and Melanorosaurs; but Anchisaurs don’t show up until late in the prosauropod fossil record, indicating that, at some point in prosauropod history, Anchisaurs downgraded from their big-boned ancestors. Anchisaurs stepped back into the limelight as recently as 2014; new research indicates that the Anchisaurs quite possibly gave birth to two groups of dinosaurs: the prosauropod Melanorosaurs and primitive sauropods. According to this view, Anchisaurs evolved ‘down’ from earlier prosauropods and then evolved back ‘up’ into the titanic sauropods. If this is true, prosauropods can no longer be considered an independent lineage, and sauropods become the Mesozoic survivors of the prosauropod extinction in the mid-late Jurassic.

Though the relationship between sauropods and prosauropods is interesting, another type of dinosaur has, at least in the view of some scientists, been slowly grafted into the prosauropod family tree: the segnosaurs (a.k.a. therizinosaurs). As far back as 1989, it was proposed that the segnosaurs – an aberrant group of Cretaceous theropods – are, in fact, descended from prosauropod stock. If this is true, then prosauropods didn’t die out in the Jurassic; rather, they continued, albeit covertly, through the Jurassic and into the Cretaceous, where they showed their heads in a revamped form. Though there are similarities between the morphologies of segnosaurs and prosauropods, more fossil evidence is needed before anyone can claim, with any degree of certainty, a familial relationship. For now segnosaurs fit pretty well into the Cretaceous theropod camp.

are these the late Cretaceous descendants of prosauropods? It's a cool thought!