On continua and categories in paleoecology. . .or, an example application of ODP data

Today we’re delighted to have a guest post from Dr. Chris Noto, a new assistant professor at University of Wisconsin-Parkside, and an old friend of mine from our graduate school days together at Stony Brook University. Chris has had a long-running interest in dinosaur paleoecology, and thus it only seemed natural for him to apply these interests to the ODP data. Enjoy!

Chris Noto, at home in the field

In describing the ecology of an organism our first inclination may be to simply go observe it in its day-to-day existence. Therefore, at its core, ecology is primarily a science of the living and this is reflected in the methods and theories one finds in the literature. Over the past couple decades there has been a growing interest in the relationship between organismal morphology and ecology, which is now often referred to as “ecomorphology”(Losos 1990; Ben-Moshe et al. 2001; Aguirre et al. 2002; Zeffer et al. 2003; Sacco and Van Valkenburgh 2004). This has opened entire new areas of research into the covariation between organisms and their environment, which is also the basic foundation for understanding evolutionary change over longer spans of time.

Some paleontologists have applied ecomorphological principles to reconstructing the paleoecology of certain extinct groups, including carnivorans (Palmqvist et al. 1999), birds (Hertel 1995), and especially ungulates (Solounias and Semprebon 2002; Meehan and Martin 2003; DeGusta and Vrba 2005; Klein et al. 2010). Changes in the types and/or proportions of ecomorphs in a fossil community have also been used as evidence of environmental and evolutionary responses to climate change(Van Valkenburgh 1995; Meehan and Martin 2003; Badgley et al. 2008; Noto and Grossman 2010). You will note though that a lot of this research relies on comparison to living analogs related to the fossil groups in question. How do we explore the paleoecology of groups that completely lack extant analogs?

If there’s one thing we’ve learned through all this research, it’s the fact that:

1. Certain morphological adaptations occur regardless of species (convergence) because of specific habitat constraints, and
2. Morphological differences between species will occur due to diverging ecologies, even if we don’t know exactly what ecological functions those morphological differences actually represent.

But, we won’t know those differences exist until we look for them. Paleoecology is first and foremost comparative: we take our fossils and compare them to other related taxa and living forms to better understand their place in the original community. Often we assign categories to taxa, such as “carnivore”, “biped”, etc.; however, differences between species are often better described by a continuum than a set of categories(Carrano 1999). The morphology of an organism reflects the amount of time it spends doing certain activities or performing certain functions. For example, a sloth can swim on occasion even if it is not particularly well adapted for it. Dinosaur paleoecology is finally moving in the direction of our mammalian colleagues by using quantitative measures of morphology (which allow for continua) instead of assigning discreet categories.

The ODP is one of the first large-scale projects to bring together the kind of dataset necessary to study dinosaur ecomorphology. In a recent paper I published looking at differences between dinosaur fossil communities (Noto and Grossman 2010), I was forced to use categories in assigning ecomorphs, which artificially restricted the analysis by forcing me to choose a category when uncertainty existed. In this case it was whether certain non-hadrosaur ornithopods were bipedal or quadrupedal. With ODP data, it is now possible to take a more quantitative (and nuanced) approach to this question.

To explore possible trends in ornithischians, I used humerus length and mediolateral width measurements to calculate Mike Taylor’s Gracility Index (GI; Taylor 2009) using only the largest individual from each species. These data were log transformed and plotted against the log of humerus length (to help minimize the effects of size and codependency). The resulting plot clearly separates the taxa, with more bipedal taxa having relatively gracile humeri and quadrupedal groups have more robust humeri. There are two ways to use this graph. First, we can look at the distribution of taxa from each group and see whether they fit more towards a bipedal or quadrupedal type; those intermediate to the extremes are referred to as facultative. These are my own divisions based on where I see breaks in the data. Basal ceratopsians, for example, occur mainly towards the bipedal or facultative ends of the spectrum, while derived Neoceratopsians are firmly on the quadrupedal end of things. Another way to look at the plot is how robust we may expect the humerus to be for a taxon of a given size. The dashed line is drawn across the middle of the plot. For a given humerus length we can compare GI between taxa. For example, Iguanodon has a more robust humerus (lower GI) than most ornithopods in the dataset. Furthermore, we can spot outliers, which may point to either extreme specialization or faulty data. The theropod Mononykus has an extremely robust humerus, approaching the level of Triceratops, which is related to its specialized digging forelimb. On the other hand, Cerasinops appears to have the most robust humerus of all, however as Mike pointed out to me, the original paper describes the humerus as extremely gracile and gives no width measurement. So where did the width measurement come from? This particular data point is worth another look.

Humeral robustness as a function of humeral length. Select taxa labeled in gray. Cera.=Cerasinops, Gypo.=”Gyposaurus”, Herr.=Herrerasaurus, Igua.=Iguanodon, Mono.=Mononykus, Post.=Postosuchus, Psit.=Psittacosaurus, Scut.=Scutellosaurus, Stego.=Stegosaurus, Thec.=Thecodontosaurus, Tric.=Triceratops.

As you can see, the distribution of humeral morphologies indicates a gradual continuum of locomotor strategies from fully bipedal to full quadrupedal. Quantitative data such as this could then be fed into a paleoecological analysis instead of categories, allowing for more refined analysis of ecological differences and similarities among paleocommunities over space and time. While evolutionary trends are certainly important, we must not forget the ecological context of the morphological patterns we are studying.

References

Aguirre LF, Herrel A, van Damme R et al. (2002) Ecomorphological analysis of trophic niche partitioning in a tropical savannah bat community. Proceedings of the Royal Society of London Series B-Biological Sciences 269:1271-1278

Badgley C, Barry JC, Morgan ME et al. (2008) Ecological changes in Miocene mammalian record show impact of prolonged climatic forcing. Proc Natl Acad Sci U S A 105:12145-12149

Ben-Moshe A, Dayan T, Simberloff D (2001) Convergence in morphological patterns and community organization between Old and New World rodent guilds. Am Nat 158:484-495

Carrano MT (1999) What, if anything, is a cursor? Categories versus continua for determining locomotor habit in mammals and dinosaurs. J Zool 247:29-42

DeGusta D, Vrba E (2005) Methods for inferring paleohabitats from discrete traits of the bovid postcranial skeleton. J Archaeol Sci 32:1115-1123

Hertel F (1995) Ecomorphological indicators of feeding behavior in recent and fossil raptors. Auk 112:890-903

Klein RG, Franciscus RG, Steele TE (2010) Morphometric identification of bovid metapodials to genus and implications for taxon-free habitat reconstruction. J Archaeol Sci 37:389-401

Losos JB (1990) Ecomorphology, performance capability, and scaling of West Indian Anolis lizards: an evolutionary analysis. Ecol Monogr 60:369-388

Meehan TJ, Martin LD (2003) Extinction and re-evolution of similar adaptive types (ecomorphs) in Cenozoic North American ungulates and carnivores reflect van der Hammen’s cycles. Naturwissenschaften 90:131-135

Noto CR, Grossman A (2010) Broad-scale patterns of Late Jurassic dinosaur paleoecology. PLoS ONE 5:e12553

Palmqvist P, Arribas A, Martinez-Navarro B (1999) Ecomorphological study of large canids from the lower Pleistocene of southeastern Spain. Lethaia 32:75-88

Sacco T, Van Valkenburgh B (2004) Ecomorphological indicators of feeding behaviour in the bears (Carnivora : Ursidae). J Zool 263:41-54

Solounias N, Semprebon G (2002) Advances in the reconstruction of ungulate ecomorphology with application to early fossil equids. Am Mus Novit:1-49

Taylor M (2009) A re-evaluation of Brachiosaurus altithorax Riggs 1903 (Dinosauria, Sauropoda) and its generic separation from Giraffatitan brancai (Janensch 1914). J Vert Paleontol 29:787-806

Van Valkenburgh B (1995) Tracking ecology over geological time – evolution within guilds of vertebrates. Trends Ecol Evol 10:71-76

Zeffer A, Johansson LC, Marmebro A (2003) Functional correlation between habitat use and leg morphology in birds (Aves). Biol J Linn Soc 79:461-484

A First Pass at Figures

Even though our paper is intended for a technical audience, it is still important to ensure that a broad range of readers can access and understand the information contained within the text. For instance, not even a competent dinosaur paleontologist is necessarily familiar with all of the intricacies of ornithischian clade names like “Ankylopolexia” or “Neornithischia.” Thus, we want to provide a brief bit of background for readers of the paper.

One option, of course, is to write out brief definitions of various clades as they are introduced. This works okay in some cases – for instance, we definitely want to briefly explain what an ornithischian is – but to do this for every term can get a little unwieldy. An old adage states, “A picture is worth a thousand words,” and this is just as true in scientific writing as it is in popular writing.

So, I suggest that Figure 1 for the paper include a simplified cladogram of the major clades discussed in the paper. A first pass at this is given below (click on the image to see at full resolution):

Figure 1. Phylogeny of Ornithischia (from multiple recent sources), showing major clades discussed in the text. Outlines show representative members of each clade; names in bold indicate clades with quadrupedal taxa.

There are a few things I should mention. First, the content of the figure is nowhere near finalized. However, there were a few principles I wanted to adhere to:

• Keep it simple. Because this is only an overview figure, I did not deem it practical to include all of the taxa that we discuss. Instead, I just chose the “important” ones that will appear over and over again.
• Terminology. In a few cases, such as Neornithischia, including only major named clades oversimplifies things just a little too much. For instance, there are a bunch of important neornithischians (e.g., Agilisaurus and Othnielosaurus) that don’t fit comfortably within ornithopods or marginocephalians, and I want to find ways to include such taxa. Thus, I’ve created terms like “Early neornithischians”. I realize that this may imply that they are a clade in their own right, where instead they form a comb or polytomy, but perhaps this is a simplification that just has to be made. If anyone has a suggestion for a better way to title the groups, please let me know. For now, I prefer “early neornithischians” over “basal neornithischians” and the like. “Basal” implies a ranking that just isn’t there for cladograms, but maybe other folks think this is less of a deal than I do.
• Notation of quadrupedal taxa. Because quadrupedalism vs. bipedalism is so important for the paper, I bolded relevant taxa as outlined in the caption. The icons (discussed next) provide an additional clue. If I recall correctly (Andrew McDonald is probably most up to speed on this of anyone who follows this blog), there are probably a few non-hadrosaurid ornithopods that should be inferred to be quadrupedal, too.
• Icons. I consider it very important to include at least a small figure for each taxon, so that readers who are not familiar with all of the terms can picture each clade in their mind. The icons that are shown here (from Mike Keesey’s Phylopic) are of generally high quality, but should be considered only temporary. Ideally, I would like to generate new images to go with our figure, if only because there has been such a hubbub over the running dinosaur pose recently.
• Orientation. I opted for portrait rather than landscape orientation for the figure, primarily because I thought it was a more efficient and readable format. Any thoughts?
• Time calibration. One option for the figure would be to time-calibrate it, and show the duration and estimated time of origin for each clade. I feel this might make things just a little too complex (and crowd other parts of the figure), but am open to alternative interpretations. Thoughts?

At any rate, that’s what we’ve got for now. Please chime in in the comments!

Image Sources: All images are from Phylopic, and are licensed accordingly under a Creative Commons License. Individual credits are as follows: Oscar Alcober & Ricardo Martinez (http://phylopic.org/image/246), Scott Hartman (http://phylopic.org/image/25; http://phylopic.org/image/48; http://phylopic.org/image/43; http://phylopic.org/image/33), Loewen et al. (http://phylopic.org/image/142), FunkMonk (http://phylopic.org/image/128), Lukas Panzarin (http://phylopic.org/image/140), Remes et al. (http://phylopic.org/image/146); Ville-Veikko Sinkkonen (http://phylopic.org/image/261)