A Jugal in the Rough: The Evolutionary Importance of the Zygoma.

Abstract

You never forget your first. So, it was for me. It was on a Wednesday in the late Fall of 1973. Around Halloween, when nothing can match the crisp, invigorating air of New England. It was my beginning term at Yale as a grad student and my first time living away from my little hamlet of Brooklyn. Everything was new, somewhat strange, and even a bit scary. Everyone was very nice to me, though, and the advanced graduate students were always very helpful. Problem was that they were just so darn smart and accomplished, especially the ones on the paleontology side of the ledger, Yale having produced a long, unbroken line of super-heavyweights in this area. Indeed, the recent and soon-to-be grads alone—Ian Tattersall, Phil Gingerich, Rich Kay, Glen Conroy, Peter Dodson, to name but a few—giants now, were publishing and already becoming well-known in the field. Earlier in the day, I tagged along after my Anatomy tablemate Peter Dodson to Kline Biology Tower to have lunch with a few recent grads and current geniuses. Ian was up from New York where he recently started as Curator of Anthropology at the American Museum of Natural History where he would produce a legacy as great as any who came before. Three of our profs joined us: the greatest primate paleontologist of the 20th century Elwyn Simons; his frequent coinvestigator and young dynamo David Pilbeam, also a Yale grad; and the only non-paleo-type, the recently hired, newly minted Ph.D. lemur biologist from Cambridge, Alison Richard (who could, btw, hold her own quite well among all these male egos; she went on to become Yale Provost and then the first woman Vice-Chancellor of Cambridge in its history and is now Dame Alison to boot!). Lunch entailed riveting stories by Elwyn or Glenn (mostly Elwyn; what a raconteur!) of finding fossil primates in the Fayum of Egypt or what hot new ideas John Ostrom was espousing on the relationship of dinos to birds. Amongst this group, they must have seen and known the name of every fossil mammal, reptile, and bird at Yale! I, on the other hand, only had familiarity with fossils from exhibits at the American Museum of Natural History, and the closest I came to a real one was being unceremoniously thrown out of that museum after trying to climb on a brontosaurus as a boy (long story; see Laitman, 2010.) I had never even held a real fossil bone up close; a pathetic, paleontological virgin! Heck, I wasn't even sure that I was going to pass Anatomy! Amongst these heavy-hitters, I felt like a total nebbish and a real pisher (my mind reverts to my ancestral language, Yiddish, at times like this: a nebbish is an unfortunate, to be pitied; a pisher is a little squirt, a nobody.) I don't think I ate, let alone spoke, for the hour we were there. All I could think about was why did I ever leave Brooklyn? They were all so interesting, sophisticated, and had seen and done so much; I was afraid I would never fit in. When was the next bus back to Flatbush? “You don't want to go back to gross lab, do you?” I was jolted out of my mental meanderings by Peter's voice. “I thought that we could go over and look at some new stuff I've been cleaning at the Museum. Want to come?” “Uh, yeah, sure. That would be great.” Peter, is, and was, always the kind and perceptive teacher who could see when a student was lost. He guided me through Anatomy lab at Yale, and has since guided thousands more as a revered Professor of Veterinary Anatomy and Vertebrate Paleontology at Penn, not to mention the world's leading expert on those funky dinosaurs with horns on their heads (see Dodson, 1996, 2009; Laitman, 2009). The “museum” was the Peabody Museum of Natural History. I learned to love this gem during my years at Yale, and it became my second home (well, third after assorted Pizza locales; New Haven has Yale and pizza, that's about it.) I always liked the idea that the museum was founded and financed by a wealthy businessman, George Peabody, so that his nephew Othniel C. Marsh (Yale Class of 1860; nobody's named Othniel any more) would have a place to keep his fossils (Marsh was appointed Professor of Paleontology at Yale in 1866, the first in the United States, and Curator of the Museum the following year; good to have a rich uncle!) So on this crisp, autumn day, with orange-red-gold leaves wafting around us, I followed Peter into the Museum, past Rudolf Zallinger's unsurpassed “Age of Reptiles” mural, through winding corridors and hallways, and eventually to a host of metal cabinets filled with a trove of skeletal fragments. Peter was a pure dinosaur paleontologist, while I studied primates, and his goodies were massive compared to the tiny, little Eocene and Pliocene paleo-primates I had been hearing about in lectures. He opened one of the cabinets that had “Marsh, 1870 something” on the door (almost all did) as the material had been transported back from the American west during the famous “Bone War” expeditions of the 1870s (see Shor, 1974). Peter lugged out a part of a face of something with some horn stubs protruding. “Go ahead, see what you can piece together,” he extolled, “make it fit.” So, with nervous hands and some trepidation I touched my first, actual, real, honest-to-goodness, dinosaur bone! “What's this thing?” I asked as I looked at something that seemed to fit around portions of an eye-socket. “Looks like a partial jugal.” “A what?” I asked, thinking he was teasing me. “A Jew-what? C'mon Peter, just because I'm from New York don't start with…” “No, Woody Allen! I said JUGAL,” he chuckled. “It's the ancestor of what you mammal-types call the zygoma. Crelin (Edmund S. Crelin, our Anatomy Professor, later my primary advisor and surrogate father) spoke about this already. We use it a lot to help position the other orbital bones. It can also give insight into who's related to whom.” Then he started. “Let's take a look at some of the Late Cretaceous ceratopsians here - open this draw here—here hold this—see another partial jugal—oh, here is a, I think from Pentaceratops…” And he was off to the races, now speaking a mile a minute. As anyone who has spent even a few moments with Peter knows, get him in room with dinosaurs, particularly ceratopsians (the horned dudes, he's their king!) and he's gone (see Dodson, 1996, 2009.) But, I was in heaven! A virgin no longer, a pisher no more. I was in a Cretaceous Park playground and felt like a real paleo-maven holding a partial jew-bone, or jugal, or whatever. Skip forward some 40-plus years and here we are in the second part of our Special Issue of The Anatomical Record, “Understanding the Zygoma: A key morphofunctional partition in the craniofacial skeleton,” Guest Edited by Paul Dechow with the assistance of Qian Wang, both of Texas A&M University College of Dentistry. As I described in my Commentary to the first part (Laitman, 2016), The Anatomical Record is proudly publishing a two-part Special Issue for the first time. The first part appeared in our December 2016 issue of the Journal, and focused on the zygoma's “Development, Adaptation, Structure and Function” (Dechow and Wang, 2016). We now dig under the rocks and look further and deeper into the nooks and crannies to see how the anatomy and biomechanics morphed and changed through the millennia in this, the continuation of our Special Issue, now exploring the zygoma's “Evolution” (Dechow and Wang, 2017, this issue.) The first part of this special issue laid out a virtual smorgasbord of studies detailing cutting-edge research on the development, adaptation, and biology of the bone. I had noted in my commentary that the zygoma could previously be considered one of the most underappreciated bones of the skull; hopefully, even a brief perusal of the excellent studies reported therein will convince any interested in what lies above the shoulders that it is no insignificant, little “yoke,” but a major functional linkage within the skull. This second chapter, so to speak, looks at how the zygoma has been altered through the varied paths of vertebrate evolution. And what a journey it has been! First off, a little on the names—zygoma, jugal, malar—as they are a little confusing. For those who study reptiles, amphibians, birds and their assorted evolutionary ancestors like primitive synapsids, dinosaurs, therapsids and the like, the bone is referred to as the jugum, jugal bone or, most often, jugal. For you etymology-lovers, it is from the Latin, jugum, meaning yoke. In many of these taxa, the jugal is quite large, and indeed can be as large, if not larger, than the maxilla. For example, in ancient anapsids like cotylosaurs (“stem-reptiles”) from the Carboniferous of the Paleozoic Era (some 360 to 300 mya) the jugal can be relatively massive, forming most of the inferior border of the orbit and most of the upper jaw posterior to the maxilla. A dominant jugal can be found in numerous fossil vertebrate taxa through the ensuing Triassic, Jurassic, and Cretaceous Periods. Indeed, while it's relative in mammals and humans may not have been the most appreciated bone to many, the jugal is a king among bones to paleontologists who need to reconstruct shards of fossil cranial remnants. The transition from mammal-like reptiles to mammals was also accompanied by reductions in the relative size of the previously dominant jugal (Hogben, 1919; see also discussion in Moore, 1981.) It is difficult to pinpoint exactly when, and for which taxa, the terminology diverged, at least my limited knowledge cannot find the edict. It may be, however, that those working on mammals found the new, relatively reduced bone of many mammalian species so different from the dominant jugal of earlier vertebrates that they were more comfortable using a Greek synonym, that is, “zygoma” (also meaning yoke) for “jugal” to describe what they saw as it was something substantively different from the jugal of nonmammalian forms. In addition, a true “zygomatic” arch with the jugal/zygoma now taking on a new function as anterior anchor of such also set it apart from non-mammals (see Moore, 1981; also Marquez et al., 2017, this volume.) These anatomic distinctions between mammals and other vertebrates may thus have determined the dichotomy of usage that was to follow: zygoma became the standard for mammal workers, while those looking outside our Class remained with jugal. (To add a little more confusion, as if any was needed, the Latin word “malar,” meaning cheek, sometimes appears. This is used most often in reference to humans. It is the term least used by evolutionary morphologists.) As they have done in the first volume, Dechow and Wang have here collected a band of merry jugal/zygoma-tologists to explore the morphings and meanderings of this bone from the earliest jawless fish (agnatha), through assorted dinosaurs and birds and their relatives (I don't know about you, but I find it a bit unsettling that some bird is a descendent of a Velociraptor and may be eyeing me for dinner), to extraordinary comparative reviews of mammals, to a number of discussions of the bone and region among human ancestors ranging from early Plio-Pleistocene australopiths to those ever pesky Neanderthals. Whether one is a seasoned craniologist with years of experience looking into every facial fossa or a neophyte to this realm, this tour-de-force on the evolutionary twists and turns of a single bone will hold you riveted. It did me, and I've already studied these lumps and bumps in hundreds of living and extinct vertebrate species. So, it's fall again and my family has felt obliged to do something for my birthday weekend. You, know, the “let's-do-something-with-dad-quality-time” routine. They choose; I go. Then they don't feel so guilty about the rest of the year. I usually hate wherever they take me. “Dad, anything special you want to do this year?” asks my eldest who thinks because he's gotten his PhD and is getting his MD from my school that seeing me in the elevator counts as quality-time. “Hey dad,” chimes in my youngest, the one in Law School, who has argued with me since she could breathe, “we found an interesting place to take you. We're going to Yale, your old haunts. There's a neat museum there. It says that they even let people come in and touch old bones and stuff. Sounds like you'll like it.” How 'bout that? They actually found a place I'd like to go. Maybe I'll sneak away and see if I can find my old jugal bone. You know, you never forget your first.