Skeletons, Specimens, Synoptic Series, Similarity, and Difference

Joyce C. Havstad writes….

A collaborator at the Field Museum once showed me something called a “synoptic series,” which he and another colleague had assembled in order to represent the avian skeleton:

On the left, a close-up of the Field Museum of Natural History’s “Avian Skeletal Collection Synoptic Series,” assembled by Keith Barker, with key produced by N. Adam Smith. On the right, a shot of the synoptic series in situ. All photos in this post by the author.

To summarize, this collection of bird bones functions as a sort of “synopsis” (a general overview) from a “series” (or subset) of avian skeletons.  There are 30 species represented in the collection, each from a different family, and altogether representing 24 different orders.  As the key to the collection specifies, “all 30 taxa are represented by skull, furcula, coracoid, sternum, pelvis, humerus, radius, ulna, carpometacarpus, digit 2 phalanx 1, femur, tibiotarsus, and tarsometatarsus.”

So, if you wanted to get a sense of which features avian skulls might have in common with one another—of what they might typically share—one thing you could do is look at this collection of crania from 30 different species.  Alternatively, if you wanted to get a sense of the ways in which avian furcula (fused clavicles) can sometimes differ from one another, again, you could take a look at this collection—in order to get a sense of the variation present amongst these 30 specimens from 30 species from 30 families and 24 orders, at least.  A working paleontologist might use a collection like this to help identify what part of the avian family tree an unidentified fossil might belong to, or to generate phylogenetic matrices that attempt to capture osteological variation across the clade.

I think that the synoptic series is a really neat tool, and it’s one I’ve been thinking about basically since I got introduced to it, 4 years ago.  I had long wondered how comparative morphologists managed to form their judgments about what is typical of a taxon or clade (i.e., what is shared in common), as well as what the significant differences are (i.e., which characteristics are key).  Philosophically, these are questions about which similarities amongst members of a group are the significant ones, and which aren’t; which differences amongst members of a group are the meaningful ones, and which aren’t.  But how do you read significance off a bone?  How do you detect meaning, scientifically?

One thing that can help is having a large assortment of samples to compare.  If you’ve got a healthy comparison class—a set of wide-ranging kinds of specimen, and multiple instances of each kind—then merely idiosyncratic similarities and differences among the kinds can often be separated from the more consistent, significant, or telling ones.  Of course, wide-ranging and repetitious collections of specimens are easier to compile for some kinds of taxa than for others.  Specimens from extant taxa are often easier to come by than specimens from extinct ones; specimens from members of large populations are often easier to acquire than apex predators, or members of other kinds of smaller (for instance, threatened) populations.

This brings me back to my initial synoptic series, the one compiled using avian skeletal specimens. One of the things that struck me when I first saw it—behind the scenes at the museum, definitely not on display—was just how different that jumbled collection of bones was from the elegantly mounted creations usually presented for public viewing. I thought the series was so cool precisely because of how useful it was. But it’s a far cry from the stunning, posed, and articulated skeletons that are so often displayed as a natural history museum’s pride-and-joy:

A bevy of beautiful Tyrannosaurus rex! On the left, the holotype of the species (CM 9380), on display at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania. In the middle, the specimen informally known as ‘Sue’ (FMNH PR 2081), on display at the Field Museum of Natural History in Chicago, Illinois. On the right, the specimen informally known as ‘Black Beauty’ (RTMP 81.6.1), on display at the Royal Tyrrell Museum of Palaeontology in Drumheller, Alberta.

These shots depict what most of us probably think of as paradigmatic “dinosaurs on display,” but scientifically speaking, a collection of 10 or more femurs might be just as useful—though probably, in a different way.  Since specimens of immensely popular dinosaurs such as Tyrannosaurus rex are so valuable as display items for museums, we don’t usually get to see them next to one another, as they are depicted in the photographs arrayed here.  It’s likely that this is as close as we’re ever going to get to having a “Tyrannosaurus rex Skeletal Collection Synoptic Series,” one displaying the similarities and differences amongst various components of actual fragments of specimens from within the species.

Ever since realizing the likely rarity and importance of finding enough specimens to generate “synoptic series” for extinct taxa, taxa with a small population size, and especially, extinct taxa with a small population size, I’ve kept an eye out for public presentation of this kind of data in natural history museum displays. Here are some of my recent favorites, from a couple of locales:

The two pictures on the left are both from the George C. Page Museum at the La Brea Tar Pits. That’s a wall of 400 dire wolf skulls, and then a full case of nothing but avian tibia—from the golden eagle Aquila chrysaetos. The two pictures on the right are both from the Utah Museum of Natural History. That’s a wall of ceratopsid skulls, and then a growth series of Allosaurus femora collected from the Cleveland-Lloyd Dinosaur Quarry.

It’s been delightful to see this kind of thing in the museums I’ve visited during the last handful of years, alongside what might be termed the more traditional “dinosaurs on display.”  (I’m not saying that museums have begun displaying this sort of collection more often in recent years—only that I notice this kind of thing now, and appreciate it, where I didn’t before.)

Learning about synoptic series has also made me appreciate certain geological & ecological features, such as predator traps: spaces where predators come to hunt doomed prey, only to be captured themselves.  As it turns out, in trapping the predators alongside the prey, a feature like a tar pit acts as a sort of trap for knowledge, too.  It’s neat the way certain natural circumstances—a sticky situation for a dire wolf, say—can actually help scientists escape from an epistemological predicament of their own.

Thanks to N. Adam Smith of Clemson University’s Campbell Geology Museum, Susan Oshima of the Natural History Museum of Los Angeles County, and Gary T. Takeucki of Rancho La Brea, for their assistance with this post.