The Morrison Titan - Part 2: Not a Diplodocus clone

In my previous post, I mentioned that Amphicoelias' known bone proportions are different from those of Diplodocus. But sadly, most reconstructions don't seem to recognize it.

Um, Carpenter, that's a big Diplodocus clone.

They don't seem to portray the actual Amphicoelias at all. They're just Diplodocus copies. They ignore the actual proportions of the animal, flying in the face of what the known fossils suggest. Let's see what's known from the Amphicoelias holotype again:

How do these compare to Diplodocus? Turns out, they're not really clones of one another.

Well, we'll start with the vertebrae.

Amphicoelias seems to have a proportionally larger neural arch compared to it's centrum than Diplodocus does. Amphicoelias also has a proportionally narrower centrum than Diplodocus. The proportional lengths of their centra are similar, though. Their neural arches, by themselves, are also similar in overall proportions.

Next, the pubis.

It's massive in proportion. Amphicoelias' pubis just blows Diplodocus' one out of the water. The 10th dorsal of Diplodocus carnegii CM 84 measures about ~26.7 centimeters long, while that of Amphicoelias altus measures about ~24 centimeters long, a little shorter than the D. carnegii centrum, however, this is due to the differing shape of the centra as preserved. Overall, the centra are essentially the same size aside from width. However, the pubis of A. altus measures about ~1.17 meters long, while that of D. carnegii is about ~1 meter long. Despite having similarly-sized posterior dorsal centra, A. altus has a significantly larger pubis. This is indication of a massive hip, and subsequently, a deep torso.

Then we get to the femora.

The femura of A. altus are notably long, with the complete right femur measuring at ~1.77 meters in length. By comparison, D. carnegii CM 84 has a femur about ~1.54 meters long. Keep in mind that D. carnegii's 10th dorsal centra is similar in size to that of A. altus. This is a clear indication of longer limbs than Diplodocus proportionally, and in the case of D. carnegii, also in absolute terms.

Another way to look at it would be to scale a Diplodocus reconstruction to the provided measurements of the individual bones, treating them as separate individuals, then comparing the results to see how much they differ.

The skeleton of Diplodocus carnegii CM 84 is estimated to be approximately ~26 meters long. Since the anterior dorsal centra of CM 84 and A. altus are about the same size, scaling CM 84 to the A. altus dorsal would give an animal practically identical in size. However, scaling CM 84 to the A. altus pubis then gives a size about 17% larger in linear dimensions, or about ~30.4 meters or so. Scaling CM 84 to the A. altus femur gives an animal about ~29.9 meters long, or about ~15% larger in linear dimensions. This is another pretty good indicator that the two diplodocoids differed quite a bit in their bodily proportions.

All in all, this animal isn't really proportioned that similar to Diplodocus. It's clearly something else.

Overall, the resulting animal would be a deep-chested, long-limbed one. It would be proportionally narrower since their proportional width across the diapophyses (the "wings" of the neural arch) are similar, while the hips and the body suggested by it are deeper and the legs longer. Tall, but not rotund. At least, for Amphicoelias altus.

What about the giant Amphicoelias fragillimus? Would it have had similar proportions? Where would it differ? As of now, we lack the data needed to give a truly concrete answer. It would depend on the affinities of A. fragillimus in the sauropod family tree, and how close it would be to A. altus. That would be the subject of the next post.

The Morrison Titan - Part 1

Argentinosaurus is commonly considered to be the largest dinosaur.

If not it, some giant Cretaceous gigapod such as Puertasaurus is given the crown. Nowadays, it seems that whether the question of what is the largest known dinosaur is brought up, most people would answer with something within Titanosauria and from the Cretaceous period. In 2014, the bones of an unnamed gigantic titanosaur, with a femur requiring three forklift pallets to hold, has been unearthed and a skeletal mount of it, restored with speculative portions, has been placed on display.

 But do the Cretaceous titanosaurs really hold the crown?

Likely not so.

A few brachiosaurs such as Breviparopus and Ultrasauripus possibly rival or exceed the sizes of supergiant titanosaurs like Argentinosaurus or Puertasaurus.

Even the good old Brachiosaurus altithorax could probably reach Argentinosaurus-esque sizes, as a large specimen (~2.42-meter humerus) in the BYU collections (Paul, 1988) suggests. This B. altithorax specimen probably massed over 70 tonnes, vindicating Walking With Dinosaurs' Brachiosaurus mass figure.

The giant Oklahoma Apatosaurus ajax specimens, which probably massed between ~50-60 tonnes or so, aren't even fully grown, they lack cervical rib fusion, which corresponds to roughly ~80% of fully-grown adult dimensions in Diplodocus and Giraffatitan. It's probable that Apatosaurus ajax adults reached the 100-tonne mark, surpassing Argentinosaurus and possibly rivaling Puertasaurus in size.

But how about something larger than even them? It turns out that such creatures probably existed. Three are known, but one is known only from a footprint (~1.5-meter wide footprint in Plagne, France). That'll be a story for another time. Another is known from an incomplete sacrum ("Brachiosaurus" nougaredi from North Africa), which will also be a story for another time.

One day, in 1878, during the Bone Wars, Edward Drinker Cope discovered the bones of a sauropod dinosaur in the Morrison Formation. He named the animal Amphicoelias, meaning "double hollow", "amphi" meaning "both sides" and "coelias" from "koilos", meaning "hollow". The binomial name was Amphicoelias altus, meaning "high double hollow", "altus" meaning "high" or "elevated".

It was far from complete. Here's what Cope found of it, along with additional elements assigned later on such as a shoulder girdle and an ulna:

More recently, analysis of the scapula material revealed a camarasaurian nature, while the ulna, being referred to A. altus largely based on disagreement with Camarasaurus and the other sauropods in Cope's collection, is probably a candidate for exclusion.

From Tschopp and Mateus, 2015:

"The holotype of Amphicoelias altus originally included a tooth, two dorsal vertebrae, a pubis, and a femur (Cope, 1877a). A scapula, coracoid, and an ulna were later provisionally referred to the specimen (Osborn & Mook, 1921). However, the strongly expanded distal end of the scapula, and the relatively deep notch anterior to the glenoid on the coracoid actually resemble more Camarasaurus than any diplodocid (McIntosh, 1990b; E Tschopp, pers. obs., 2011). The same accounts for the single tooth stored at AMNH (Osborn & Mook, 1921). The tooth has already been excluded from scores of A. altus in recent phylogenetic analyses (Whitlock, 2011a; Mannion et al., 2012), which is followed here. Mannion et al. (2012) furthermore excluded the referred forelimb elements. Given that personal observations confirmed the rather camarasaurid than diplodocid morphology of the scapula and coracoid, but not particularly the ulna, two different preliminary phylogenetic analyses were performed with a reduced (excluding the tooth, the scapula and the coracoid, but including the ulna) and the extended holotype Amphicoelias altus OTU (including all referred elements other than the tooth). Because both analyses yielded the same position for the specimens, the reduced holotype was preferred in the final analysis. The risk of adding dubious information from potentially wrongly referred material was thus circumvented. More detailed analysis is needed in order to refine these assignments."

 Removing the referred shoulder girdle and ulna, this is what's left from the holotype, the original specimen that Cope found of it:

Not that much. But it's enough to give us a bit of an idea about the animal's size and proportions.

Now, this doesn't seem like an animal anywhere near Argentinosaurus size, let alone larger. But here's the thing - it's not alone. It had a cousin that was much, much larger.

Later in 1878, Cope, back in the Morrison, found a huge bone. A fragmentary neural arch from a posterior dorsal, around 1.5 meters tall. A single bone fragment, from a bone that isn't really very big by itself compared to the rest of the body, as tall as a person. Whatever animal bore this bone must have been truly immense in life.

This gigantic bone was assigned by Cope to a new species, Amphicoelias fragillimus, meaning "fragile double hollow".

Had this fossil survived to the present, Giraffatitan would have never taken fame as the "largest known dinosaur" for decades, and Tyrannosaurus would have never been called "the king of the dinosaurs". Argentinosaurus would not have had the fame for it's size that it has now, and the Chubut monster titanosaur wouldn't have looked so impressive. But this was not to be.

The bone was in very bad condition and was crumbling. It's condition deteriorated to the point that it was soon discarded, possibly by Cope himself. The fragility that gave it it's name, was the cause of it's destruction.

The legendary giant of the Jurassic, was lost, and then forgotten by almost all but a few. Ok, this animal has gained a little popularity recently, but is still very obscure. The big reason for this recent gain in popularity is Carpenter's 2006 paper, which attempts to estimate size based on the smaller A. altus dorsal and Diplodocus.

He arrives at a total last dorsal height of ~2.7 meters, a total length of ~58 meters and a mass of ~122.4 tonnes, approximately ~30 or so tonnes more massive than the average blue whale, or probably 50-70 tonnes more than Argentinosaurus.

However, his reconstruction depicts the animal as an upscaled clone of Diplodocus. Amphicoelias' actual known proportions are significantly different from those of Diplodocus. I will compare their proportions bone-by-bone in the next post.