Morphometrician of the Month (April)

This month we would like to introduce a new shape-lover, friend, and future collaborator Dr. Robert Z.(Zac) Selden Jr. from the crhr:archaeology

photo

Name: Dr. Robert Z. Selden Jr.

E-mail: selden3d@gmail.com

Institution: Stephen F. Austin State University

What is your research on?

Geometric morphometrics of ceramic vessels, Caddo ceramics primarily, but I am also engaged in a bit of an affair with projectile points (less Paleoindian, and more Archaic). I’m interested in assemblage-level variation in ceramic shape and size (to include decorative elements) as these elements may help us to further extend our archaeological inferences related to cultural transmission, craft theory and the—potential—identification of communities of practice.

I see promise in the capacity of geometric morphometrics to assist in identifying specific vessels that might represent the beginning (or end) of a morphological tradition. While I don’t subscribe to evolutionary archaeology, my perhaps-too-lofty goal is to eventually identify (to the extent possible) the ceramic equivalent of a transitional species.

image 1

Network of ceramic types (circles/nodes) plotted by archaeological sites (lines/edges) within the study area. If there is an edge (or multiple edges) between two nodes, they both appear at the same (or multiple) site(s). If two or more types appear at multiple sites, the lines/edges between them are larger (weighted).

Upon completion of the quantitative analysis, shape attributes are ascribed qualitative identifiers (vessel shape numbers) based on the outcome of the analysis, which are used in a network analysis.

In practice, I use those data garnered from the integrated approach of geometric morphometrics and network analyses as something akin to a hypothesis engine, where the results of the analyses (which can—and should—be viewed independently of one another) tend to generate more questions than they answer. At the end of each run, I spend time reflecting on the process, and the method, while writing like a madman (I have dozens of journals full of observations, notes and possible future projects at this point), as I try to capture inferences and ancillary observations from each cycle before moving on to the next.

image 2

The same network as above, but with shape numbers added. The different colours represent different communities (defined by modularity) in which more/less vessel shapes are associated with each of the ceramic vessel types. I am currently working to plot these spatially and temporally (using R) to see how they articulate with known Caddo periods and phases.

Those questions raised—at least in part—in each iteration are helping me to refine and redress additional ancillary observations like vessel tilt and rotational asymmetry, along with a host of others. Over the past year (the last six months in particular), I have spent a lot of time on Skype asking folks whether they are seeing the same thing as I am. Many of the ancillary observations look to be quite useful, and I want to ensure that we capitalize on each of them as time comes available.

image 3

Rotational asymmetry – where the widest vessel radius is rotated 360-degrees to generate a nominal surface that is contrast with the mesh to calculate (and illustrate) the deviation from rotational symmetry—a.k.a., asymmetry. Note: I have shifted to a new method for the formal asymmetry analysis (more on that below), but this method may be useful in finally quantifying the differences for rotational asymmetry between coil-built and wheel-built vessels (where wheel-built vessels are generally thought to be more symmetrical).

What got you into morphometrics?

The genesis of my interest in geometric morphometrics began at a pub with my graduate school roommate (imagine that), who is a biological anthropologist. He was (is, rather) using geometric morphometrics to look at a variety of biological structures (from ape jaws to rodent post-crania), and we began a discussion of how the various methods might be applied to ceramics to answer questions related to cultural transmission.

image 4

Results of assemblage-level variation in vessel shape from three Caddo sites.

At that time, I had been working to insert myself between the repatriation and reburial process for Caddo burial vessels; particularly those that fell under the purview of the Native American Graves Protection and Repatriation Act (NAGPRA), as I wanted to document these important cultural items before they were no longer available for study. It was at this point that I really began contemplating the potential contributions of geometric morphometrics to my current research design. However, the bulk of our discussions remained centred upon how to properly identify landmark and semi-landmark data points on a series of specimens that—for all intents and purposes—have only a single homologous landmark (the central base), which made me question whether this was even feasible.

https://sketchfab.com/models/42620534f6384e36b61728467367e190/embed

CNO 3SV10 81-89-1
by Dr. Robert Z. Selden Jr.
on Sketchfab

 

Where would you insert landmarks and semi-landmarks on the vessel above? This is an interactive 3D image; press play to activate it, then click/drag to rotate.

At this point, I want to echo Professor Collard’s statement from Morph2015, in that I see one of the principal challenges of employing a study of geometric morphometrics in archaeology as defining homologous (or for that matter, even semi-homologous) landmarks. It took me quite a while to settle on my current method of applying landmark and semi-landmark data points to the sample of 3D ceramics.

Initially, we used Cartesian coordinates that were subjectively applied, then exported those to Morphologika for analysis. While I was pleased to have taken a first stab at geometric morphometrics, I would be remiss if I didn’t mention my disappointment at the lack of replicability that I came to see as blatantly obvious in the pilot study.

In its current form (see an example in the YouTube video below), my method for applying landmark/semi-landmarks to ceramics has evolved substantially (after dozens of iterations), and continues to be refined. I have also shifted over to the geomorph package in R for the analysis.

YouTube video that outlines the process used to populate landmark/semi-landmark data points on a ceramic vessel using reference geometry in Geomagic Design X.

Subsequent to alignment, I now use Geomagic Design X (reverse-engineering software) to insert a revolving vector (defined by an algorithm—not subjectively [although note that the algorithm is still representative of a bias; just not my own]), then the single landmark (the only homologous point that I see as transcending all of the various vessel classifications [bowl, bottle, etc.]) at the central base, now defined by projecting a single point at the intersection of the 3D mesh and the revolving vector.

The basal plane (defined during alignment) is used to orient the vessel as if it were sitting on a planar surface—like the ground—which I assume to be the intent of the maker. This plane serves as the basis for a mesh sketch, used to generate and extrude a cylindrical surface around the circumference of the vessel. Deviations are calculated between the cylindrical surface and the mesh, making it possible to identify (consistently – ergo, replicable) the widest point of each vessel. That point is then used to insert a plane—coplanar to the central vector—along the widest profile of the vessel (defined by that widest point on the mesh surface).

The plane inserted along the widest profile is used as the base plane for a 3D mesh sketch, where 12 splines (six full vessel profiles) are inserted at equidistant intervals around the circumference of each vessel. Those splines are cut at the point of highest curvature along the rim. As an aside, I want to mention here that one of my goals was to generate a landmark/semi-landmark configuration that I could use for both 2D and 3D data, since we have hundreds of images in publications that can (and will) augment this initial study. The splines on the interior of the vessels were deleted (primarily because I use surface scanners, and cannot scan the interior of carinated bowls, bottles, etc.), and equidistant semi-landmarks were inserted along those splines from the central base (LM1) to the highest point of curvature on the rim along each radius, while rotating the vessel in a clockwise direction (although see note below). Once populated, those data are then exported to a .csv file.

Note: I have since incorporated a minor alteration to this method, and am now splitting each of the splines at the base/body and body/lip junctures as well. This provides a means by which I can explore the correlation between the base, body and lip, and explore things like shifts in basal morphology through time (which turned out to be important—remember those ancillary observations?). Additionally, I can use those divisions to figure out which component best discriminates between the various vessel shapes.

From this point, you’re likely familiar with the remainder of the analytical process – import raw data to your favourite software, then dig in.

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Another quick note here – I am now using those landmarks associated with the widest vessel profile (a) to look at variations in (b) fluctuating and (c) directional asymmetry. I think that both measures of asymmetry have some potentially interesting applications in archaeology (for a variety of artefact classes—certainly not limited to ceramics).     

Do you have any advice for budding morphometricians/shape-lovers?

Ask questions – lots of them, and be skeptical of your own work. Also, be comfortable enough in your own skin to genuinely laugh at your mistakes, shake them off, and keep failing forward. I used to skateboard when I was younger, and still remember one of my best friends telling me, “if you’re not falling, you’re not pushing yourself hard enough.” I think that the same logic applies here (but do yourself a favour and try to do your falling in the lab, rather than in print—so, full-circle back to my initial answer; ask lots of questions).

Some of my biggest gains have come from posting my ideas online and soliciting feedback from the larger community of practitioners. Join the MORPHMET list, and participate in discussions about topics or concerns that interest you. Also try to think through issues that others are having; it helps to conceptualize the process outside of those artefacts or specimen categories that you become comfortable with.

What is your go-to guide/article?

It’s not a guide or an article, but a rather short read that I think many of you would enjoy; The Shape of Time by George Kubler. If I find myself discouraged, or feeling stagnant, this gets me right back into the swing of things. Also, if you have not read The Writing Life by Annie Dillard, you should. Plenty of guidance in there that can be appreciated by graduate students and professionals alike—truly transcen­ds disciplines.

Favourite software?

I’m a really big fan of geomorph. I had been working with Emma Sherratt for about a year before heading to Portugal last fall for a workshop with Dean Adams, Michael Collyer and Antigoni Kaliontzopoulou. These folks have been absolutely wonderful, and I cannot thank them enough for their guidance and support as I have made my way through the various analyses.

Favourite online reference (besides this)?

I always have one eye on the geomorph blog, and frequently find myself checking the new meetings, workshops, courses, etc. page on the SUNY Stony Brook website.

Additionally, I assembled the beginnings of what I’m hoping will be a nice crowdsourced bibliography of geometric morphometric resources—for archaeology and beyond—beginning with Sarah and Christian’s list, then expanding on it a bit (access that here—generate a .pdf of the bibliography by clicking on “PDF” at the top of that screen, and please share widely), and I’m hoping that this community will help me to get this resource up to date. Send additional references to selden3d@gmail.com with Morph2016 as the subject line (include DOI and ISSN where possible), and I’ll get them added!

Many thanks to Sarah and Christian for putting this resource together for the archaeological morphometric community. It’s nice to see more folks beginning to work through and apply these methods to archaeological problems.

^C&S