Hallmarks amongst these are Owen's and Peyer's seminal works, and the volumes edited by Miles and Teaford et al. However, large gaps still persist in the literature regarding the development and evolution of actinopterygian and squamate dentitions. Where and when did teeth arise?
According to the classical theory, teeth are derived from skin odontodes dermal denticles that came to reside within the oral cavity when competent odontode-forming cells invaded the latter in conjunction with the origin of jaws e. Both teeth and odontodes are composed of a dentin cone with a base of acellular or cellular bone-like tissue. In most cases the dentin is covered by a hypermineralized layer consisting of enamel or enameloid Reif, Both structures have a pulp cavity that contains odontoblasts, connective tissue, nerve fibres and blood vessels.
Extant chondrichthyans sharks and rays have retained odontodes, called placoid scales Reif, This hypothesis, with M. Further support for their hypothesis was seen in the idea that the splanchnocranium to which pharyngeal denticles are attached should be considered different in origin from the integumentary skeleton and skin denticles cf.
It is worth noting however, that in addition to pharyngeal denticles, thelodonts also possess an integumentary skeleton characterized by numerous, minute odontodes similar to chondrichthyan odontodes Janvier, ; Sire et al. In zebrafish, the presence and position of teeth on the last branchial arch has led to the widespread acceptance that pharyngeal teeth develop from endodermal epithelium. Clearly, the debate is ongoing.
Here, we propose an alternative hypothesis that integrates both paleontological and embryological data, and is consistent with the above described findings. We remove, however, the conodonts from the discussion given that only the Euconodonta are currently recognized as vertebrates, yet that there is still no consensus about the structural homology of their denticles with any kind of vertebrate odontode Scott, ; Morris, ; Pridmore et al.
These ectodermal odontodes developed inside the oropharyngeal cavity as a result of competent ectoderm migrating inwards — not only via the mouth, but also via each of the gill slits Fig. The odontogenic potential of the ectodermal epithelium may have been subsequently transferred to the endoderm, provided there was an intimate contact between these two germ layers, such as where mouth and gill slits form. Consequently, provided pharyngeal denticles or teeth indeed develop from endodermal epithelium, the adjacent presence or even physical contact of ectoderm with endodermal epithelium would have been a requirement for their development.
Yet, unequivocal evidence for an endodermal origin of pharyngeal teeth or denticles in extant primitive gnathostomes, and by inference in extinct thelodonts, still needs to be collected.
To date, strong evidence for endodermal participation in tooth formation has been collected only for urodele amphibians. However, careful examination of the reports of the many experimental studies that have been carried out on salamanders in the last century, indicates that ectoderm and endoderm are both required to form teeth e. Working with Pleurodeles waltl , Chibon , , on the other hand, noted that the odontoblasts exert an inductive action upon the epithelium, which can be an ectodermal epithelium in anterior teeth , an endodermal epithelium in posterior teeth , or one of mixed origin.
A recent study on the axolotl Ambystoma mexicanum using GFP transgenes has confirmed Chibon's hypothesis that the epithelial component of the teeth can be both ectodermal and endodermal Soukup et al. Nevertheless, the experiments of Soukup et al. In these experiments, it remains to be clarified whether the participation of endoderm in the enamel organs is not just an expression of a potential not normally displayed during regular development, rather than the normal in vivo capacity. Other not normally odontogenic tissues have indeed been shown to be able to express an odontogenic potential.
For example, using the same species A. Graveson's experiments were nevertheless not a test of whether teeth form from endodermal or ectodermal epithelium. Among mammals, Imai et al. Comparison of the arrangement of the gill slits, the position of the branchial arches and of the gills in agnathans A and gnathostomes B,C. Odontodes and dermal bones are indicated in red.
A,B Extent of ectoderm blue and endoderm yellow as usually assumed. In C we postulate contra B that the ectoderm penetrates further inwards arrowheads , and possibly covers the endoderm, as observed by Edwards cf.
The key issue of our revised hypothesis is the assumption that odontogenic competent ectoderm invaded the oropharyngeal cavity through both the mouth and the gill slits. Even if odontogenic competence would have been transferred from ectoderm to endoderm i. Paleontological data can be reconciled with this view as long as it can be shown that the taxa known to possess pharyngeal denticles possessed gill slits, or other structures permitting ectoderm to penetrate into the body, such as the nasopharyngeal duct including Rathke's pouch or the spiraculum corresponding to the opening between mandibular and hyoid arch cf.
One may thus assume that they could develop because of an ectodermal contribution through this duct. Most osteostracans have slit-shaped external gill openings, the condition in tremataspidids with small rounded openings being derived; extant agnathans hagfish and lampreys and most extinct agnathans anaspids, astrapidids and galeaspids have small rounded-shaped external gill openings Janvier, , p. However, even the possession of reiterative slits is not a requirement, as physical contact between ectoderm and endoderm, without the formation of an open gill slit proper, could be sufficient for interactions to occur between the two tissue layers.
In the zebrafish, the first tooth anlage appears after a connection has been established between ectoderm and endoderm in the form of an elongate, initially two-cell-thick strand bridging the skin with the pharynx on both sides, well before the actual formation of the gill slit within this strand Fig. Interestingly, Jackman et al. In our view, the altered tooth morphogenesis likely results from an altered signaling from the pouch epithelium. In , Edwards performed a detailed embryological study of mouth and pharynx formation in the carp Cyprinus carpio , a family member of the zebrafish that also forms pharyngeal teeth.
By careful analysis of histological sections, he concluded that in the tooth-forming region, the pharyngeal epithelium was composed of a superficial layer of flattened cells, derived from migration of ectoderm, overlying a layer of endodermal epithelium. Edwards also reported that the enamel organs were derived from the deep endodermal layer, but clearly illustrated the close contact between ectodermal and endodermal layers Fig. Three stages in the development of the gill slits and pharynx of the carp Cyprinus carpio.
A—C respectively 36, 56 and 78 h post-fertilization. An ectodermal plug blue invaginates the endodermal pharyngeal folds yellow A and forms a layer of flattened ectoderm-derived cells blue on top of the columnar endoderm-derived epithelial cells yellow B,C. The latter produce the enamel organs of the teeth. Semithin sections of forming pharyngeal pouches in zebrafish Danio rerio at 56 h A and 72 h B post-fertilization. A An epithelial connection black arrowheads is seen between the epidermis white asterisks and the foregut black asterisk.
B The first tooth germs white arrowhead are forming whilst the gill slits are still closed black arrowheads , and the pharyngeal lumen opens black asterisk.
Evolutionary and developmental origins of the vertebrate dentition
Upon closer investigation, we view these interpretations as doubtful based on the following: Also, whereas differential gene expression patterns in oral vs. In our view there is no proof for germ layer-related fundamental developmental differences between teeth and odontodes. In addition, first-generation teeth, widely considered to be representative of an ancestral type of teeth, develop from the superficial epithelium in most non-mammalian osteichthyans examined so far Sire et al.
These teeth form a series with a whorl-like arrangement due only to space constraints. A successional lamina is virtually non-existent in these early tooth generations; it only becomes prominent at older developmental stages Huysseune, Finally, one should also consider the possibility that patterning of pharyngeal denticles into families as exemplified in thelodonts may well be the outcome of a heterochronic shift of patterning of the crowns of skin denticles.
Merging of pharyngeal denticles into a single unit would yield a single crown with a herring bone-like morphology comparable to that of skin denticles. Interestingly, this tooth morphology is similar to that of most pharyngeal denticles in basal chondrichthyans and osteichthyans. Different types of dental lamina black arrowheads during tooth replacement. Replacement tooth formation without the presence of a dental lamina A, Atlantic salmon, Salmo salar ; with a transient, successional dental lamina B, zebrafish, Danio rerio , and C, jewel cichlid, Hemichromis bimaculatus , or with a permanent dental lamina D, Pleurodeles waltl , a urodele amphibian; E, Chalcides sexlineatus , a scincid lizard; F, human first lower deciduous molar.
F , courtesy of Ralf J. The presence of teeth in the pharyngeal cavity could thus have been an early event in vertebrate evolution whereby ectoderm invaded through the gill slits, possibly interacting with endoderm, thereby being involved in pharyngeal denticle teeth development. One obvious question to ask is why pharyngeal teeth did not evolve more often?
Denticles cover the median part of the oral roof in some osteostracans, which, together with thelodonts, are the only agnathans with oral or pharyngeal denticles Janvier, The relative rarity of pharyngeal denticles in agnathans may be related to differences in the distribution of ectoderm and endoderm at the gill slits, as evidenced by the differing origins of gill filaments in modern lampreys and gnathostomes.
Early in the 20th century it was discovered that lamprey gill filaments are endodermal in origin, and, alongside the branchial nerves and blood vessels, reside medial to the skeletal gill arches Goette, In contrast, among gnathostomes, gill filaments are ectodermal in origin and, at least in osteichthyans, are positioned lateral to the skeletal gill arches Goette, ; Janvier, If ectoderm penetrated also less deeply into the gill slits in extinct agnathans, or only late, this could be a reason why pharyngeal denticles are not more widespread in agnathans.
Our hypothesis states that teeth originated prior to jaws, but only when the formation of gill slits allowed for a relatively deep invasion of ectoderm into the oropharyngeal cavity and extensive physical contact between ectoderm and endoderm. Such a scenario may explain why pharyngeal teeth, located deeply within the oropharyngeal cavity, were lost in early tetrapods and maintained only on the margins of the jaws and roof of the oral cavity.
We propose that loss of gill slits, and the ability for the ectoderm to invade, leads to the disconnection of the competent ectodermal epithelium and the endodermally lined pharynx.
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Data gleaned from the fossil record supports this conclusion. For example, among Tetrapodomorph fish, the group of lobe-finned fish sarcopterygians that is generally believed to have given rise to tetrapods, with Eusthenopteron late Devonian, Ma as an important representative, the internal surfaces of the gill arch elements are covered with tooth plates Nelson, ; Carroll, The branchial skeleton of Ichthyostega , an upper Devonian Ma amphibian close to the ancestry of all later terrestrial vertebrates, is poorly known, but the animal possibly had a small gill slit Janvier, Acanthostega falls outside the Neotetrapoda, a group that is characterized by the closure of the gill slit Janvier, Branchial tooth plates have also been reported for the basal temnospondyl amphibian Colosteus scutellatus upper Carboniferous, Ma Hook, Interestingly, Schoch noted the coincidence between the loss of branchial denticles, considered to be homologous to teeth, and the loss of gill slits in temnospondyls Fig.
Schoch suggested a functional explanation for the simultaneous loss of both branchial denticles and gill slits: Paleontologists indeed tend to use the presence of denticulated pieces of bone branchial ossicles, sensu Schoch, in temnospondyl amphibians as evidence for the presence of a cartilaginous branchial skeleton Boy, and open gill slits e.
Berman, ; Schoch, We propose that the simultaneous loss of gill slits and of pharyngeal denticles teeth is not just functionally but also developmentally related, as the loss of gill slits could prevent the invagination of odontogenic, or inductively competent, ectoderm. However, it is important to note that the presence of gill slits does not necessarily predict the development of branchial denticles.
For example, lungfish Dipnoi lack pharyngeal denticles. This is assumed to be a secondary loss, possibly related to the extensive evolution of the dentition during the early history of the group Ahlberg et al. The loss could be analogous to the loss of teeth on the different gill arches in teleosts.
Branchial denticulated plates branchial ossicles in the pre-metamorphosis stage of Onchiodon labyrinthicus , a temnospondyl amphibian from the lower Permian reproduced from Schoch, , Fig. Also significant to our hypothesis is that in no other tetrapods have pharyngeal teeth, or branchial denticles, ever been observed, despite the presence of pharyngeal endoderm, and despite the likely presence of segmentally arranged ectodermal—endodermal contacts. We speculate that such contacts are constituted solely of outpockets of endoderm abutting the ectoderm, and that without ectodermal invagination into the body, tooth development is not initiated.
This will be easy to test once a reliable marker for endoderm is available. In the urodele amphibian Ambystoma mexicanum , endodermally derived oral teeth are formed in close proximity to the invaginated ectoderm Soukup et al. In our view, the concomitant lack of gill slits and pharyngeal teeth in tetrapods is a strong argument for the ectodermal origin of teeth. Even if the competence to form teeth had been transferred during evolution from ectoderm to endoderm, and the ectoderm would still be required as an inductive tissue, tooth formation would be blocked because of the loss of extensive contact between the two embryonic layers ectoderm and endoderm.
Major Events in Early Vertebrate Evolution - CRC Press Book
Given that the spiraculum, similar to other gill slits, could allow ectoderm to migrate inwards, it is interesting to note that Eusthenopteron retains denticles inside the spiracular canal Jarvik, Most tetrapod stem group members are now assumed to retain an open spiraculum Clack, Possibly, the rise of the tympanic membrane could have been the ultimate event that definitively sealed off this route for ectodermal migration. Systematics Association Special Volumes.
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Already read this title? Please accept our apologies for any inconvenience this may cause. Add to Wish List. Toggle navigation Additional Book Information. Limit the search to the library catalogue. Dialipina and the characters of basal actinopterygians English. Coneference, Major events in early vertebrate evolution: Major events in early vertebrate evolution: For LUH campus users we will be happy to check if free access is available for you. Ask for availability for LUH. Table of contents conference proceedings The table of contents of the conference proceedings is generated automatically, so it can be incomplete, although all articles are available in the TIB.
The origin and early fossil history of the acustico-lateralis system, with remarks on the reality of the echinoderm-hemichordate clade.