In addition to his many contributions to the basic anatomy and nomenclature of the osteohistology of extant vertebrates, Armand de Ricqlès has been more instrumental than any other researcher of the past half century in elucidating the structure and anatomy of the bone tissues of extinct vertebrates and in guiding the field in interpreting their meaning and application to a variety of important paleobiological problems. As a result of his pioneering work, which began with his doctoral thesis and has continued through five decades of collaborative research, we are now able to answer definitively many questions about the growth, physiology, function, and paleoecology of extinct tetrapods. In some cases we can even clarify their taxonomic status in ways unavailable through gross anatomical studies. This would have been unimaginable several decades ago, and it demonstrates how, thanks largely to the work and influence of Armand de Ricqlès, palaeohistology has been thoroughly integrated into palaeobiology.
Outre ses nombreuses contributions à l’anatomie et à la nomenclature de base de l’ostéohistologie des vertébrés existant encore, Armand de Ricqlès a joué, plus que quiconque, un rôle décisif dans le demi-siècle passé, en élucidant la structure de l’anatomie des tissus osseux de vertébrés disparus, en étant le guide en ce domaine, par son interprétation de leur signification et leur application à nombre d’importants problèmes paléobiologiques. Le résultat de son travail de pionnier, qui a commencé par sa thèse de doctorat et qui s’est poursuivi au long de cinq décades de recherche en collaboration, est que l’on peut à présent répondre définitivement à nombre de questions sur la croissance, la physiologie, la fonction et la paléoécologie des tétrapodes disparus. Dans certains cas, il est même possible de clarifier leur statut taxonomique, ce qui n’eût pas été possible par des études anatomiques grossières. Cela eût été inimaginable quelques décades plus tôt et montre combien, en grande partie grâce au travail et à l’influence d’Armand de Ricqlès, la paléohistologie a été complètement intégrée dans la paléobiologie.
Palaeontology, like many other sciences, began to take shape during the enlightenment, and it did so in fits and starts. Fossilized objects (literally, those “dug up” from the Earth) originally included minerals, meteorites, and archaeological remains, as well as those to which we would now restrict the term “fossil”. Once it was established that these were not sports of nature or works of the devil but remains of formerly living plants and animals, it was possible to begin to interpret their meaning. By the late
The field of paleohistology developed with similar fits and starts, and like the science of paleontology in general, it relied to a great extent on the actualistic assumption: that is, that extinct organisms can be understood through what is known of extant ones. Insights into the ecology and functional morphology of extinct vertebrates have historically been based on the general (and sometimes specific) resemblances of their body parts to those of extant counterparts, closely related or not (
The later studies of Foote, Gross, Seitz, Ørvig, and others elucidated the microscopic structure of the tissues of more and more extinct vertebrates, but special mention must be made of the landmark studies of Donald H. Enlow (
The results of Armand de Ricqlès's doctoral dissertation were published as a series of twelve papers in
When the bone tissue types of tetrapods were sorted by phylogenetic groups, it appeared that some types of bone were restricted to some kinds of tetrapods, and that some tetrapods seldom if ever produced certain kinds of bone tissues. Within particular major lineages, there were wholesale transitions between tissue types in the long bones that clearly reflected a rise in growth and presumably metabolic rates; these were seen in synapsids (
Armand was keenly interested in the connection between bone tissue type and metabolic physiology, even from his earliest work. One of his first papers (
In the late 1980s, Armand began a collaborative project with John R. Horner of the Museum of the Rockies at Montana State University, Bozeman. Jack Horner's discoveries of nests, eggs, and skeletons of embryonic and recently hatched dinosaurs in the Late Cretaceous formations of Montana had stimulated his interest in the growth and development of dinosaurs (
It has really only been in the past two decades that access to materials for paleohistological study has been anything but strictly limited. There is a simple reason for this. Until the recent series of papers by Armand and his colleagues, with their insights into the growth rates and skeletochronology of bones and their implications for many aspects of the biology of fossil tetrapods, palaeohistology attracted very little interest. It could still be said that most colleagues in the field are generally innocent of its basic principles and insights, and that not much attention is given to them in the training of vertebrate paleontologists. This is nothing new: since the mid 19th century, there has seldom been more than one or a very few specialists in this field, from Owen and Quekett to Foote, Seitz, Gross, Ørvig, Enlow, and of course de Ricqlès. Consequently, few curators in charge of fossil vertebrates have had much sympathy with the prospect of seeing their prize specimens destroyed under the rock saw for the arcane benefit of paleohistological shamans and their obscure and trivial stores of knowledge. At best a researcher could hope for an isolated long bone of good quality to section; at worst, a fragment of a rib or shaft of sometimes ambiguous provenience. The advantage of the collaboration between Armand and Jack was that Jack had collected all his dinosaurs and was very sympathetic to the goal of understanding their growth, so he had no reservations about sectioning them as needed. Paleohistological research could thus proceed systematically.
The importance of this advance cannot be overestimated, because for the first time a comprehensive study of the ontogeny of a group of extinct vertebrates could be undertaken with the assurance that researchers could control the exact skeletal element and location of sections taken in individuals from the earliest growth stages to adults, and that they could be compared with their relatives and with other taxa under the same controlled conditions. Before this, even in the classical paleohistological literature, plates of photographs typically presented sections taken from a femur here, an ulna here, a rib there, and all from various positions within the bone.
By the early 1990s the two researchers had sketched out a plan to study the growth and evolution of bone tissues in dinosaurs – not simply individual dinosaur taxa, but the entire group, as well as their extinct relatives among the archosaurs. They identified four major signals (or “factors” or “influences”) on the appearance of bone tissue in any region of a skeleton at any given time. These were ontogeny, which, echoing the great insight of Rodolfo
Other workers such as Robin Reid and Anusuya Chinsamy were beginning to make valuable contributions to the histological knowledge of individual dinosaurs, and to formulate hypotheses to explain the distribution of bone tissue types among taxa.
One of the first projects de Ricqlès and Horner and their colleagues undertook was a small one, but it underscored the potential of bone histology to approach paleobiological problems. Bob Harmon of the Museum of the Rockies had collected the wing skeleton of a Late Cretaceous pterosaur about 2 m in wing span. Its morphology showed that it belonged to the Azhdarchidae, a group of Late Cretaceous pterodactyloids that included
Armand's vast knowledge of bone tissue structure, development, and evolution formed the basis for a research program that would realize the vision that he and Jack Horner shared for understanding the life history strategies of dinosaurs. They determined to study the bone histology of as many archosaurian taxa as possible in their ontogenetic and phylogenetic frameworks. Although researchers from This line of research may have been inspired, in part, by the successful use of histology in the taxonomy of Paleozoic finned vertebrates, whose dermal skeleton includes, at least in some cases, a variety of tissues (enamel or enameloid, dentine, spongy and compact bone), some of which form complex structures (isolated odontodes, ridges, cosmine, etc.) (
To establish which bones were best for histological analysis, sections from all of the major long bones of the hadrosaurid dinosaur
The preservation of a nearly complete growth series, from embryo to adult, of the hadrosaur
In later years de Ricqlès, Horner, and their colleagues extended their research to a comparison of embryonic bone tissues in dinosaurs and a variety of living and fossil reptiles (including birds); the evolutionary changes in growth rates during the evolution of birds from dinosaurs and during the early history of birds; and in the evolution of trends in growth rates and size in theropod and ornithischian dinosaurs, including some “bizarre structures” in thyreophorans (
In addition to many works on the bone histology of archosaurs, Armand had a strong interest in all tetrapods, as his dissertation research indicates. Some of the principal groups that attracted his interest include temnospondyls (
These works have generally laid the groundwork for placing palaeohistology in broad ontogenetic and phylogenetic context, in combination with many other contemporary researchers such as Jacques Castanet, Anusuya Chinsamy, Jorge Cubo, Kristina Curry Rogers, Greg Erickson, Michel Laurin, Martin Sander, and Torsten Scheyer. The review here of course does not encompass the full scope of his work in paleontology; for other references please consult the bibliography of Armand de Ricqlès's works prepared for this volume and
From his earliest work, Armand de Ricqlès incorporated analysis and review of major evolutionary concepts into his work. The value of bone histology in interpreting thermal physiology (
The perspective from his work on fossil tetrapods brought substantial insight to collaborative reviews on the bone microstructure of extant tetrapods. In particular, the kinds of bone tissues and their distribution among extant tetrapods show a disjunct distribution, but when actualistic data are complemented with information from extinct taxa, gaps are bridged and other possibilities not seen in the living fauna are revealed. Four papers necessary to the education of every bone histologist are those by
It would be remiss not to consider Armand de Ricqlès's contributions to evolutionary theory through his understanding of palaeobiology and macroevolution. Particularly in France, where the Modern Synthesis of Evolution did not have as strong a reception as in America and Britain, his papers have informed and educated the French scientific community as well as the public. He wrote a great many papers in popular French science magazines and journals, as well as in professional venues (e.g.,
Armand de Ricqlès has left an unparalleled legacy in French biology, and his legacy has extended worldwide through his work in many aspects of paleontology and palaeobiology, including the description of specimens, the analysis of tooth replacement patterns, the evolution of dermal skeletal elements, the secondary return of tetrapods to an aquatic existence, and the evolution of mineralized tissues in general. But his greatest body of work, of course, is in deciphering the ontogenetic, phylogenetic, physiological, and mechanical signals left in bone tissues, both fossil and extant. He has taught generations of students and colleagues how to decipher these signals, and in so doing has made the palaeohistology of bone a more popular, fruitful, understood, and integrated field of study than ever before. Future advances in this field will be laid pre-eminently at his doorstep.
I thank Armand de Ricqlès, first and foremost, for decades of instruction, collaboration, friendship, and recreation with various forms of the internal combustion engine and the graphic novel (Vive Blake et Mortimer!). I am grateful to Michel Laurin and Jorge Cubo for inviting me to participate in this symposium and to all the students and staff who made it happen. Funding was graciously provided by the Collège de France, the Muséum National d’Histoire Naturelle, and the University of California Museum of Paleontology. This is UCMP contribution No. 2026.