Stepping aside from the issue of actual measurement, I continue from the last post in this series with a discussion on what and how to call the elements we are measuring. As may have been indicated before, I am a func-morph kinda guy. That’s functional morphology, and is intimately associated with the argument that expressions of morphology ultimately derive from constraints imposed by biomechanical principles, and that for the most part, spandrels (Gould & Lewontin, 1979) don’t really exist.
While I am very fond of a standardized system of osteological nomenclature (Baumel & Witmer, 1993), the names for things are incorrigible in general (I’m paraphrasing Wittgenstein), and sometimes off-putting to English speakers. I’d like a technical nomenclature that is both standardizable, but also approachable. If Latin be it, I’d just re-adapt the system to use that lingo. For now, though, I will use a functional English system. I will use this to do three things: First, name the bones. This one’s super-easy, as “dentary” and what-not are simplistic. Second, I will use a system that extends my short-lived Precision in Nomenclature series, starting with my system of annotating phalanges. I will adapt the system used for vertebral lamina (Wilson, 1999) (and the adapted vertebral fossae system, Wilson et al., 2011) by using a shorthand for positional and element terms, combine them, and apply it — that is, it will follow a basic adjective-noun-noun form (e.g., “rostral dentary process”). Third, and this will comprise the next post in the series, I will use a similar system to describe the axes of orientation and measurement that this post and the last were building to.
In the figures that follow, and into the next post, there will be several types of labels used, and none of them will use a full English word. Everything will be abbreviated. To avoid confusion, I will use three types of abbreviations: The names of bones are in UPPERCASE (or “all-caps”), and comprise only three-character terms (the full list is below); terms for “features” such as fossae, fenestrae, processes, etc., are in lowercase, and follow the adjective-noun-noun form (e.g., “rdp” for “rostral dentary process”); and the terms for axes and measurements are in italicized lowercase. To further separate them, the terms for bones and features are emboldened, while those for axes are not. This will avoid confusion as to what is considered more important in a given figure, save where only axes are being referred to in the figure, in which case they will be the emboldened ones.
To make it clear that certain closely-associated terms are not confused, a system for the abbreviation for bone names in terms will also be used apart from one to label bones, thus allowing a non-repeating set of terms: “d” in rdp means “dentary,” not “dorsal,” but “dorsal” will still appear as it is the most effective term for the orientation toward the margin of the jaw (“marginal” might work, but is ambiguous due to lack of use). Ligatures “ar,” “an” etc. will separate terms for bones that start with the same letter (articular, angular); this method is better to me than using a unique letter for each bone address in the term, and follows what is typically done. Finally, some features use similar letters, and these will also be separated by ligatures: “f,” “fo” and “fe” denote “foramen,” “fossa” and “fenestra,” respectively; “prong” is not used, nor is “crest” — these will be systematized to “process” (“p”) and “ridge” (“r”), although the words in Latin (processus and crista) are the same for either synonym. Where useful, “articular facet” is the term given to the region of bone where two bones contact one another, such as the complex relationship of the dentary with the postdentary bones, and is abbreviated “[bone]af.”
As before, I will be using CMN 8776, holotype of Caenagnathus collinsi Sternberg (1940) as the basis for my illustrations, as modified from Currie et al. (1994).
The mandible as shown above has not been modified to fit the orientation process I mentioned previously. This is because I simply modified it directly from a published source; but, it gets the job of explaining what things are done.
ART (ar) – Articular, os articulare
ANG (an) – Angular, os angulare
COR (c) – Coronoid, os coronoideum
DEN (d) – Dentary, os dentale
PRD (pd) – Predentary, “os predentale”
PRE (pa) – Prearticular, os prearticulare
SPL (sp) – Splenial, os spleniale
SUD (sd) – Supradentary, “os supra-dentale“*
SUP (sa) – Suprarticular, “os supra-articulare“*
SUR (su) – Surangular, os supra-angulare
* These bones only appear in a small number of archosaurian taxa, and may be limited to just one taxon. In some cases, they are the product of ossification of ligaments or cartilage, although the “supradentary” has been considered part of the ancestral tetrapodal coronoid system (of which there are three distinct elements, only the last of which remains in archosaurs as an ossified element). The predentary is known in some birds, but is not consistent to a particular general clade, while it is universally present in all Ornithischia (some doubt has been raised to its presence in Pisanosaurus mertii (Casamiquela, 1967)). They are included here for consistency, and because they do not add to the list too much to be distracting.
adsap – anterodorsal surangular process
adsys – anterior dentary symphysis sulcus
amfo – anterior mandibular fossa**
avsap – anteroventral surangular process
cp – coronoid process
ctc – chorda tympani groove
ctf – chorda tympani foramen
des– dental sulcus
emfe – external mandibular fenestra
icoarr – intercondylar articular ridge
ider – interdental ridge
imfe – internal mandibular fenestra
laco – lateral articular cotylus
maco – medial articular cotylus
mf – Meckel’s foramen (Meckelian foramen)
mg – Meckel’s groove (Meckelian groove or channel)
mmnf – major mandibular nutrient foramen
mpmvfo – m. pterygoideus medialis ventralis fossa***
nf – nutrient foramen
ng – nutrient groove
pddp – posterodorsal dentary process
pdsys – posterior dentary symphysis sulcus
pledf – posterolateral external dentary foramen
pmfo – posterior mandibular fossa**
ptm – posterior tomial margin
pvdp – posteroventral dentary process
pvsap – posteroventral surangular process
rarp – retroarticular process
rarph – retroarticular process “heel”
rarr – retroarticular ridge (posterior extent of the surangular ridge, sar)***
sap – surangular process
sdpl – subdental platform
tm – tomial margin
vdsys – ventral dentary symphyseal sulcus
vsap – ventral surangular process
** A fossa typically surrounds the external mandibular fenestra (emfe) on the lateral surface of the mandible. This expands onto the lateral face of thedentary (anterior mandibular fossa, amfo) and the lateral surface of the postdentary complex (posterior mandibular fossa, pmfo). These are the apparent portions of the fossa that also extends dorsally and ventrally, to minor degrees, and is clearly bounded in the dorsal direction at the sap by a small ridge that divides an anterior and posterior facet; this ridge is absent in oviraptorids, and appears to be present only in some caenagnathid jaws, regardless of the presence of the sap. The dorsal mandibular fossa (dmfo) is present on the anterior facet, and is limited by the ridge further posteriorly. In oviraptorids, this facet is posterior to the emfe, but is divided from the pmfo not by a ridge but by the lateral bowing of the surangular; while this feature may be analogous to the ridge described in caenagnathid jaws, it is not clear, and I am choosing not to label it here.
*** This feature is typically bounded dorsally by the surangular ridge (sar, not included), due to the prevalence of attachment sites for ligaments on both the dorsal and ventral facies of the ridge. These dorsally-attaching ligaments are not actually present in this region, and the fossa is located far back and beneath the overhanging lateral articular facet (laco), so that the boundary for attachment is the articular itself, rather than the surangular. A slight ridge on the retroarticular process (rarp) forms part of this boundary site, but posterior to the articular I will refer to it as the retroarticular ridge (rarr), although it is located on the surangular and is homologous with the sar.
This list is not exhaustive, as it lacks features that describe the mandibles of other taxa, including and especially those with teeth, multiple tooth rows, etc. When it comes time to discuss such a list, it will be more comprehensive and thus more useful than this.
Now consider something:
Virtually all of these features refer to elements of the mandible that vary among Oviraptorosauria. With over 35 features listed, and the mean feature potentially having a diagnostic relationship in phylogeny to at least two discrete “characters,” it is likely that such a mandibular index can provide a low end of 70 mandibular characters that vary among oviraptorosaurs. This list excludes the list of proportional and positional relationships, description of the “shapes” of things, and measurement axes which I will be getting to, but which may meaningfully triple the total number of “characters.” This list also does not include the terms of “faces” of bones, their explicit distinction from one another, the position and extent of ligament attachment scars (with a few exceptions, see “mpmvfo” above), the relationship and shapes of the hyoid apparatus and the muscles of the jaw that attach to it, and the overall relationship of the mandible to the body skeleton. Whether these characters are meaningful, however, remains to be known. Including those would simply add what I estimate to be about 100 additional “characters.” In case this seems high, I suspect given the totality of variation in the mandibles of terrestrial vertebrates, I suspect it is rather middling, as I am also excluding the very LARGE range of tooth-based characters that could be added to assess variation. I’d probably then cut that total number by a third for the “phylogenetically meaningful” characters, for which the distinction is based solely on post-analysis examination of the list. Thus, I’d get around 350 characters, and maybe above 200 phylogenetic characters for the mandible alone, for archosaurs (and like I said, this is the middling value).
A strong emphasis in this analysis has been on the bony anatomy of the jaw, but because it’s designed to help describe the functional anatomy of the jaw, I cannot discard discussion of the soft-tissue anatomy of the jaw, and in this respect, I will expand the discussion past that of the mandible by segueing into that of the likely rhamphothecal covering (“beak”) that would have altered the appearance and the relationship of the jaw margin to a degree that alters some of the things I’ve so far discussed. The relevance of this issue is, in fact, likely more important than that discussing the terms for the jaw, but I felt it necessary to set the ground rules in how I would be calling things and what those things are.
Baumel, J. J. & Witmer, L. M. 1993. Osteologia [Osteology]. in Baumel, King, Breazile, Evans & Berge (eds.) Handbook of Avian Anatomy: Nomina Anatomica Avium. Publications of the Nuttall Ornithological Club 23:45-132.
Casamiquela, R. M. 1967. Un nuevo dinosaurio ornitisquio triásico (Pisanosaurus mertii; Ornithopoda) de la Formación Ischigualasto, Argentina [A new Triassic ornithischian dinosaur (Pisanosaurus mertii; Ornithopoda) from the Ischigualasto Formation, Argentina]. Ameghiniana 4(2):47-64.
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.]
Gould, S. J. and Richard C. Lewontin. 1979. The spandrels of San Marco and the panglossian paradigm: A critique of the adaptationist programme. Procedings of the Royal Society of London, B 205:581-598.
Sternberg, R. M. 1940. A toothless bird from the Cretaceous of Alberta. Journal of Paleontology 14(1):81-85.
Wilson, J. A. 1999. A nomenclature for vertebral laminae in sauropods and other saurischian dinosaurs. Journal of Vertebrate Paleontology 19(4):639-653.
Wilson, J. A., D’Emic, M. D., Ikejiri T., Moacdieh, E. M. & Whitlock, J. A. 2011. A nomenclature for vertebral fossae in sauropods and other saurischian dinosaurs. PLoS One 6(2):e17114. doi:10.1371/journal.pone.0017114