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Dilophosaurus wetherilli, from the Hettangian or Sinemurian Kayenta Formation of Arizona, to many seems to be a sort of quintessential early theropod.
And there are a number of good reasons for that:
Firstly, it’s been known for a long time. Welles dug up several nice, articulated skeletons as early as the 1940s. Secondly, it’s from the US…that often (though not always) helps things along.  Thirdly, it’s morphology was so clearly different from other theropods known at the time, and is still so unusual compared to other well-known theropods, that it’s easily recognizable to anyone, probably leading to its inclusion, albeit in a very inaccurate form, in the original Jurassic Park movie.

And finally, it is true that Dilophosaurid-grade theropods, a number of stem-neotheropod taxa with very similar morphology (while probably not all forming a natural clade), were widespread components of upper Triassic and Lower Jurassic faunas worldwide.
Gojrasaurus quayi from the Norian of New Mexico, Zupaysaurus rougieri from the Norian of Argentina and Liliensternus liliensterni from the Norian of Germany are among the earliest theropods (note Herrerasaurids’ status as theropods is very questionable) that we could consider "large-bodied", and which filled the apex-predatory role in their ecosystems.
This type of theropod persists into the Lower Jurassic: The Elliot Fm. in South Africa (Hettangian/Sinemurian) has yielded Dracovenator regenti. The Hettangian Lufeng Fm. has Sinosaurus triassicus (formerly "Dilophosaurus sinensis"). And perhaps most famously, the Hanson Formation of Antarctica has produced Cryolophosaurus ellioti (although this taxon might actually be a basal Tetanuran, making it much more advanced compared to the other taxa on this list). And of course Dilophosaurus wetherilli itself.
All of these seem to share weird head crests, more or less developed premaxillary notches and relatively slender postcranial body proportions.

Zupaysaurus rougieri by theropod1



As the largest predators in their various respective ecosystems, naturally we must wonder about the size of these animals.
Commonly given mass estimates tend to be extremely low for the proposed dimensional measurements. This is generally explained by the very gracile, coelophysoid-grade morphology of these taxa. Paul (³ and 2009) estimates the holotype of Dilophosaurus at 283 kg and the largest specimen at 400 kg. Smith et al. 2007 estimate Cryolophosaurus at 465 kg based on femur circumference (claiming it to be the largest Lower Jurassic theropod), even though the specimen has a femur length similar to an adult Allosaurus’ massing 1.5 t.
So were these animals really that small?

Most specimens of Dilophosaurus are, in fact, juveniles. That includes the 6 m long holotype skeleton (UCMP 37302, 53 cm skull, 56 cm femur) described in detail by Welles (1984), as well as one of the original paratypes (Welles 1970), another partial skeleton referred by Tykoski (2005) and part of the fragmentary material described by Gay (2001).

The only substantially complete adult individual is one of the paratypes, UCMP 77270 (Welles 1970). This individual has dorsal and sacral vertebrae on average 18% longer than the holotype, although its precaudals in general are 14% longer, owing to the negatively allometric neck length. It also has an appreciably larger skull, at 62 cm, and a slightly longer femur, at 59 cm (all measurements for this specimen from the theropod database², holotype from Welles 1984).
Dilophosaurus tends to get portrayed as downright ridiculously skinny, likely mostly inspired by Greg Paul’s shrinkwrapped skeletal.
To test this, I made a volumetric 3D model based on ScottHartman’s composite Dilophosaurus skeletal (which is considerably more robust-looking than Paul’s), scaled to 6 m total axial length (that measurement by Welles is probably reliable, as the skeleton was found in articulation and is very complete). For the body width, I adapted the dorsal view skeletal of Syntarsus rhodesiensis from Paul (2016), which is probably quite conservative.
For the holotype, I’m getting a volume of 366 l, which translates to a mass of 330 kg (assuming a density of 0.9). So very light for its length, but a bit heavier than the 283 kg Paul and Mortimer list it at.

Compil by theropod1



We can estimate total length of the adult UCMP 77270, using the length ratios of all overlapping vertebrae, as ~6.9 m. I’d suggest scaling body mass from only the dorsal and sacral vertebrae, because the largest part of the mass is located in the torso, which gives a mass estimate of ~544 kg.

A couple of other speculative size estimates for more or less related species:

>Liliensternus liliensterni (larger individual, subadult, von Huene 1934):
Femur length: 42 cm
Total length (based on all precaudal vertebrae, scaling from UCMP 37302): 5.1 m
Body mass (based on dorsal  and sacral vertebrae): 178 kg

>Gojirasaurus quayi (UCM 47221, subadult, Carpenter 1997):
Tibia length: 47 cm
Pubis length: 50 cm
Total length (based on 3 mid-posterior dorsal vertebrae, scaling from UCMP 37302): 5.0 m
Total length (based on tibia): 5.1 m
Body mass (based on dorsal vertebrae and tibia): 187-199 kg
(long pubis might imply deeper body shape)

>Zupaysaurus rougieri (PULR-076, ?adult, Arcucci & Coria 2003):
Skull length e. 42 cm (measured in Fig. 2E)
Total length (based on skull and axis length, scaling from UCMP 37302): 5.3 m
Body mass (same method): ?221 kg
(very long axis might just be due to proportionately long neck)

>Dracovenator regenti (BP/1/5243, ?adult, Yates 2005):
Skull length e. 56 cm
Total length (based on skull length): 6.3 m
Body mass (based on skull length): 389 kg

>Cryolophosaurus ellioti (FMNH PR1821, late subadult/young adult, Smith et al. 2007):
Femur length: 77 cm
Total length (based on all precaudal vertebrae, scaling from UCMP 37302): 7.9 m
Body mass (based on the dorsal  and sacral vertebrae): 794 kg
Body mass (based on reverse-calculated femur-circumference of 233mm from Smith et al. 2007, using Anderson et al. 2009 and cQE-phylocor from Campione et al. 2014): 687 kg


Discussion and Conclusions (for now):
Dilophosaurus, and probably most other coelophysoids, were probably a little heavier than commonly cited, and quite a lot when taking into account that many are immature. Adult Dilophosaurus seem to have approached or exceeded 500 kg, not the 400kg from Paul or 330 from Mortimer. However this estimate is very sensitive to placement of the vertebrae in question, and there is some serious confusion because Welles confusingly labels dorsals 1 through 5 as pectorals, while Mortimer confusingly labels what I think is probably dorsal/pectoral 1 as cervical 10. So take the size of the larger individual with a grain of salt. Welles (1970) claims the specimen was one third larger; if he was referring to linear dimensions, then this estimate is quite conservative, although if he meant volumetrics it is too high.

The commonly cited length estimate for Liliensternus are pretty much spot-on when scaling from vertebral lengths of Dilophosaurus. The mass estimates however, such as Paul’s (2009 and ³ 130kg) appear too low based on the relative length of the dorsal and sacral vertebrae. Furthermore, the known specimens are both subadults (Tykoski 2005), which implies larger sizes for adult individuals.

Gojirasaurus is possibly slightly larger than Liliensternus, based on the vertebral and tibial lengths, although the lack of available cervicals results in a slightly lower total length estimate based on the dorsals alone. Its pubis is even slightly longer than that of the Dilophosaurus holotype, which could imply a slightly deeper-bodied morphology. Carpenter’s estimate of 5.5 m could be roughly accurate.

Zupaysaurus has a relatively small skull based on the restoration in the description paper, but a very long axis (11 cm). The mass estimate presented here might be too high due to this. There are more vertebrae preserved, which sadly don’t have any reported measurements, so I currently can’t make a better estimate.

I agree with Smith et al. 2007’s claim that Cryolophosaurus is the largest Lower Jurassic theropod, but I think their mass figure and most other size figures for this taxon are underestimates. The holotype may well have approached 8m and 800 kg (which is also much more consistent with the reported lengths of its vertebrae and the femur). Finally, considering its poorly fused sacrum (Smith et al.), it may well be a young individual that was still well below average adult size.

References:
Anderson, J. F., A. Hall‐Martin, and D. A. Russell. 1985: Long‐bone circumference and weight in mammals, birds and dinosaurs. Journal of Zoology 207:53–61.
Arcucci, A. B., and R. A. Coria. 2003: A new Triassic carnivorous dinosaur from Argentina. Ameghiniana 40:217–228.
Campione, N. E., D. C. Evans, C. M. Brown, and M. T. Carrano. 2014: Body mass estimation in non-avian bipeds using a theoretical conversion to quadruped stylopodial proportions. Methods in Ecology and Evolution 5:913–923.
Carpenter, K. 1997: A Giant Coelophysoid (Ceratosauria) Theropod from the Upper Triassic of New Mexico.(With 8 figures and 3 tables in the text). Neues Jahrbuch fur Geologie und Palaontologie-Abhandlungen 205:189–208.
Gay, R. 2001: New specimens of Dilophosaurus wetherilli (Dinosauria: Theropoda) from the early Jurassic Kayenta Formation of northern Arizona. Mesa Southwest Museum Bulletin 8:19–23.
von Huene, F. F. 1934: Ein neuer Coelurosaurier in der thüringischen Trias. Palaeontologische Zeitschrift 16:145–170.
Paul, G. S. 2009, 2016: The Princeton field guide to dinosaurs. Princeton University Press, Princeton.
Smith, N. D., P. J. Makovicky, W. R. Hammer, and P. J. Currie. 2007: Osteology of Cryolophosaurus ellioti (Dinosauria: Theropoda) from the Early Jurassic of Antarctica and implications for early theropod evolution. Zoological Journal of the Linnean Society 151:377–421.
Tykoski, R. S. 2005: Anatomy, ontogeny, and phylogeny of coelophysoid theropods.PhD Thesis, University of Texas at Austin.
Welles, S. P. 1970: Dilophosaurus (Reptilia, Saurischia), a new name for a dinosaur. Journal of Paleontology 44:989.
Welles, S. P. 1984: Dilophosaurus wetherilli (Dinosauria, Theropoda). Osteology and comparisons. Palaeontographica Abteilung A:85–180.
Yates, A. M. 2005: A new theropod dinosaur from the Early Jurassic of South Africa and its implications for the early evolution of theropods.

¹Hartman, Scott. 2015: New-look Dilophosaurus. DeviantArt. Downloaded from www.deviantart.com/scotthartma… on 9 September 2018.
²Mortimer. : Coelophysoidea. Downloaded from theropoddatabase.com/Coelophys… on 11 September 2018.
³Paul, Gregory S. 2010: Data - Gregory S. Paul’s Dinosaur Mass Table. Downloaded from gspauldino.com/data.html on 11 September 2018.

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:iconthaanonymousperson:
ThaAnonymousPerson Featured By Owner Oct 1, 2017
Are you from Carnivora
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:icontheropod1:
theropod1 Featured By Owner Nov 24, 2017  Student Traditional Artist
yes
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:icondinopithecus:
Dinopithecus Featured By Owner Edited Mar 19, 2017
This may be quite a really, really stupid question, but I've been wondering about it.

Let's say a cat or bear grappled onto the neck of an equal-sized predatory theropod (of what clade (carnosaurs, tyrannosaurids, megalosaurids, etc.) you decide). Would the latter's cervical flexibility and cranial flexibility from the atlas be sufficient for it to turn its head/neck to the side and bite down on the carnivoran's forelimb (theropod necks were particularly flexible, right?)? Would some with powerful necks have the power and flexibility to free themselves through vigorous shaking?

I'm so sorry. Spending >4 years debating on animal vs. animal forums can make you think a little hard about this stuff.
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:icontheropod1:
theropod1 Featured By Owner Mar 23, 2017  Student Traditional Artist
First of all, depends on the theropod in question, they aren't that similar across the ranks in that regard. Some had more flexible necks than others. And of course it depends on what angle the opponent would be relative to the theropod.

So rather than giving you an overgeneralized answer to that question, here’s a very helpful visualisation from Snively et al. 2013:
palaeo-electronica.org/content…

Shown (3) is the maximum lateral range of motion of the head and neck of Allosaurus.
A couple of things to note: Of course Allosaurus is considered one of the large theropods with a rather high degree of cervical flexibility. I don’t know much about Megalosaurs in this regard. A derived tyrannosauroid would almost certainly be less flexible, but of course it would also have a shorter neck to make up for that, which would make grappling it even more difficult.
As you see the individual joints, including the craniocervical articulation, aren’t all that flexible, but if you add up the angles over several you end up able to flex that neck quite a bit rather quickly. So it’s very important to take into consideration where that hypothetical grappling animal has its hold on the neck, and how much of its length is still free to move. That’s of course also relevant because grappling more anteriorly would give the grappler much better leverage and make escaling its grasp much more difficult.
So perhaps theropod flexibility was sufficient for what you describe, perhaps not, depending on where the attacker is standing compared to the theropod and what theropod we are talking about.

As for shaking vigorously, there are certainly theropods I could see doing that, though of course the neck is a vulnerable region and doing so would be risky. But again, depending on the attacker I suppose it could work in some cases.
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:icondinopithecus:
Dinopithecus Featured By Owner Mar 24, 2017
"As you see the individual joints, including the craniocervical articulation, aren’t all that flexible"

Yeah, I find that puzzling. I read that the large articular surfaces of theropod zygapophyses promoted neck flexibility; I imagined it would be by making the individual joints flexible. The opisthocoelus morphology of carnosaur cervicals should also have helped with this. Lastly, Snively's reconstruction doesn't look like the cranium is flexed all that much from the atlas, but theropods had more or less spherical occipital condyles, allowing for great mobility at the joint.

Did these things just really not help that much to make the individual joints flexible in absolute terms?
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:icontheropod1:
theropod1 Featured By Owner Mar 25, 2017  Student Traditional Artist
I didn’t mean to say Allosaurus’ neck wasn’t flexible. For a cervical skeleton, it’s in all likelyhood very flexible. But individually, intervertebral joints or the craniocervical joint still don’t have huge ranges of movements.
They never do, probably those in Allosaurus already have a comparatively large range of movement (as you correctly point out, they are in fact adapted for that), but individual vertebrae simply don’t move all that much. It’s the summation of many small rotations that ends up making the neck flexible.
What I meant to say by that: it really matters how much neck there is to make that turn you were referring to, because it can’t just bend 90° in a single spot.

Simply put, the measure of what’s flexible for an intervertebral joint isn’t the same as that for a jaw joint, or knee joint etc., because they don’t have to be and vertebrae are loaded in a very different way from jaw or limb bones. They need those support structures, like zygapophyses, cervical ribs and neural spines to provide adequate support to the spine without excessive muscular effort despite its mostly horizontal posture. Allosaurus does have a comparatively flexible neck. Part of the reason why is that its skull likely wasn’t very heavy compared to some other theropod skulls, meaning it needed less rigid support. But still individual vertebral joints will generally not be as flexible as some other joints in the body, and this is why.
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:icondinopithecus:
Dinopithecus Featured By Owner Edited Sep 24, 2016
Hey theropod.

You might be aware of what I asked you on WoA* (i.e. seeing if my argument on theropod agility on Carnivora was sound). If I was asking for too much, then I apologize.

I just have one question for you now regarding the topic. Since theropod legs were pretty much at the center of mass, theropods would have produced little torque compared to quadrupedal animals. Did they have a way of solving this problem?

Thanks. :) (Smile) 

*Or if you're not, well, you could still check out hippo vs. Carnotaurus on Carnivora to at least see what I'm talking about.
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:icontheropod1:
theropod1 Featured By Owner Sep 27, 2016  Student Traditional Artist
Just because it would be a shame to leave this unanswered, even though this is pretty much just what I wrote on WoA:
I think their way of solving that problem was simply by producing more force.
Quadrupeds have better leverage because they have two leg pairs, but most theropods have humungous musculature powering their hindlegs, i.e. more force to make up for that. Whether they could produce just as much torque I don’t know, but even if not, how fast a quadruped can turn may be limited for other reasons than the sheer ratio between torque and RI.
The rotational inertia would likely be somewhat greater due to their more elongated body shape than the typical quadruped, but that’s certainly highly dependant on the theropod and quadruped in question (theropods do have more or less effective means to lighten the ends of their bodies).
This is partly offset by what we already discussed; potentially quicker capacity of excerting torque because of shorter moment arm, and flexing of the body (also more effective in some theropods than in others) while turning in order to reduce RI. I’ve got no quantification of this, but I doubt this is enough to offset their long body shapes and make them able to turn on the spot just as well as a same-sized quadruped. However as I wrote
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:icondinopithecus:
Dinopithecus Featured By Owner Edited Dec 6, 2016
About that topic (or really to deviate from it):

Remember how you said elephants are kind of a (rough, at least) sauropod analogue when it comes to dealing with predators (they're neither behaviorally nor anatomically adapted for dealing with predators as massive as they are)? Well, another person I asked on Carnivora agrees that their body plan takes advantage of being so large.

Do you know of any ways this might hold true? That is, what about their anatomy makes them so reliant on size? Having tusks makes it kind of hard to believe (although, I recently read a publication saying that elephant tusks have a rather low tensile strength and are susceptible to fracture, for whatever it's worth).
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