Denis O. Lamoureux

HISTOLOGY OF TOOTH ATTACHMENT TISSUES IN THE LATE CRETACEOUS MOSASAURID PLATECARPUS

Abstract We present new data on the tooth attachment histology of the Late Cretaceous marine lizard Platecarpus (Mosasauridae). Examination of thin sections of a right dentary reveals the presence of a woven-fiber bone matrix that forms the margins and floor of the tooth alveolus; this bony matrix is traditionally identified as bone of attachment. We identify it as alveolar bone based on its histologic and topologic similarities to archosaurian and mammalian alveolar bone. We also identify a cribiform plate, a structure usually associated with the periodontal ligament. Parallel fibers present in multiple, non-resorbed generations of alveolar bone are tentatively identified as remnants of mineralized portions of collagen fiber bundles, or Sharpey’s fibers. Along the sides of the dentine root we identify a thin layer of acellular cementum. The acellular cementum is surrounded by an enormous mass of cellular cementum tissue that fills the alveolus. This cementum mass is composed of two histologically distinct forms: (1) a loosely organized cellular cementum ground matrix; (2) a laminar form surrounding the vascularization (cementeons) that we term osteocementum. Mosasaurs possess the attachment tissues that are used to diagnose thecodont ankylosis. Mosasaur thecodonty is derived within the Mosasauroidea (aigialosaurs + mosasaurs).

Tooth histology in the cretaceous ichthyosaur Platypterygius australis, and its significance for the conservation and divergence of mineralized tooth tissues in amniotes

Ichthyosaurs are an extinct group of secondarily aquatic reptiles that show ligamentous tooth attachment to the jaw in some derived forms. Here, we provide a modern description of tooth histology in ichthyosaurs, using Platypterygius australis, a large ichthyosaur from the Cretaceous of Australia. Our study supports evolutionary conservation of the principal mineralized tooth tissue types in amniotes with ligamentous tooth attachment: enamel, dentine, cellular, and acellular cementum. This is the first time that the latter tissue has been located in ichthyosaurs. Vascularized cementum (osteocementum) is reduced or absent in amniotes in which the teeth are ankylosed to the jaw bone, such as basal ichthyosaurs, and raises questions regarding the function of this tissue and the potential developmental or selective conditions leading to its convergent evolution. J. Morphol., 2011.

The Structure and Phylogenetic Distribution of Amniote Plicidentine

ABSTRACT Plicidentine, infolded dentine around the pulp cavity at the base of a tooth, has been histologically characterized in sarcopterygians, basal tetrapods, and the actinopterygian Lepisosteus. Within amniotes, however, its phylogenetic distribution is much less well documented and its morphological variability has not been described, in spite of the fact that plicidentine is considered a synapomorphy of three major clades of amniotes (Neochoristodera, Ichthyosauria, and Varanoidea). In this study, we provide a summary of the taxonomic distribution of plicidentine in amniotes, and also describe the morphological variation encompassed by this term. We conclude that plicidentine, as defined here, has evolved independently several times in amniotes, and that each occurrence is histologically distinct.

Tooth histology, attachment, and replacement in the Ichthyopterygia reviewed in an evolutionary context

Ichthyosaurs, an extinct group of Mesozoic marine diapsids, show a relatively small range of tooth crown morphologies. With few exceptions, members of the group bear a large number of conical teeth and show only minor heterodonty within a jaw. This uniformity in gross morphology masks a high degree of variation in both the quantity and arrangement of the mineralized tooth tissues. Here, we describe tooth tissue structure and distribution in derived ichthyosaurs. We synthesize these new observations with the historical literature, to map changes in the quantity and arrangement of tooth tissues. These changes affected tooth attachment, tooth replacement, plicidentine morphology, and the amount and distribution of cellular cementum. The amount of variation detected in features relating to ichthyosaurian dentition is not surprising given the geological longevity and morphological disparity of the group, but does emphasize the importance of extensive taxon sampling in studies of tooth histology and evolution. This study is important in that it incorporates morphological and histological information in a phylogenetic and developmental context, something that is rarely done for marine reptile dentitions.KurzfassungDie Ichthyosaurier, eine ausgestorbene Gruppe von mesozoischen marinen Diapsiden, zeigen nur geringe morphologische Unterschiede der Zahnkronen. Die Mitglieder der Gruppe tragen, mit wenigen Ausnahmen, eine große Anzahl von konischen Zähnen, und zeigen nur geringfügige Heterodontie innerhalb eines Kiefers. Diese Einheitlichkeit in der Gesamtmorphologie verbirgt ein großes Ausmaß an Variation, sowohl was die Anzahl als auch die Anordnung der mineralisierten Zahngewebe betrifft. In der vorliegenden Arbeit beschreiben wir die Struktur und Verteilung des Zahngewebes bei Ichthyosauriern. Wir vergleichen diese neuen Beobachtungen mit solchen aus der Literatur, um Veränderungen der Zahngewebe bei Ichthyosauriern darzustellen. Diese Veränderungen betrafen Zahnbefestigung, Zahnersatz, Plicidentin-Morphologie und die Menge und Verteilung von zellulärem Zement. Der Grad an Variabilität, der bei den Merkmalen des Ichthyosauriergebisses festgestellt wurde, ist in Anbetracht der geologische Langlebigkeit und der morphologischen Unterschiede der Gruppe nicht überraschend, unterstreicht aber die Bedeutung der umfangreichen Probenahme an Taxa bei Untersuchungen der Zahnhistologie und -entwicklung. Diese Studie ist im Hinblick darauf wichtig, dass sie morphologische und histologische Informationen in einen phylogenetischen Zusammenhang und einen Entwicklungszusammenhang setzt, was sonst nur selten bei Gebissen von Meeresreptilien getan wird.

Histology of tooth attachment tissues and plicidentine in Varanus (Reptilia: Squamata), and a discussion of the evolution of amniote tooth attachment

Few recent studies have examined the histological basis for tooth attachment in squamates. In the past few years, a surge of interest in this topic has led to the intriguing suggestion that the major tissues derived from the tooth germ (enamel, dentine, cementum and alveolar bone), are conservative and are present in all amniotes. In this study, we describe the histology and development of the tooth attachment complex in Varanus rudicollis, the rough‐neck monitor. We provide the first published evidence for the role of cementum and alveolar bone in tooth attachment in varanoid lizards. In Varanus, cementum is deposited on the external surface of the tooth root as well as at the base of the tooth, where it plays a role in the attachment of the tooth to the jawbone. Alveolar bone is also involved in tooth ankylosis. Our results support the hypothesis that the major tooth germ tissues are found in all amniotes. We provide insights into the structure and development of plicidentine, defined as infolding of the dentine around the tooth base. This feature is unique to varanoids among extant tetrapods and is the third tissue implicated in tooth attachment in Varanus. Plicidentine develops asymmetrically along the labial‐lingual axis of a tooth. Varanus is characterized by the presence of both primary and higher‐order lamellae, which anastomose to form a honeycomb‐like surface that then interacts with the more basal attachment tissues. J. Morphol. 2011.

Evolutionary Creation: Moving Beyond the Evolution Versus Creation Debate

Evolutionary creation offers a conservative Christian approach to evolution. It explores biblical faith and evolutionary science through a Two Divine Books model and proposes a complementary relationship between Scripture and science. The Book of Gods Words discloses the spiritual character of the world, while the Book of Gods Works reveals the divine creative process. This view of origins recognizes that the Bible features an ancient conceptualization of nature, and consequently rejects concordism (or scientific concordism). It understands biblical revelation in the light of the Incarnation and suggests that Scripture was accommodated for an ancient Near Eastern mindset. Evolutionary creation holds a traditional notion of natural revelation. The reflection of intelligent design extends to the process of evolution, rejecting the God-of-the-gaps creative method, and declaring the faithfulness of the Creators evolutionary mechanisms.

Was Adam a Real Person

Belief in the historicity of Adam has been held firmly throughout the history of the church. In the light of modern biblical criticism and the evolutionary sciences, some conservative Christians are now questioning whether or not Adam was a real person. This paper argues that the existence of Adam in the opening chapters of scripture reflects an ancient understanding of biological origins. More specifically, the quick and complete (de novo) creation of life is the result of retrojecting an ancient phenomenological perspective of living organisms back in time to the origin of the world. The apostle Pauls references to Adam are rooted in this ancient scientific understanding of human origins. In moving beyond the belief in the historicity of Adam, this paper concludes that Adam is an incidental, though necessary, ancient vessel that transports inerrant messages of faith regarding the human spiritual condition.

Histological evidence for a dynamic dental battery in hadrosaurid dinosaurs

The first histological study of an entire hadrosaurid dental battery provides a comprehensive look at tooth movement within this complex structure. Previous studies have focused on isolated teeth, or in-situ batteries, but this is the first study to examine an entire dental battery of any dinosaur. The absence of direct tooth-to-tooth contact across the entire battery and a unique arrangement of the dental tissues in hadrosaurids led us to compare their teeth with the ever-growing incisors of mammals. The similarity in the distributions of tissues along the incisor, coupled with continuous eruption, make for helpful comparisons to hadrosaurid teeth. The mammalian ever-growing incisor can be used as a model to extrapolate the soft tissue connections and eruptive mechanisms within the hadrosaurid dental battery. Serial sections across the adult dental battery reveal signs of gradual ontogenetic tooth migration. Extensive remodeling of the alveolar septa and the anteroposterior displacement of successive generations of teeth highlight the gradual migration of tooth generations within the battery. These eruptive and ontogenetic tooth movements would not be possible without a ligamentous connection between successive teeth and the jaws, underscoring the dynamic nature of one of the most unique and complex dental systems in vertebrate history.

Mosasaurs and snakes have a periodontal ligament: timing and extent of calcification, not tissue complexity, determines tooth attachment mode in reptiles

Squamates present a unique challenge to our understanding of dental evolution in amniotes because they are the only extant tooth‐bearing group for which a ligamentous tooth attachment is considered to be absent. This has led to the assumption that mammals and crocodilians have convergently evolved a ligamentous tooth attachment, composed of root cementum, periodontal ligament, and alveolar bone, whereas squamates are thought to possess a single bone of attachment tissue that fuses teeth to the jaws. The identity and homology of tooth attachment tissues between squamates, crocodilians, and mammals have thus been a focal point of debate for decades. We provide a novel interpretation of the mineralized attachment tissues in two focal taxa in this debate, mosasaurids and snakes, and compare dental tissue histology with that of the extant crocodilian Caiman sclerops. We identify a periodontal ligament in these squamates that usually exists temporarily as a soft connective tissue anchoring each tooth to the alveolar bone. We also identify two instances where complete calcification of the periodontal ligament does not occur: in a durophagous mosasaur, and in the hinged teeth of fossil and modern snakes. We propose that the periodontal ligament rapidly calcifies in the majority of mosasaurids and snakes, ankylosing the tooth to the jaw. This gives the appearance of a single, bone‐like tissue fusing the tooth to the jaw in ankylosed teeth, but is simply the end stage of dental tissue ontogeny in most snakes and mosasaurids.

Tooth germ initiation patterns in a developing dentition: An in vivo study of Xenopus laevis tadpoles

The transparency of soft tissue in Xenopus laevis tadpoles and the anterior‐posterior orientation of their developing tooth germs in the upper jaw offer a unique opportunity for the in vivo charting of the first 15–20 days of the developing dentition. Twenty‐two X. laevis tadpoles were anesthetized daily and their mouths opened to record the first appearance, position, and development of tooth germs in the upper jaw. The initiation patterns revealed considerable variability between animals, and even between the jaw quadrants in the same animal. This variability appears within a structural boundary and the results are consistent with the presence of an odontogenic band. The final length of dental rows far exceeded the jaw growth for each quadrant during the recording period. This in vivo investigation underlines the limits of cross‐sectional studies, and in particular the assumption that tooth germs initiate at the same position in the dental row. The tooth germ initiation patterns in this study did not align with the predictions of standard models for the development of the dentition—Zahnreihen, Clone, and New Progress Zone theories.