It’s been a while since I’ve added to my overarching plan to describe the diet of oviraptorosaurs, and now is time to continue. But first, a recap and linkfest: Part 1 introduced the series looked at the development of the jaw in oviraptorosaurs in general. Part 2 looked at the jaws of the basal toothed oviraptorosaurs in specific. Part 3 looked at the broad picture of basal toothed and toothless taxa. Part 4 isn’t really labeled as such, but should have been, and discusses the sliding jaws of caenagnathoid oviraptorosaurs. Part 5 looked at the diversity of oviraptorosaur skulls, most of which are oviraptorids, to discuss in brief the variation in jaw form. Part 6 looked at oviraptorid palates, bizarre things that they are. And here I slightly crossed over by proposing 20 reasons that Gigantoraptor erlianensis could be a caenagnathid. We restart by … setting some standards.
I want to measure the jaw of oviraptorosaurs in detail, largely because it is important to have a large set of data points to assess variation in morphology and proportion, figure out which of these are evolutionarily informative, and most of all determine how, what, and why these jaws are like this (which, of course, has to do with diet). But measuring the jaw is not just as simple as taking the bones and pulling a tape across them, or using the “new” way of digitally measuring it with a ruler tool in Photoshop or something. You have to know what you’re actually measuring, and by this I mean you have to have some intrinsic aspect that what you’re measuring is not only useful, but comparable.
A mandible is a pair of levers (left and right) with holes in it, to put it bluntly, and as a mechanical device, it follows rules on where the pivot is, the effect and quality of force acting on it, and through which it can act on other objects, and the relationship of parts of that lever. This is a very simplistic description, but it’s apt. Typically, scientists who measure bones draw from extremes: the anteriormost point to the posteriormost point, the dorsalmost point to the ventralmost point, etc. These can differ from scientist to scientist, from standard to standard, and even paper to paper from a single set of researchers. Even settling on the effectiveness and place to use terms like “apical” and “basal” can be tricky, and it leads to confusion when following a paper trail. Often, it means every new scientist must remeasure the material at hand; relying on another’s measurements can be a problem for the sole reason that you don’t know how they actually made the measurement, what yardstick was used, or that they actually estimated the size based on an object in the photograph. Then there’s the issue of perspective, which distorts an image even if it’s right in front of you. In a mandible, the far side will always be smaller than the near side relatively, and thus the far mandibular ramus will be “shorter” and “shallower” than the near ramus.
So what we want to do is measure an object, and get as close to perfect and adaptable as we can. And we start this process by finding the correct alignment for the mandible to then compare variables such as the perpendicular or the parallel. These are useful when we want to know the height of an element relative to its length, and not have to deal with the fact that this height is measured at an angle below 90° to the same element’s length.
Step 1. Choose.
Actually, we start this process by finding a mandible. Now, in retrospection, we can use ANY mandible — that’s how this thing is supposed to work. But I want to illustrate this while using as few unknowns as possible, and for me, this means using a taxon in which all margins of the bones of the mandible are visible from the perspective angle. As the above image might spoil, I’ve chosen CMN 8776, holotype of Caenagnathus collinsi Sternberg (1940), and almost certainly because it’s an oviraptorosaur, but also that I’m leading into discussing the caenagnathid mandible and this really is the best one to choose, but also because unlike noncaenagnathoid archosaurs the articular surface is complete exposed in both medial and lateral views. This means I can choose a measurement axis such as “ventralmost point in the articular cotylus to the ventralmost point on the mandible perpendicular to the long mandibular axis.” We’ll get to that next post. This is harder to measure in a jaw such as that of AMNH FR 5356, holotype of Dromaeosaurus albertensis Matthew and Brown (1922), where the lateral surface of the surangular (and a portion of the lip of the articular) obscures this point in lateral view, and thus leaving aside this issue makes this job simpler — or at least until I must confront it.
Step 2. Align.
So what is the best alignment? To determine alignments, I took CMN 8776 and applied a variety of benchmarks to it:
Benchmark A: Where the mandible is oriented so that the rostral tip of the dentary and the coronoid process form a horizontal line, instead of using characteristics of the ventral margin.
Benchmark B: Ventral margin of the jaw is oriented horizontally, while the ventral curvature of the dentary and the deflection of the retroarticular process posteriorly are excluded. This means the benchmark is the rest of the mandible except those features.
Benchmark C: Where the mandible is oriented so that the rostral tip of the dentary and the dorsal surface of the articular form a horizontal line.
Benchmark D: Ventral margin of the jaw is oriented horizontally, and includes the ventral curvature of the dentary and the ventral point of the retroarticular process. This is the position that reflects the “typical” orientation in papers, where the jaw is aligned how it would sit on a worktable or other flat surface and viewed from the side.
Benchmark E: Where the mandible is deflected ventrally from the position in C by 15° (as explained below this section). The gma (gross mandibular axis; see figure) becomes nearly horizontal in this position, but is actually horizontal in a position rotated slightly ventrally (not shown).
Benchmark F: Where the apex of the tomial margin directly rostral/mesial to the coronoid process (the “peak” in the sigmoid curvature of the dentary’s dorsal margin) and the dorsal surface of the articular form a horizontal line.
There are several reasons to prefer any of these, because they can all be useful benchmarks. But I wanted an independantly useful benchmark to affirm the alignment, so I looked at the skull’s alignment.
Primarily, there are several useful benchmarks in the skull. First, there’s the bsl (basal skull length), which is reconstructed here on the left as a blue line from the ventral margin of the quadrate to the tip of the rostrum. Due to the incompleteness of the material, however, I have to guess at the relationship of the mandible, and this is tricky. Second, there’s the lpa (long palatal axis), which is here shown to be roughly parallel to the bsl. There’s a third benchmark, and an elegant one, which I require using an oviraptorid to show:
The ebl (endocranial basal length) refers to a line drawn along the ventral floor of the foramen magnum rostrally into the ventral floor of the endocranial vault. This is roughly (~3°) from the ventral jugal axis which I typically use to determine skull attitude, and so it serves as a very functional benchmark. Moreover, it roughly parallel to the tma (tomial mandibular axis), which is depicted in F above. This has functional use for setting a baseline for measurement largely because it corresponds to the static, “alert” posture of the head in most terrestrial vertebrates (birds, mammals, and quadrupedal reptiles such as crocodilians and turtles: de Beer, 1947; Dujim, 1951; Erichsen et al., 1989), which I should clarify is when the head is held at rest or in a relaxed condition, rather than during “active” situations, such as feeding. I should note that this posture is inferred for some taxa relating to the lpa, and then deflected from the horizontal around 15°. In oviraptorids at least, this is problematic: When the head is held in “alert” posture, where the ebl is horizontal (B, in the figure directly above), the lpa is declined to nearly 30°. This is due to the unusual ventral deflection and “eversion” of the palate in lateral view, such that virtually every palatal bone is exposed below the maxillary/jugal margin. In caenagnathids, this is untested, because all we possess (described) is a maxilla and possible palatine, and these are shaped differently from that of oviraptorids. Despite this, resolving the basal skull attitude requires making an assumption towards the “general” attitude, and this can only be done by making a substantive stab at resolving general postures during behaviors (such as drinking, feeding, “alert,” etc.); we can only resolve the correlates described by others and equate them to the most stable baseline, that of the endocranial floor, and thus the ebl. While de Beer (1947) and Dujim (1951) implicated a close association with this to the orientation of the horizontal semicircular canal, there is considerable variation involved that those authors noted (see also Taylor et al., 2009), and I further cannot use the position of the horizontal semicircular canal because this information is unavailable.
When positioning the skull to the ebl benchmark, we arrive at the F benchmark of the mandible (horizontal red line in B, above), the tma, and this gives us a useful function position for the mandibular attitude from which to calculate length, height, etc. But because the skull of a caenagnathid is virtually unknown, we must hypothesize the possible positions of the relevant elements that correspond to our benchmark just to figure out how the head was oriented. Meanwhile, we get to settle on placing the mandible with the tma at the horizontal, which looks like this:
This will be the main benchmark I will be using when I begin working on the measurements part of this project. Note that while this posture alters the relative height to length ratios we’ll be getting, it will not alter the relative length ratios by more than 3%, due to the relatively low level of rotation required to place the jaw in this position. It does, however, substantially alter relative angles, such as the mandibular symphysis angle relative to either gma or lma. And this is why getting your benchmarks set will effect how others can reproduce your results and allow you to alter your paradigm to correspond.
Abbreviations used above, just to use as a reference:
bsl – basal skull length. This is a line drawn from the posteriormost point on the lateral articular condyle of the quadrate to the rostral and ventral tip of the premaxilla. In taxa with teeth, the mean of the tips of the first pairs of teeth are typically used, but I would position this line instead toward the rostral end of the relevant alveolus (base of the crown) rather than the tip. But we’ll get there eventually.
ebl – endocranial basal length. This is a line drawn along the floor of the foramen magnum from the occipital condyle through the floor of the basal endocranium.
gma – gross mandibular length. This is a line drawn from the midline of the jaw, and from the absolute rostral to absolute posterior points on the mandible. We’ll be getting to this in detail next post in the series.
lma – long mandibular axis. This is a line drawn from the rostral end of the mandible to the posterior extent of the articular face. In taxa with concave articular faces, this value is different, which I will get to on the follow-up post.
tma – tomial mandibular axis. This is a line drawn from the apex of the dorsal “peak” of the sigmoidal curvature of the tomial margin of the dentary to the posterior extent of the articular face.
Clark, J. M., Norell, M. A. & Rowe, T. 2002. Cranial anatomy of Citipati osmolskae (Theropoda, Oviraptorosauria), and a reinterpretation of the holotype of Oviraptor philoceratops. American Museum Novitates 3364:1-24.
Currie, P. J., Godfrey, S. J. & Nessov, L. A. 1994. New caenagnathid (Dinosauria: Theropoda) specimens from the Upper Cretaceous of North America and Asia. Canadian Journal of Earth Sciences — Revue de Canadienne des Sciences de la Terre 30:2255-2272. [Published in 1994, dated 1993.]
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Duijm, M. 1951. On the head posture in birds and its relation to some anatomical features. II. Proceedings of the Koninklijke Nederlandse Akademie van Wetenschappen (Series C) 54:260–271.
Erichsen, J. T., Hodos, W., Evinger, C., Bessette, B. B. & Phillips, S. J. 1989. Head orientation in pigeons: Postural, locomotor and visual determinants. Brain Behavior Evolution 33:268–27.
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