|The typical look of Deltadromeus,|
but is this what it really looked like?
Now I know what you're going to say; "Why Deltadromeus? For a Theropod it's not that weird. Why not those fin-backed Spinosaurs, or those short-faced Abelosaurids?" Well, that's what you probably think now, but you see, in the past few years some information on a bizarre group of dinosaurs has changed our perception on what we thought we knew about Deltadromeus, and suggests that this animal might actually be far different than what anyone expected.
Deltadromeus: The Predator You Know
Deltadromeus agilis was named by Paul Sereno in 1996. It has been a mystery since its discovery, known only from two partial skeletons with elements of the legs, pelvis, tail, ribs, and bits of the arm bones. Sadly no elements of the skull or the hands of Deltadromeus have ever been found, leaving its business end completely unknown.
|Known Elements of Deltadromeus|
Deltadromeus was described by many as having unusually long legs. Deltadromeus has some of the longest legs of any large theropod, probably only beaten by advanced albertosaurids when comparing leg ratios. Having such long legs might have permitted Deltadromeus to hunt down faster game in the Sahara than Carcharadontosaurus. Another interesting feature is its shoulder girdle, which is massively constructed, suggesting powerful forelimbs.
For a long time Deltadromeus was considered a type of large coelurosaur due to some similarities and the fact that its proportions best matched small members of the group. However, more recent analyses have found it to instead represent a species of ceratosaur, although its exact placement remains unknown. A study in 2003 suggested that it was a very large member of the noasaurids, which were small-sized ceratosaurs like Noasaurus and Masiakasaurus. But more recent studies have instead come up with a different conclusion, and found Deltadromeus to be more closely linked to a strange group of Late Jurassic ceratosaurs, sometimes referred to as the Elaphrosauridae.
Elaphrosaurs: The Ceratosaurs You Didn't Know About
What's an elaphrosaur? Actually, I didn't know that answer myself until a few months ago, and being such odd dinosaurs you'd expect them to get much more attention for their quirkiness. Elaphrosaurs were a type of basal ceratosaur, but were very lightly built. They had long and powerful legs, a gracile build, robust shoulder girdles, and most members of the group are missing their skulls (sound familiar at all?). This family of dinosaurs is named after the mysterious Elaphrosaurus bambergi.
Elaphrosaurus lived in the Late Jurassic of Tanzania, and would've lived alongside other dinosaurs in the Jurassic, such as Giraffititan brancai, Kentrosaurus aethiopicus, and even possibly Allosaurus. A possible skeleton of Elaphrosaurus has also been found in the lower part of the Morrison Formation, but it awaits further study. The name Elaphrosaurus means "lightweight lizard," and if you were to see the skeleton, you'd know why.
|Mounted Skeleton of Elaphrosaurus|
Skull and limbs are speculative
Elaphrosaurus was about as gracile and lightweight as an Ostrich. Its tibia-to-femur ratio is also very large, suggesting it was very fast, perhaps even the fastest runner of the Jurassic. At abut 20 feet long, 4 feet tall at the hips, and a weight of about 450 pounds, this animal was clearly built for speed.
Elaphrosaurus was first discovered in 1920, but it wasn't until much later that we found our next member of this weird group. Known as Spinostropheus, it was actually first thought to represent a new species of Elaphrosaurus during the 1960s, but was given the name Spinostropheus in 2004. Living in Niger during the late Jurassic, it was going the same way as all the other Saharan theropods and is very fragmentary. Another possible candidate for an elaphrosaur includes the Early Jurassic Berberosaurus liassicus, however some scientists have since found this fragmentary animal to be more closely related to the dilophosaurs. Only time will tell if it is actually related to this odd group.
The Mystery is Solved by Limusaurus
In 2009, another species of elaphrosaur was revealed. Dubbed Limusaurus inextricabilis, it is the most complete elaphrosaur yet found. Limusaurus had the typical elaphrosaur-like body plan, but it was very small, only about 6 feet, making it among the smallest of theropods outside the mainiraptors, and possibly the smallest ceratosaur. Not just that, but Limusaurus was the first elaphrosaur found with a skull and arms intact! We now finally know what the skulls of these strange, lightweight predators looked like! Limusaurus, as it turns out, looked like this:
|Wait.... that's a Ceratosaur?|
Well, yes as this turns out to be the case, Limusaurus didn't look like any of its larger carnivorous kin, and more closely resembled the ornithomimids and small ornithopods in the later Cretaceous than the horned Ceratosaurus or the gigantic Majungasaurus. Yet, an analysis showed that it was indeed a type of ceratosaur, a bizarre offshoot of the group that evolved towards a different lifestyle -- one biased towards plants.
Their are multiple pieces of evidence suggesting that Limusaurus gave up the meat-filled diet of its relatives. It has a short, deep skull, a slender lower jaw with a downward-curved tip, small, vestigial forelimbs with only two functional digits (like tyrannosaurs), it's entirely toothless, and has a beak at the front of its jaws. This beak actually shares many traits in common with small ornithischian dinosaurs like Hypsilophodon. But the smoking gun for finding out that Limusaurus was a herbivore was the discovery of a patch of gastroliths in both of the known specimens. Gastroliths are used by herbivorous birds to help grind up plant matter within the bird's gut, and are also found in many other types of herbivorous dinosaurs. This all provides strong evidence that Limusaurus was an herbivore/omnivore that ate and digested plant matter.
|With the discovery of Limusaurus, our idea of what|
Elaphrosaurus looked like changed dramatically.
The discovery of Limusaurus has opened a window into the entire elaphrosaur family of dinosaurs. If Limusaurus was biased towards a herbivorous lifestyle with the discovery of its skull, what if other elaphrosaurs were as well? All other members of the family have their skulls unknown, so it is certainly possible that they were in fact an entire family of herbivorous ceratosaurs. In fact, it's interesting to note that Elaphrosaurus was first thought to represent a Jurassic ornithomimid, fueling the idea of herbivory in that species as well.
But wait! If elaphrosaurs were herbivorous theropods, and recent studies have found that Deltadromeus might have an affinity with this group, then.....
What No One Expected
Yes, there is a possibility that Deltadromeus actually represents a type of herbivorous ceratosaur within the elaphrosauridae. You don't think so? Well I could hardly believe it either when I heard the idea myself, but it makes sense to me when I looked through the evidence for it.
Deltadromeus is only known from 2 partial skeletons, and only its legs, pelvis, parts of the tail, and a few bones from the forelimbs are known. We only knew about the same from Elaphrosaurus when it was first discovered, meaning that the really odd parts like the head and neck are still missing for Deltadromeus, and we still don't know what Deltadromeus' teeth looked like. (Some rock shops claim to sell Deltadromeus teeth, but the truth is that there is no way of possibly knowing that it came from this animal without any skull material. I fell for that trick years ago, but I don't mind. Besides, it just toys at your imagination to think of what the teeth are actually from.)
|Reconstruction of the head of Limusaurus|
Note just how thin and fragile the bones are.
In fact, the lack of the skull of Deltadromeus might actually support the idea that it was an elaphrosaur! Typically, the smaller the dinosaur's head, the harder it is for it to be preserved. Take sauropods for example, they were giant herbivores with long tails, long necks, and heavy skeletons, but the head in comparison is tiny, occupying less than 1% of the animal's total mass in things like diplodocids. Not just that, but the muscles and ligaments that supported a sauropod's neck were also very weak, and the skull bones were fragile, meaning that not long after death the skull might just fall away from the skeleton and crumble. When looking at Limusaurus, we see a similar thing. The skulls of elaphrosaurs were just as (if not more) delicate than sauropod heads, and the ligaments supporting the necks of such creatures were likely very weak, meaning that the heads would normally fall off soon after death and disintegrate.
No wounder we hadn't found the skulls of these creatures for so long, seeing how delicate they likely were. The only reason Limusaurus had its skull perfectly preserved was due to the fact that it died trying to free itself from a predator trap. (In fact it was found within the same predator trap that caught the first Guanlong specimens. Perhaps the Guanlong were after the Limusaurus when they got trapped in the mud, and they ended up getting trapped themselves.) All the other elaphrosaurs are thought to have died in typical environments, like dried up riverbeds, meaning that when the water started to flow, it took the skulls with it.
Other evidence exists besides this. As I said before, Deltadromeus shares many similarities with the elaphrosaurs, such as its robust shoulder girdle, its long legs with very elaphrosaur-like proportions to them, and the gracile build. The upper arm bone of Deltadromeus is known, but the rest is missing. However, looking at the proportions, the arms seem to be rather short, not quite as short as those of Limusaurus, but this could be explained by the size difference between the two.
Speaking of size difference, wouldn't this be a record breaker if Deltadromeus was actually a giant herbivorous ceratosaur? The smaller holotype specimen was eastimated to be about 26 feet long, but the larger specimen was supposedly closer to 40 feet! Not just that, but with the longer neck and legs of the elaphrosaurs, it could be over 15 feet tall at the top of the head! Even better, what about Baharisaurus? As I explained above, Baharisaurus has been shown to be a close relative of Deltadromeus, if not its synonym. Baharisaurus was even more fragmentary than Deltadromeus, and then we lost the only known specimen in WW2. Given its past, fragmentary nature, and being such a close relative to Deltadromeus, it could've probably been a giant elaphrosaur as well!
So, given all this new information, this is what I think Deltadromeus might have resembled:
So, given all this new information, this is what I think Deltadromeus might have resembled:
Seems like there is a good chance that Deltadromeus didn't resemble many typical large theropods, but was instead something entirely different. From the smaller sleek carnivorous ceratosaur, to a giant, herbivorous theropod that dwarfed most around it. At this size and mass, I'd expect it might have been competing with sauropods for food! Some scientists have been puzzled as to why there are so many Theropod species found in the Sahara, yet we actually know of very few herbivore species from this time. Well, if we are to accept this theory, that's one less carnivore, and one more herbivore for the history books!
Given some more recent studies, it seems that Deltadromeus could very well represent a type of herbivorous elaphrosaur, and the evidence for such a notion continues to rise. The change from what was thought to be a typical large carnivorous dinosaur to a herbivorous theropod with a small head, tiny arms, and a beak is among the most extreme transformations of any dinosaur.
Given the size of the creature, it likely fed on high foliage, using its long neck to feed at the tops of trees. It likely used its long, gracile legs not to catch prey, but to escape predators such as Carcharadontosaurus and Spinosaurus. A high-browsing theropod might come as a surprise, but given how we've discovered Therizinosaurus cheloniformis, Gigantoraptor erlianensis, and the mysterious Deinocheirus mirificus, it really shouldn't. However, this would then be the first high-browsing theropod known from outside of Mongolia, and shows us that large herbivorous theropods might be more common that we originally believed.
I will admit that older studies on both Deltadromeus and Bahariasaurus found them to be a type of very large, primitive noasaurid, and many scientists still agree with this idea. But I personally favor the idea that it is a herbivorous elaphrosaur, due to the great number of similarities I pointed out.
Now I'd like to hear your thoughts about Deltadromeus. Do you think it's an herbivorous elaphrosaur, or maybe you still think it's a predatory noasaurid or some other type of ceratosaur? I look forward to hearing your opinions.