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

Radius, Not Ulna

As you may recall from your own background knowledge or from this tutorial, the forearm includes two bones: radius and ulna. One of the core analyses we’re going to be looking at is the proportions of the forearm relative to the arm (“upper arm”). Of course, we have to use bone length as a proxy for the true length of the different limb segments. For the arm, it’s a no-brainer. There’s only one bone, the humerus. For the forearm, we can choose between the radius and the ulna. At first glance, it might seem like the bones are interchangeable. But, it’s not really that simple.

Forelimbs of Othnielosaurus (left) and Triceratops (right), modified after the Marsh 1896 originals. In each, the radius is green, the main body of the ulna is blue, and the olecranon process of the ulna is yellow.

You see, some ornithischians (especially the big ones, it seems) have this giant sticky-outey thing on the proximal end of the ulna: the olecranon process. The triceps, among other muscles, attaches here. Because our spreadsheet only records maximum length of the ulna (and this is all most researchers ever report, anyhow), we encounter a complicated situation. The olecranon sticks up past the end of the humerus – so that a naive ulna : humerus ratio doesn’t really accurately describe forearm : arm length. It actually covers forearm + a little bit of the arm : arm length.

It doesn’t make much of a difference for some dinosaurs – for one Psittacosaurus, for instance, we’re talking ratios of 0.66 vs. 0.67. But what about an animal like Triceratops? In some specimen, it’s a matter of 0.99 vs. 0.71!  Obviously, we would get very different results if we apply this across the board.

For this reason, I would propose that we’ll want to use radius:humerus instead of ulna:humerus. In a quick look through the data, it also looks like we wouldn’t really lose out on any specimens, either; radius length is reported just as frequently as ulna length, if not more! And, of course, radius:ulna would be an interesting way to (indirectly) examine the relative size of the olecranon process in various bipedal and quadrupedal species. Which is an important issue in its own right. . .

Beware the Shifting Catalog Number

Those who have contributed to the ODP over the last few months know that a single specimen might have measurements featured in 2, 3, 4, or more separate scientific papers. In order to keep data entry and verification as transparent as possible, we’ve included the presentation from each scientific paper as a separate entry. Now, though, it’s time to combine these separate entries into composite entries that can be analyzed as a single unit (see this post for how you can help).

But, we do face some real challenges in cobbling this information together. One major problem concerns different specimen numbers or museum abbreviations for the same specimen. For those who aren’t familiar with the museum world, every specimen in a museum gets a unique number. This helps us to keep track of the data with each specimen (not just measurements, but locality information, storage location, etc.). Rather than saying “that big T. rex skull on display in that big New York museum,” we just say “AMNH 5027″. This means that it’s specimen number 5027 at the American Museum of Natural History; there’s only one specimen with that number. Believe it or not, some people memorize such minutia (maybe you’re one of them). I know the specimen numbers for most of the well-known ceratopsian skulls (just mention the phrase “YPM 1822″, and Triceratops prorsus springs to mind), but still have a tough time remembering my wife’s birthday. Believe me, I catch grief for that one.

At any rate. . .in some cases, it’s pretty easy to figure out multiple presentations of the same specimen. AMNH FR5240 (American Museum of Natural History Fossil Reptile #5240) is pretty certainly the same as AMNH 5240. There are just a few extra letters (to distinguish 5240 in the fossil reptile collection from 5240 in the modern fish collection, for instance).

Sometimes things get complicated. For instance, museums change names. The old “Geological Survey of Canada” specimens eventually became “National Museum of Canada” specimens, which then morphed into “Canadian Museum of Nature” specimens when the institution changed its name. So, the Chasmosaurus skeleton that started out as GSC 2245 became NMC 2245 became CMN 2245. “CMN” seems to be the abbreviation of choice nowadays, and luckily the specimen numbers stayed the same. Sometimes historic abbreviations are carried on through sheer inertia. For instance, “USNM” stands for “United States National Museum.” Yet, it hasn’t been called that in decades – today we know it as the “National Museum of Natural History” (or just “The Smithsonian” to most of the general public). But, for various reasons (including overlap in abbreviations with all of the other countries’ national museums), “USNM” still stands. When different publications use different abbreviations, we still have to sort out what’s going on.

AMNH 6471. . .I mean, CM 9185. This file is licensed under the Creative Commons Attribution ShareAlike 3.0 License, modified from an original posted to the Wikimedia Commons by Kevmin.

Sometimes things get really complicated. Did you know that the Protoceratops skeleton listed as AMNH 6471 by Brown and Schlaikjer’s 1940 paper is now known as CM 9185? This happened when the specimen was sent from the American Museum of Natural History to the Carnegie Museum in Pittsburgh. The only reason I know of this is because Matt Carrano had noted this in one of his data entries, and also through a chance reading of a 1981 publication on dinosaurs of the Carnegie by Jack McIntosh.

And sometimes things get just flat-out twisted. Back in the day, the Royal Ontario Museum completely renumbered their fossil collection. What was once known as the Corythosaurus ROM 5505 is now ROM 845. The Lambeosaurus ROM 6474 is now called ROM 1218. Thankfully, some papers indicate the old and the new catalog numbers. But not always. There are measurements from old papers of certain specimens (e.g., ROM 5167 and ROM 5971, specimens of Edmontosaurus regalis and Prosaurolophus maximus, respectively) that just aren’t clear. So, we’ll either hope that someone out there reading this knows the current specimen number, or we’ll have to contact a curator at the museum to find out. (feel free to chime in in the comments, if you know the answer)

These sorts of things are hugely important for the utility of our dataset, and we’re depending on each other to get these details ironed out. That’s the real strength of an open project like the ODP – anyone can contribute!

Key Concepts: Scaling

Anyone who has dealt with puppies, kittens, or human babies has probably noticed their freakish (“cute”) body proportions relative to adults. The heads are too big and the limbs too small! Fortunately, most of us grow out of this condition as we mature. The change (or lack of) in the proportions of an organism as its overall size changes is called scaling.

Scaling relationships can be compared between species, within species, or even within the same individual over the course of its development. In the present round of the ODP, we’ll be focusing on the first comparison, although many of the examples that follow will draw on within-species or within-individual scaling.

Scaling, as a whole, can be divided into two general forms: isometry and allometry. The key difference between these terms is how the different parts of an organism change in proportion to each other with increasing or decreasing overall size. Isometry (a.k.a., isometric scaling) means that everything stays the same. Here, imagine a condition in which a baby maintains its proportionately large head size into adulthood. Contrast this with the actual condition, allometry. In humans (and most other vertebrates), the head becomes proportionately smaller to the rest of the body. When different parts of the body scale at different rates, this is called an allometric scaling relationship. You can see this in the puppy and adult golden retrievers at right.

Allometry is divided into two classes – negative and positive. Our oft-cited baby’s head is a classic example of negative allometry—the head becomes relatively smaller with increasing body size. Other features might show positive allometry—for instance, the horns of male bighorn sheep grow at a much faster rate than the rest of the body. In a lamb, the horns are tiny compared to the skull. In the adult, the horns are huge!

Scaling comparisons are pretty straight-forward for linear measurements (e.g., comparing scaling of femur length relative to humerus length). Isometry is indicated by a one-for-one change in size between the two elements. But, the complications of mathematics mean that things are a little less intuitive when considering areas or volumes. Consider a cube measuring 1 mm along each side. Its volume (length x width x height) is 1 cubic mm. If we double the size to 2 mm along each side, its volume doesn’t just double, but leaps to 8 cubic mm (2 mm x 2 mm x 2 mm). Bumping the size up to 3 mm along each side, the volume is now 27 cubic mm (3 mm x 3 mm x 3 mm). Length along any given dimension has changed by a factor of 3, but volume has changed by a factor of 27 (3 cubed). Similarly, the cross-sectional area has changed by a factor of 8 (2 cubed). We won’t be doing much with areas or volumes for the present study, but this is an important attribute of scaling to keep in mind.

So, one important analysis to be considered in ODP 1.0 is how limb bone lengths scale against each other. For instance, does the humerus increase its length at the same rate as the ulna, or metacarpals, or phalanges? Or, does the humerus get relatively shorter in larger animals? The departure from simple isometry might be for any number of reasons – perhaps larger animals need relatively more surface area for muscle attachment. Or perhaps weight limitations mean that certain bones can’t increase their size too quickly. We’ll just have to analyze the data and see what the trends are!

Image credits: Puppy image licensed under Creative Commons License 2.0; adult golden retriever in the public domain.

Key Concepts: Scaling
Anyone who has dealt with puppies, kittens, or human babies has probably noticed their freakish (“cute”) body proportions relative to adults. The heads are too big and the limbs too small! Fortunately, most of us grow out of this condition as we mature. The change (or lack of) in the proportions of an organism as its overall size changes is called scaling.

Scaling relationships can be compared between species, within species, or even within the same individual over the course of its development. In the present round of the ODP, we'll be focusing on the first comparison, although many of the examples that follow will draw on within-species or within-individual scaling.

Scaling, as a whole, can be divided into two general forms: isometry and allometry. The key difference between these terms is how the different parts of an organism change in proportion to each other with increasing or decreasing overall size. Isometry (a.k.a., isometric scaling) means that everything stays the same. Here, imagine a condition in which a baby maintains its proportionately large head size into adulthood. Contrast this with the actual condition, allometry. In humans (and most other vertebrates), the head becomes proportionately smaller to the rest of the body. This is called an allometric scaling relationship.

Allometry is divided into two classes – negative and positive. Our oft-cited baby's head is a classic example of negative allometry—the head becomes relatively smaller with increasing body size. Other features might show positive allometry—for instance, the horns of male bighorn sheep grow at a much faster rate than the rest of the body. In a lamb, the horns are tiny compared to the skull. In the adult, the horns are huge!

Scaling comparisons are pretty straight-forward for linear measurements (e.g., comparing scaling of femur length relative to humerus length). Isometry is indicated by a one-for-one change in size between the two elements. But, the complications of mathematics mean that things are a little less intuitive when considering areas or volumes. Consider a cube measuring 1 mm along each side. Its volume (length x width x height) is 1 cubic mm. If we double the size to 2 mm along each side, its volume doesn't just double, but is cubed (2 mm x 2 mm x 2 mm) to 8 cubic mm. Bumping the size up to 3 mm along each side, the volume is now 27 cubic mm (3 mm x 3 mm x 3 mm). Length along any given dimension has changed by a factor of 3, but volume has changed by a factor of 27 (3 cubed). Similarly, the cross-sectional area has changed by a factor of 8 (2 cubed). We won't be doing much with areas or volumes for the present study, but this is an important attribute of scaling to keep in mind.

So, one important analysis to be considered in ODP 1.0 is how limb bone lengths scale against each other. For instance, does the humerus increase its length at the same rate as the ulna, or metacarpals, or phalanges? Or, does the humerus get relatively shorter in larger animals? The departure from simple isometry might be for any number of reasons – perhaps larger animals need relatively more surface area for muscle attachment. Or perhaps weight limitations mean that certain bones can't increase their size too quickly. We'll just have to analyze the data and see what the trends are!

Key Concepts: Osteology II (The Hindlimb)

In a recent post, we gave an introduction to the osteology of the forelimb. Now, we’ll round out that series with a consideration of the hind limb. Fortunately, many of the concepts are the same, so we’ll be able to move more quickly.

As you may recall, the forelimb was divided into a pectoral girdle, a big proximal bone (the humerus), two more distal long bones (radius and ulna), and a hand (manus) consisting of some carpals, metacarpals, and phalanges (with some modified into unguals). The same pattern follows for the hind limb, with a pelvic girdle, a big proximal bone (the femur), two more distal long bones (tibia and fibula), and a foot (pes) consisting of some tarsals, metatarsals, and phalanges (again, with some modified into unguals). Easy, isn’t it?

Centrosaurus hind limb (after Brown 1917)

First, let’s take a look at the pelvic girdle. In dinosaurs, as in humans and pretty much every other limbed vertebrate, the pelvis includes three elements on each side: ilium, pubis, and ischium. Looking at the whole structure in side view, the ilium is the top bone, and the latter two are on the bottom. Where the three bones meet, their surfaces form the limb socket, which is called the acetabulum. The ilium is a pretty big, usually flat structure, that anchors the pelvis (and thus the limb) to the vertebral column. Lots of thigh and butt muscles also attach to it. Of the two bottom bones, the pubis is the front (anterior, sometimes called cranial) one. A big deal has been made of its difference in its orientation between ornithischian and saurischian dinosaurs – in ornithischians, most of the bone is directed backwards, and in most saurischians (with the exception of birds and their close allies) the bone is directed forwards. Finally, there is the ischium (which a classically-grounded anatomy professor of mine liked to note is correctly pronounced with a hard “k” sound, rather than the “ish-ee-um” that most folks use). For various reasons (namely, all of the processes and extra bumps render accurate comparison of measurements difficult), we won’t be doing much with the pelvis in the present study. So, let’s move on to the femur.

The femur, just like the humerus, is a single robust bone that articulates with the limb girdle, its head fitting into the acetabulum proximally, and with two other elements distally. Sometimes, there is a little backwards-directed hangy process from the middle of the shaft, called the fourth trochanter.

The tibia and fibula are the developmental homologues of the forelimb’s radius and ulna. Unlike mammals, dinosaurs lack a kneecap (patella) floating over the proximal end of the tibia and distal end of the femur.

Centrosaurus pes (after Brown 1917)

The “foot” is called the pes (Latin for “foot”), and is very slightly differently configured than for the manus. The hind limb’s equivalent of carpals are called tarsals – and unlike the condition up front, the tarsals are actually rather important and frequently ossified elements. In fact, they are so ossified that they usually fuse right on to the tibia and / or fibula. The two major tarsals in ornithischians are the astragalus (capping the tibia) and the calcaneum (capping the fibula, or at least floating in its general vicinity). Because the astragalus is so often fused to the tibia, many authors measure tibia length with the astragalus included.

Instead of metacarpals, we now have metatarsals. The numbering system is the same as for the manus, except they’re abbreviated as “MT.” So, the first (innermost) metatarsal, equivalent to the one associated with our big toe, is MT-I. Phalanges are handled quite similarly, with IV-2 being the second most proximal phalanx on the fourth digit. And once again, the final phalanges are often modified into unguals.

And that’s all there is to know about ornithischian dinosaur limb osteology!

Centrosaurus pes (after Brown 1917)

Key Concepts: Osteology I (The Forelimb)

Forelimb of the horned dinosaur Centrosaurus apertus

Osteology is the study of bones. Recognizing that not everyone here is completely familiar with all of the relevant names and features, this post will cover a brief tutorial of limb osteology and terminology in dinosaurs.

Broadly speaking, anatomists usually divide the skeleton into three sections: cranial (the head); axial (the vertebral column and ribs, although embryological and evolutionary histories mean that parts of the skull are sometimes lumped in here); and appendicular (the limbs). Presently, we’re only interested in the latter.

The appendicular skeleton includes forelimbs and hind limbs. Let’s start at the front in this post, and work back in a subsequent post. But before we start that, we need to introduce one more set of terms: proximal and distal (see image for their context within the forelimb). This just refers to the position along a structure relative to the main part of the body. Proximal is close to the body, and distal is away from it. Considering the humerus (upper arm bone), the elbow is at the distal end and the shoulder is at the proximal end. Within the entire leg, your toes are at the distal end and the thigh bone is at the proximal end.

The forelimb includes the pectoral girdle as well as the limb bones themselves. In dinosaurs, the pectoral girdle includes a scapula, a coracoid and a sternal plate on each side. Humans have scapulae too (most of us know them as “shoulder blades”), but our coracoids have shrunk down to little nubbins (the coracoid processes) that are fused onto the scapulae themselves. We also have clavicles (“collar bones”) as part of our pectoral girdle, but ornithischians lack this bone (although theropods preserve part of it in the furcula, or “wishbone”). In all adult ornithischians, the scapula and coracoid are fused together, and the area where they meet forms the glenoid, or shoulder socket. If the bones are fused, their total combination is then called a “scapulocoracoid.”

The humerus (or “upper arm bone”) fits into the glenoid. It’s a long bone, expanded at both ends for various muscle and bony attachments. Lots of muscles—including the famous deltoids, lats, biceps, triceps, and pectoral muscles—attach here. The “midshaft” of the humerus is exactly that – the point at the middle of bone.

A pair of bones – the ulna and radius – form the forearm. They articulate with the distal end of the humerus. They’re pretty simple, rod-like bones in most cases. The ulna usually has a process (i.e. a sticking-out bit), called the olecranon, at its proximal end for attachment of the triceps muscle.

Finally, we have the hand – more properly called the manus (Latin for “hand,” strangely enough). The manus has carpals (wrist bones), metacarpals (joining the wrist to the digits), and phalanges. Each digit (or finger) is numbered starting at the thumb. The thumb (innermost digit, for ornithischians) is I (note the Roman numeral), the index finger II, middle finger III, ring finger IV, and pinkie V.

The most proximal elements within the manus (just distal to the ulna and radius) are called the carpals. They’re often just cartilage, and even when ossified are rarely preserved (they tend to float away if the skeleton becomes disarticulated). At any rate, they’re usually non-descript little round elements in ornithischians, and we’ll pretend these bones don’t exist for the purposes of our study.

Right manus of the horned dinosaur Centrosaurus (modified after Brown 1917)

Next, we have the metacarpals. If you squeeze the palm of your right hand between the thumb and index finger of your left, these are the bones you’re feeling. The number of metacarpals is variable in many dinosaurs. Humans, and most ornithischians, have five metacarpals (and hence, five fingers in most cases). Most theropods have fewer. “Metacarpal” is often abbreviated as MC. So, the first metacarpal would be MC-I, and so on.

Finally, we come to the phalanges. A single element is most properly called a phalanx (not a “phalange,” although this archaic spelling is not technically incorrect – many older publications use the terminology). The phalanges are numbered by digit (I-V) as well as their position relative to the metacarpals (given by an Arabic numeral). For instance, I-1 is the first phalanx on the first digit, and III-2 is the second phalanx on the third digit. The second-to-last phalanx is sometimes referred to as the “penultimate” phalanx.

The distal-most (terminal) phalanx is often modified into a hoof or claw. These specially modified phalanges are usually called unguals, but they are numbered just the same as regular phalanges. Even if the third and final phalanx on the third digit is a huge claw, it’s still called manual phalanx III-3.

Finally, we should mention the sternal plates. These odd bones (probably equivalent to the sternum, or breast bone, of mammals) are usually floating at the front of the chest wall. The sternals sometimes look like kidney beans (in ceratopsids) or hatchets (in other ornithischians).

It’s a blizzard of terms, but a little practice should help you become completely conversant with all of the parts of the forelimb. In an upcoming post, we’ll tackle the hindlimb. Don’t worry – many of the concepts are the same!