Taller dinosaurs, cautionary cartilaginous tales, and happy endings

Casey Holliday measuring ostrich bones (2001).

A project that got its start way back in WitmerLab’s Triassic Period finally came to fruition, thanks to the professionalism and perseverance of lead author Casey Holliday. Today, in the freely available, open-access online journal PLoS ONE, we published an article (Holliday et al. 2010) on the caps of cartilage (known as epiphyses) that form the articulations between the long bones of dinosaurs and their modern-day archosaurian relatives (birds and crocs). For a long time, paleontologists had looked at the ends of dinosaur bones like the femur or humerus and suspected that something might be missing. The bones’ ends looked poorly formed, almost too simple, or were covered with a rough pattern of bumps and grooves. The logical conclusion was that many of these bones must have been covered with pretty significant amounts of cartilage.

Cladogram (genealogical diagram) of dinosaurs and their modern-day relatives showing the femur. Note how the ends of the bones are poorly formed, reflecting the missing cartilage caps, in all but the most birdlike theropods. Also note that this is Ryan Ridgely's first ever scientific illustration. (from Holliday et al. 2010: Fig 8)

Business end of the bone
So, we’re missing some cartilage…so what? Death, decay, and the millennia strip off all the soft tissues. Nothing new here, right? True, but it turns out that this missing cartilage is hugely important for interpreting all kinds of things about dinosaurs, because virtually all movements are dependent on the details of how these bones connect to each other. It’s pretty hard to understand the business of being a dinosaur without knowing about the business end of the bone.

Taller, faster…but maybe a bit more enigmatic

Alligator humerus (above) and ulna (below) with cartilage intact before skeletonization (left) and after skeletonization (right). Note how much important anatomical structure is lost after the cartilage cap is removed. (from Holliday et al. 2010: Fig 5)

Casey Holliday has presented some great resources on his website, including a common language summary and a nice narrative of what we did and why, and what we learned. In a nutshell, we turned to the modern-day relatives, as we often do, to make quantitative assessments of just how much cartilage is missing. Adding back in appropriate amounts of cartilage to dinosaur limb bones based on gator and ostrich “cartilage correction factors” (CCFs…it ain’t science without a TLA) makes some dinosaurs a good bit taller and maybe even a bit faster. Personally, I was even more struck by how much of the detailed structure of the joint is lost. Our study does indeed provide a cautionary tale: for many dinosaurs, we need to develop new criteria to make detailed inferences of function at many joints. Basically, the bones mounted in museums aren’t necessarily what the animals actually used, and so we need to use modern-day relatives to constrain how we reconstruct the cartilaginous ends of bones.

Casey Holliday molding freshly dissected alligator limb bones, complete with intact cartilage caps. (Fall 2000)

The humble beginnings of the cartilage project
This project began almost as soon as Casey Holliday walked into my lab in 2000 to start his doctoral research. During the upcoming summer of 2001, many of us would be attending the International Conference of Vertebrate Paleontology (ICVM) in Jena, Germany, and, since the ICVM is indeed a congress to love, I wanted new-guy Casey to attend, too. But he was so new that he had no ready-made project to present…so we cooked one up: the cartilage project. The idea had been percolating in my brain for years, no doubt because of all the gator skeletons I had prepared. When Casey joined us, he had just finished a stint at the Field Museum’s satellite preparation facility in Orlando, cleaning the rock from the vertebrae and limb bones of their new T. rex specimen, Sue. He also had been well trained in molding and casting, so it seemed to be a natural idea for him to cast the freshly dissected bones of an alligator with all the cartilage still in place, which we could then compare to the same bones after complete skeletonization.

Team Cartilage: (from left) Ryan Ridgely, Jayc Sedlmayr, Casey Holliday (Witmer is holding the camera). 2001.

Assembling Team Cartilage
The project quickly expanded beyond this one alligator to many alligators, and then to multiple species of birds, especially ostrich (which were delicious, I might add). Early on in the project, we brought in Jayc Sedlmayr, who was also a doctoral student in our lab, and Ryan Ridgely, who joined us in the fall of 2000. [Jayc got his PhD in 2002, did a postdoc at UCLA, and is now faculty at Louisiana State University’s med school. Ryan has been with us ever since, rising to the full-time position of Research Associate and is co-PI on our NSF grants.] Jayc and Ryan helped Casey with all the dissection, measuring, skeletonization, remeasuring, etc. Casey indeed presented the project at the ICVM. But later that year, in the fall of 2001, Casey presented the project again at the annual meeting of the Society of Vertebrate Paleontology in Bozeman, Montana. There, it created a stir. We knew we were on to something.

Damn dissertations are nothing but trouble
The major problem with the cartilage project was that it was a “side project.” It wasn’t Casey’s dissertation, and so it languished. The manuscript surfaced every once in a while. For one, too-long pe

Knees of an adult ostrich with fully ossified joint surfaces (above) and a subadult ostrich with rough, pitted surfaces reflecting the lost cartilage. (Courtesy of WitmerLab at Ohio University.)

riod, it languished in my hands, and so blame me, not Casey, for any delays in publication.

Happy Ending!
Fortunately, Casey is a professional…and is nothing if not persistent. Once he started his current position at the University of Missouri’s med school, he banged together an outstanding, mature draft that we both took up with enthusiasm. As is often the case, the delays resulted in a better manuscript. So, a decade after the project started, it’s finally out! I’m very proud of the published article, but I’m far prouder of the people with whom I’ve had the pleasure to work and from whom I’ve learned so much.  (sniff, sniff…)

–Larry Witmer