This Blog Entry is Dedicated to the Natural History Caucus, Many of Whose Members work in Collections Serving Paleontology: You Help Promote the Enduring Fascination of Kids of All Ages with Dinosaurs, and Deserve the Nation's Thanks for the Careers in Science this Inspires
We all pretty much believe in things like gravity and inertia, even if we cannot do the math behind them. We also tend to believe professional engineers when they make more sophisticated claims about a given metal beam being able to support a skyscraper, or a truck of a certain rating being able to haul a cargo of so many tons at a particular top speed, because we know that these engineers have a firm grounding in the laws of physics that govern those situations. But structural and automotive engineers are not the only ones who can do these calculations and make pretty near exact analyses of what is possible and what is not. These days, vertebrate paleontologists are doing some pretty good biomechanical engineering analysis themselves on the fossil record of predatory dinosaurs. A look at one of the year 2007’s top dinosaur stories gives us some good examples of how things that seem totally conjectural to laypersons such as ourselves, are actually based on calculations of what kind of body structure it would take for dinosaurs to overcome gravity and inertia routinely and thrive daily, millions of years ago.
So Just How Big Were the Big Meat-Eating Dinosaurs & How Do We Know?
One of the biggest stories this year was the announcement by a team led by Xu et al. (2007) from the Chinese Academy of Sciences that they had found a gigantic bird-like dinosaur Gigantoraptor erlianensis. Based on about 16 well preserved bones, including a mandible (basically a jaw), they showed that this animal was about 24 ft. long, about 12 feet high at the hip and weighed about 3,000 pounds. This creature was taller than a giraffe , was longer than the longest living crocodile ever measured, and weighed about as much as a black rhino. Yet, at least in its erect stance, it resembled Sesame Street’s Big Bird, assuming that Big Bird also had very heavily clawed feet and wings that also ended in very serious claws. It was largely meat-and –mollusc eating, but apparently could cross-over into plants. The posture in which it apparently walked (or ran) featured a head held somewhat aloft and forward of the body, shoulders leaning forward (like a charging football player running almost parallel to the ground) with its tail held out stiffly in back (no analogy with the football player there. )
How did the paleontologists infer all this? First, they were able to determine it was bird-like owing to both its wing-like bone structure, certain feather-like patterning on those bones and histological (microscopic bone tissue level ) data that matched it to other bird-like dinosaurs (the latter detail showing the porosity of the bones also explains why even at 3,000 pounds, Gigantoraptor was light, and probably fast on the ground, for its size) . Second, they had samples of more complete skeletal remains from other smaller bird-like ancient animals, so they had a head start by being able to scale up using structural analogies (or “homologies”), when filling in some of the remaining major bio-architectural blanks. Third, the grooving and tilt of the bone surfaces indicated the leaning and standing aspect of the bird-like dinosaur. Fourth its ultimate live weight was calculated using measurements like the major bones’ circumference, length, and the ratios among them: this is not unlike bringing in an a broken auto part to an auto body parts shop and the staff there being able to tell you what kind of car or pick up truck you were driving, and how much it could tow behind itself or carry in its cargo box. Fifth, the jaw structure showed that it was somewhat beak-like, in a kind of rounded goose-like manner, but there were places for teeth in the back. The creature could grab and break off a chunk of prey with the front end and masticate with the back. While this structure is not widely seen in extant animals, it has been found in related dinosaurs. In those other dinosaurs the bones of smaller eaten dinosaurs were found within the predator’s fossilized body frames where the stomach of the predator that ate them would have been located, although a smaller number of specimens showed occasional fossilized plant remains instead. As with many current dinosaur studies, CAT scans and computer graphics were used to help visualize details and help visualize matching “missing pieces.”
So, is Gigantoraptor the biggest meat-eating dinosaur?
A review paper by Therrien and Henderson (2007) suggests that earlier bird-like dinosaurs found by Xu’s team (and by inference a dinosaur like Gigantoraptor ) are probably not going to be the body mass leaders of the carnivorous pack, as it were, because that honor goes to the theropods (the “beast-footed”). This group includes Giganotosaurus/ Carachadontosaurus (Coria & Salgado 1995, Coria & Currie 2006), Spinosaurus (Dal Sasso et al. 2005, Sereno et al., 1996, Sereno et al. 1998), or everyone’s Jurrasic Park movie & museum favorite Tyrannosaurus rex (Henderson & Snively 2004). To get a great and quite accurate size comparison chart that includes these dinosaurs click on the enigmatically authored (“Dinoguy2”) link that follows:
Therrien & Henderson suggest that for a vertebrate paleontologist, the luckiest fossil body part to find and begin modeling your theropod is a complete, or near complete skull. They strongly favor the use of a skull-length- based formula for estimating body length (about 97% reliability of prediction) and somewhat different but still skull-length-based formula for estimating body mass or weight (about 87% reliability). The authors’ preferred engineering modeling approach is shown to be the best available when competing methods of estimating body size are tested against dinosaur fossils that are known to be skeletally complete from head to toe and tail. In other words, their formula can be proven to work better than others , because the proof of correlating measurements is already on display “in the skeletal flesh” as it were in the museum to check against.
They also use a kind of structural engineer’s “strength of materials vs. likely stresses” argument that suggests that there probably was an overall upper limit to the size and weight of theropods, based partly on their type of bone structure, and the requirement that they be able to walk , run and hunt on only two feet . Given what is known from fossils found thus far, they suggest that the two very closely related dinosaurs Carcharodontosaurus and Giganotosaurus would have been about 39 feet long and weighed about 14 tons, eclipsing Tyrannosaurus rex in length and overall body size by a small but persistent margin of a few feet and a couple of tons.
If ---- and this is a big if in those author’s eyes, the partial skull of Spinosaurus is as big as some dinosaur reconstructors have made it out to be through their filling in of absent material----- the overall length of Spinosaurus would have been about 42 feet, and the calculated weight a seemingly incredible, and biomechanically hard-to-rationalize 20 tons. Therrien and Henderson posit that as a practical matter a body length of about 39 feet and a weight of 14 tons was about as big as they see as sustainable for these predators before they would simply become too slow to hunt or simply start breaking down of their own weight.
Biomechanics Is at the Core of Vertebrate Size & Motion Studies in the Present as Well as the Past
Are you still unconvinced that these vertebrate paleontologists were not just making it up? Well, it turns out that their predictive biomechanical models of weight-bearing capacity and style of locomotion work very well with living animals, and that these studies of living animals tend to reinforce the reasonable certainty of the paleontologists. In addition, animal biomechanics also works in comparisons of modern man vs. the great apes vs. prehistoric man. Orthopedic surgeons rely on biomechanics when designing replacement knee and hip joints, and collegiate kinesiology brings biomechanics right down to the analysis and enhancement of athletic performance .
Because, in the end, gravity and inertia have been around and working a lot longer than they have been doing arthroplasty, longer than we humans have been around , and yes, even longer than when these large carnivorous dinosaurs roamed (or more likely ran on their hind legs) across the earth.
A Brief Bibliography on the Use of BioMechanics In the Estimation of Size, Strength, Mobility & Speed of Fossil & Contemporary Vertebrates, with special interest in Large Predatory Dinosaurs
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