Arrays in rays: terminal addition in echinoderms and its correlation with gene expression

The echinoderms are deuterostomes that superimpose radial symmetry upon bilateral larval morphology. Consequently, they are not the first animals that come to mind when the concepts of segmentation and terminal addition are being discussed. However, it has long been recognized that echinoderms have serial elements along their radii formed in accordance with the ocular plate rule (OPR). The OPR is a special case of terminal growth, forming elements of the ambulacra that define the rays in echinoderms. New elements are added at the terminus of the ray, which may or may not be marked by a calcified element called the terminal plate (the "ocular" of sea urchins). The OPR operates in every echinoderm, from the occasionally bizarre fossils of the Cambrian to the most familiar extant taxa. Using the OPR and other criteria of recognition, echinoderm body wall can be divided into two main regions: extraxial components are associated with the somatocoels, axial components (formed in accordance with the OPR) with the hydrocoel. We compare patterns of development in axial regions of echinoderms with those found in the anterior-posterior axes of the earliest echinoderms as well as other invertebrates. Although axial and extraxial skeletons appear to be composed of the same biomineral matrix, the genes involved in patterning these two skeletal components are likely distinct. During development of the axial skeleton, for instance, the genes engrailed and orthodenticle are expressed in spatial and temporal patterns consistent with the OPR. Other genes such as distal-less seem to demarcate early ontogenetic boundaries between the axial rudiment and the extraxial larval body. There is a complex and pervasive reorganization of gene expression domains to produce the highly divergent morphologies seen in the Echinodermata. We integrate morphological and genetic information, particularly with respect to the origins of radial symmetry in the rudiment, and the concomitant development of the rays.     

Evolution Within a Bizarre Phylum: Homologies of the First Echinoderms

SYNOPSIS. The Extraxial/Axial Theory (EAT) of echinoderm skeletalhomologies describes two major body wall types: axial and extraxial.The latter is subdivided into perforate and imperforate regions.Each of the regions has a distinctly different source in earlylarval development. Axial skeleton originates in the rudiment,and develops in association with the pentaradially arrangedhydrocoel according to specific ontogenetic principles. Perforateand imperforate extraxial regions are associated with the leftand right somatocoels respectively, are not governed by ontogeneticprinciples of plate addition, and are products of the non-rudimentpart of the larval body. The morphology of even the most bizarreof the earliest echinoderms can be explored using the EAT. Amongthese, edrioasteroid-like taxa best fit the idea that formsexpressing archimery in the sequential arrangement of axial,perforate extraxial, and imperforate extraxial regions are thefirst echinoderms. Metamorphosis is especially marked in cladesthat have a high axial to extraxial skeleton ratio because structuresdeveloping from the non-rudiment part are suppressed in favorof the developing axial elements during this process. However,inearly echinoderms, extraxial skeleton makes up a far largerproportion of the body wall than axial, implying that metamorphosiswas not as significant a part of the developmental trajectoryas it is in more recently evolved taxa. Echinoderm radiationconsists of a succession of apomorphies that reduced the expressionof extraxial components but increased the influence of axialones, with a concomitant increase in the prominence of metamorphosis.     

Evolution to the extreme: origins of the highly modified apical system in pourtalesiid echinoids

The apical system of the genus Pourtalesia displays a plate architecture that falls so far outside that typical of other echinoids that plate homologies remain problematic. A new approach using the Extraxial–Axial Theory (EAT) that develops homologies for the Echinodermata is proposed. The exploration of apical plate patterns throughout ontogenetic sequences shows that the typical holasteroid pattern found in the youngest specimens undergoes a series of disturbances that result in a multiple disjunction accompanied by isolation or disappearance of certain genital plates. We propose a new interpretation of the apical architecture of the genus that agrees with: (1) the plate addition processes as predicted by the EAT; (2) patterns observed in other genera of the Pourtalesiidae as well as in its sister‐group (plexechinids); and (3) the patterns known from Palaeocene holasteroids. In the context of the EAT, the genus Pourtalesia appears to represent the extreme in a reduction of the extraxial part of the body wall. © 2004 The Linnean Society of London, Zoological Journal of the Linnean Society, 2004, 140 , 137–155.     

Occurrence of a microcanalicular system within the ossicles and dermal tissue of Asterias rubens and Marthasterias glacialis (Echinodermata: Asteroidea)

Summary A microcanalicular network is demonstrated within the ossicle stroma and the dermal tissue of two asteroid species. Microcanaliculi are presumed to be mesodermal structures. They consist of convoluted tubular ducts lined by epithelial cells associated with scattered basiepithelial nervous processes. Such a microcanalicular system has not been reported previously from any echinoderm species. Its discovery in asteroids entails some conceptual changes, especially considering the physiology of the body wall.Research assistants of the National Fund for Scientific Research (NFSR, Belgium)     

Arms versus brachioles: Morphogenetic basis of similarity and differences in food-gathering appendages of pelmatozoan echinoderms

The similarity in the skeleton model of the brachiolar food-gathering system of Blastozoa and the arm system of Crinozoa, including the apical growth with enantomorphous displacement of skeletal ele-ments, is explained by the primary organizing role of the radial ambulacral canals, which have the same branching model for ambulacral tentacles. The difference in the positions of brachioles and arms relative to the theca (exothecal and endothecal) is associated with the formation of the primary ambulacral tentacles directly on the body surface of the majority of Blastozoa, particularly, the closed vestibular cavity of crinoids. The supporting skeleton of brachioles arose as a branch of the plates covering the floor of the ambulacrum, if they were present, or formed similarly as a new formation outside the theca. The supporting skeleton of arms, brachials, developed as a result of the serial growth of plates positioned radially at the boundary of the aboral skeleton and tegmen formed due to the appearance of the vestibulum. The hypothesis of the inductive role of hydrocoel and its radial ambulacral appendages, which organize the arrangement of skeletal elements in the morphogenesis of echinoderms, enables the refinement of the principle of skeleton division into the axial and extraxial parts. The axial skeleton has a developmental model formed under the control of the radial ambu-lacral canals. Remaining skeleton is extraxial, subdivided into the symmetrized part arranged under direct or indirect organizing effect of the hydrocoel and unregulated, nonsymmetrized part, which is not connected initially with the influence of the hydrocoel.     

Increased Epicardial Adipose Tissue Is Associated with the Airway Dominant Phenotype of Chronic Obstructive Pulmonary Disease

Background

Epicardial adipose tissue (EAT) has been shown to be a non-invasive marker that predicts the progression of cardiovascular disease (CVD). It has been reported that the EAT volume is increased in patients with chronic obstructive pulmonary disease (COPD). However, little is known about which phenotypes of COPD are associated with increased EAT.

Methods

One hundred and eighty smokers who were referred to the clinic were consecutively enrolled. A chest CT was used for the quantification of the emphysematous lesions, airway lesions, and EAT. These lesions were assessed as the percentage of low attenuation volume (LAV%), the square root of airway wall area of a hypothetical airway with an internal perimeter of 10 mm (√Aaw at Pi10) and the EAT area, respectively. The same measurements were made on 225 Vietnamese COPD patients to replicate the results.

Results

Twenty-six of the referred patients did not have COPD, while 105 were diagnosed as having COPD based on a FEV₁/FVC<0.70. The EAT area was significantly associated with age, BMI, FEV₁ (%predicted), FEV₁/FVC, self-reported hypertension, self-reported CVD, statin use, LAV%, and √Aaw at Pi10 in COPD patients. The multiple regression analyses showed that only BMI, self-reported CVD and √Aaw at Pi10 were independently associated with the EAT area (R² = 0.51, p<0.0001). These results were replicated in the Vietnamese population.

Conclusions

The EAT area is independently associated with airway wall thickness. Because EAT is also an independent predictor of CVD risk, these data suggest a mechanistic link between the airway predominant form of COPD and CVD.     

Pale biological implications of some Upper Ordovician juvenile asteroids (Echinodermata)

Blake, D. B. 1990 10 15: Pale biological implications of some Upper Ordovician juvenile asteroids (Echinodermata). Lethaia , Vol. 23, pp. 347–357. Oslo. ISSN 0024–1164.
Two patterns have been recognized in the early dorsal skeletal development of modern asteroids. The skeleton of well-preserved juveniles of the Late Ordovician species Promopalaeaster finei is similar in fundamental ossiclar arrangement to one of these patterns, suggesting continuity of developmental sequence. Similarities include presence of large terminal ossicles and double rows of marginal ossicles. Ancient and modern juveniles differ in that in P. finei , podial pores leading to the interior of the arms are lacking, thus providing an ontogenetic argument that such pores are phylogenetically derived within the class. An unpaired interbrachial marginal, or axillary, is present distal to the oral frame; positioning supports earlier suggestions that the axillary is the homologue of the odontophore ossicle of modern asteroids. Comparisons between P. finei and another Ordovician species, Macroporaster matutinus , suggest the need for greater mouth frame flexibility contributed to the evolution of the modern odontophore. Axillary development is linked to the common absence of actinal ossicles from Paleozoic species. ▭ Asteroidea, Echinodermata, functional morphology, ontogeny, phylogeny, Ordovician, Paleozoic .     

The fine structure and development of calcified skeletal elements in the body wall of holothurian echinoderms

The calcareous ossicles and iron-rich calcified granules in the integument of sea cucumbers (Echinodermata: Holothuroida) have been examined by light and electron microscopy. Most ossicles are perforated, platelike structures that measure 100–500 μm long and consist of magnesium-rich calcite. The formation of ossicles occurs within multinucleated syncytia of sclerocytes that are situated in the dermal layer of the body wall. Sclerocytes possess fairly large numbers of mitochondria and are characteristically enveloped by an external lamina. Each ossicle arises as a minute rodlike spicule that branches and develops into a fenestrated skeletal element. When viewed by SEM, fully developed ossicles appear non-crystalline. Following decoration with synthetically grown calcite seeds, however, each ossicle reveals an ordered array of crystallites that seem to be aligned perpendicular to the ossicle's original plane of growth. Examinations of sectioned ossicles indicate that the mineral phase in each skeletal element lacks a well developed matrix of EDTA-insoluble organic substances. Ossicles first arise in the ciliated, lecithotrophic larvae of Eupentacta quinquesimita at about 1 week postfertilization. Two-week-old specimens adopt an epibenthic existence and subsequently become enveloped by imbricated ossicles that are secreted by sclerocytes resembling those found in adults. In molpadiid holothurians, the adult body wall contains numerous reddish-brown granules that are chiefly composed of iron-rich subunits. The dermal granules differ from calcitic ossicles in developing extracellularly in association with finely filamentous material.     

Oral Region Homologies in Paleozoic Crinoids and Other Plesiomorphic Pentaradial Echinoderms

The phylogenetic relationships between major groups of plesiomorphic pentaradial echinoderms, the Paleozoic crinoids, blastozoans, and edrioasteroids, are poorly understood because of a lack of widely recognized homologies. Here, we present newly recognized oral region homologies, based on the Universal Elemental Homology model for skeletal plates, in a wide range of fossil taxa. The oral region of echinoderms is mainly composed of the axial, or ambulacral, skeleton, which apparently evolved more slowly than the extraxial skeleton that forms the majority of the body. Recent phylogenetic hypotheses have focused on characters of the extraxial skeleton, which may have evolved too rapidly to preserve obvious homologies across all these groups. The axial skeleton conserved homologous suites of characters shared between various edrioasteroids and specific blastozoans, and between other blastozoans and crinoids. Although individual plates can be inferred as homologous, no directly overlapping suites of characters are shared between edrioasteroids and crinoids. Six different systems of mouth (peristome) plate organization (Peristomial Border Systems) are defined. These include four different systems based on the arrangement of the interradially-positioned oral plates and their peristomial cover plates, where PBS A1 occurs only in plesiomorphic edrioasteroids, PBS A2 occurs in plesiomorphic edrioasteroids and blastozoans, and PBS A3 and PBS A4 occur in blastozoans and crinoids. The other two systems have radially-positioned uniserial oral frame plates in construction of the mouth frame. PBS B1 has both orals and uniserial oral frame plates and occurs in edrioasterid and possibly edrioblastoid edrioasteroids, whereas PBS B2 has exclusively uniserial oral frame plates and is found in isorophid edrioasteroids and imbricate and gogiid blastozoans. These different types of mouth frame construction offer potential synapomorphies to aid in parsimony-based phylogenetics for exploring branching order among stem groups on the echinoderm tree of life.     

A re‐interpretation of the ambulacral system of Eumorphocystis (Blastozoa,Echinodermata) and its bearing on the evolution of early crinoids

Recent debates over the evolutionary relationships of early echinoderms have relied heavily on morphological evidence from the feeding ambulacral system. Eumorphocystis, a Late Ordovician diploporitan, has been a focus in these debates because it bears ambulacral features that show strong morphological similarity to early crinoid arms. Undescribed and well‐preserved specimens of Eumorphocystis from the Bromide Formation (Oklahoma, USA) provide new data illustrating that composite arms supported by a radial plate that bear a triserial arrangement of axial and extraxial components encasing a coelomic extension can also be found in blastozoans. Previous reports have considered these arm structures to be restricted to crinoids; these combined features have not been previously observed in blastozoan echinoderms. Phylogenetic analyses suggest that Eumorphocystis and crinoids are sister taxa and that shared derived features of these taxa are homologous. The evidence from the arms of Eumorphocystis suggests that crinoid arms were derived from a specialized blastozoan ambulacral system that lost feeding brachioles and strongly suggests that crinoids are nested within blastozoans.     

Energetic composition,reproductive output,and resource allocation of Antarctic asteroids

Summary Thirteen species of antarctic asteroids were collected in McMurdo Sound during the austral spring and summer of 1984–1985 and 1985–1986. Variability in the energetic composition of the body wall, pyloric cecum, and gonad was directly related to differences in levels of protein, carbohydrate, lipid and ash in these tissue types. Reproductive output was generally similar between males and females when expressed in gravimetric units. When considered in energy units, females generally had greater reproductive output than males. Reproductive output values of antarctic asteroids were similar to published values for temperate, tropical, and subpolar species. In a given year, the majority of antarctic asteroids have the greatest energy resources in the body wall, a tissue representing a long-term energy investment. Lesser amounts of energy are devoted to annual nutrient storage (pyloric ceca) and reproduction (gonads).     

Toxicity of shallow-water Antarctic echinoderms

Summary The toxicity of 23 species of shallow-water antarctic echinoderms from McMurdo Sound was investigated using Gambusia affinis (Vertebrata: Pisces) as a test organism. Ichthyotoxicity assays were conducted on the body wall tissues of thirteen species of asteroids and three species of holothuroids, the tests of three species of echinoids and the arms of three species of ophiuroids and one species of crinoid. Patterns of toxicity were class-specific, with varying degrees of toxicity occurring exclusively among the Asteroidea and Holothuroidea. Seven of the thirteen species of asteroids were mildly to highly toxic (54%), while one of the three holothuroids was mildly and one highly toxic. Toxicity was not related to the level of calcification of the body wall and energy level in asteroids. The only armored asteroid, Notasterias armata, was non-toxic. The occurrence of toxins in the body tissues of antarctic echinoderms is similar in pattern and frequency with that of temperate and tropical species.     

Ultrastructure of Axial Vascular and Coelomic Organs in Comasterid Featherstars (Echinodermata: Crinoidea)

Abstract The spongy body of Davidaster rubiginosa, D. discoidea, and Comactinia meridionalis, is an axial haemal plexus consisting of two structurally similar, but positionally distinct, regions: an oral circumesophageal part and an aboral part which lies lateral to the axial organ. The axial organ is a large axial blood vessel which is infiltrated by hollow cellular tubes lined with monociliated epithelial cells. The spongy body plexus is a tangle of small blood vessels overlain by podocytes and myocytes. The spongy body and the axial organ are situated in the axial coelom, which is confluent with the perivisceral coelom, the water vascular system, and the parietal canals. The parietal canals open to the exterior via ciliated tegmenal ducts and surface pores. The crinoid spongy body is morphologically similar to the axial gland of asteroids, ophiuroids, and echinoids (AOE). Although the axial glands of these three classes of echinoderms are mutually homologous structures, the homology of the crinoid spongy body and the AOE axial gland is questionable because of differences in organization and developmental origin. Alternatively, the crinoid spongy body may be homologous to asteroid gastric haemal tufts, which are podocyte-covered blood vessels suspended in the perivisceral coelom. The functional organization of the spongy body suggests a filtration nephridium and predicts an excretory function. An alternative hypothesis is that the spongy body is a site of nutrient transfer from the blood vascular system to the perivisceral coelom.     

Control of water uptake by rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.): role of the outer part of the root

A new pressure-perfusion technique was used to measure hydraulic and osmotic properties of the outer part of roots (OPR) of 30-day-old rice plants (lowland cultivar: IR64, and upland cultivar: Azucena). The OPR comprised rhizodermis, exodermis, sclerenchyma and one cortical cell layer. The technique involved perfusion of aerenchyma of segments from two different root zones (20-50 mm and 50-100 mm from the tip) at precise rates using aerated nutrient solution. The hydraulic conductivity of the OPR (Lp(OPR)=1.2x10(-6) m s(-1) MPa(-1)) was larger by a factor of 30 than the overall hydraulic conductivity (Lp(r)=4x10(-8) m s(-1) MPa(-1)) as measured by pressure chamber and root pressure probe. Low reflection coefficients were obtained for mannitol and NaCl for the OPR (sigma(sOPR)=0.14 and 0.09, respectively). The diffusional water permeability ( P(dOPR)) estimated from isobaric flow of heavy water was smaller by three orders of magnitude than the hydraulic conductivity (Lp(OPR)/ P(fOPR)). Although detailed root anatomy showed well-defined Casparian bands and suberin lamellae in the exodermis, the findings strongly indicate a predominantly apoplastic water flow in the OPR. The Lp(OPR) of heat-killed root segments increased by a factor of only 2, which is in line with the conclusion of a dominating apoplastic water flow. The hydraulic resistance of the OPR was not limiting the passage of water across the root cylinder. Estimations of the hydraulic properties of aerenchyma suggested that the endodermis was rate-limiting the water flow, although the aerenchyma may contribute to the overall resistance. The resistance of the aerenchyma was relatively low, because mono-layered cortical septa crossing the aerenchyma ('spokes') short-circuited the air space between the stele and the OPR. Spokes form hydraulic bridges that act like wicks. Low diffusional water permeabilities of the OPR suggest that radial oxygen losses from aerenchyma to medium are also low. It is concluded that in rice roots, water uptake and oxygen retention are optimized in such a way that hydraulic water flow can be kept high in the presence of a low efflux of oxygen which is diffusional in nature.     

Water permeability and reflection coefficient of the outer part of young rice roots are differently affected by closure of water channels (aquaporins) or blockage of apoplastic pores

The relative contribution of the apoplastic and cell-to-cell paths to the overall hydraulic conductivity of the outer part of rice roots (LpOPR) was estimated using a pressure perfusion technique for 30-d-old rice plants (lowland cultivar, IR64, and upland cultivar, Azucena). The technique was based on the perfusion of aerenchyma of root segments from two different zones (20-50 mm and 50-100 mm from the root apex) with aerated nutrient solution using precise pump rates. The outer part of roots (OPR) comprised an outermost rhizodermis, an exodermis, sclerenchyma fibre cells, and the innermost unmodified cortical cell layer. No root anatomical differences were observed for the two cultivars used. Development of apoplastic barriers such as Casparian bands and suberin lamellae in the exodermis were highly variable. On average, matured apoplastic barriers were observed at around 50-70 mm from the root apex. Lignification of the exodermis was completed earlier than that of sclerenchyma cells. Radial water flow across the OPR was impeded either by partially blocking off the porous apoplast with China ink particles (diameter 50 nm) or by closing water channels (aquaporins) in cell membranes with 50 micro M HgCl2. The reduction of LpOPR was relatively larger in the presence of an apoplastic blockage with ink ( approximately 30%) than in the presence of the water channel blocker ( approximately 10%) suggesting a relatively larger apoplastic water flow. The reflection coefficient of the OPR (sigmasOPR) for mannitol significantly increased during both treatments. It was larger when pores of the apoplast were closed, but absolute values were low (overall range of sigmasOPR=0.1-0.4), which also suggested a large contribution of the non-selective, apoplastic path to overall water flow. The strongest evidence in favour of a predominantly apoplastic water transport came from the comparison between diffusional (PdOPR, measured with heavy water, HDO) and osmotic water permeability (PfOPR) or hydraulic conductivity (LpOPR). PfOPR was larger by a factor of 600-1400 compared with P(dOPR). The development of OPR along roots resulted in a decrease of PdOPR by a factor of three (segments taken at 20-50 and 50-100 mm from root apex, respectively). Heat-killing of living cells resulted in an increase of PdOPR for both immature (20-50 mm) and mature (50-100 mm) root segments by a factor of two. Even though both pathways (apoplast and cell-to-cell) contributed to the overall water flow, the findings indicate predominantly apoplastic water flow across the OPR, even in the presence of apoplastic barriers. Low diffusional water permeabilities may suggest a low rate of oxygen diffusion across the OPR from aerenchyma to the outer anaerobic soil medium (low PO2OPR). To date, there are no data on PO2OPR. Provisional data of radial oxygen losses (ROL) across the OPR suggest that, unlike water, rice roots efficiently retain oxygen within the aerenchyma. This ability strongly increases as roots/OPR develop.     

Linear growth in spines from Acanthaster planci (L.) involving growth lines and periodic pigment bands

We report the first observations of a linear growth pattern in aboral spine ossicles of adult Acanthaster planci (L.). This is unlike the spine development of other echinoderms. Growth in aboral spine ossicles of A. planci is essentially by addition of stereom at the base and the spine's growth history is preserved along its length. There are numerous growth lines perpendicular to the long axis of the ossicle. These are clearly evident in longitudinal spine sections and apparently caused by frequent growth episodes. There are periodic pigment bands which are parallel to the growth lines and evident on the surface of the ossicle. Basal growth of the spine ossicle and the nature of the growth lines were confirmed by tetracycline staining. Size/frequency analyses of a population of A. planci from Davies Reef (GBR) found spine ossicle growth, but not body diameter growth, over the six month period between sampling dates. The additional pigment banding in spine ossicles of 4 individuals recaptured after 6 months suggests that pigment bands are laid down seasonally. If pigment band cyclicity is validated, it offers a simple method for ageing adults of A. planci in field populations.     

Involvement of EAT/mcl-1, an anti-apoptotic bcl-2-related gene, in murine embryogenesis and human development

Apoptosis plays an important regulatory role in mammalian embryogenesis and development. EAT/mcl-1 (EAT), an anti-apoptotic bcl-2-related gene, was isolated during the early differentiation of a human embryonal carcinoma cell line, an event which serves as a model of early embryogenesis. EAT is involved in apoptotic regulation and is believed to also function as an immediate-early gene. Thus it was hypothesized that EAT would be expressed during early embryogenesis and would be involved in the regulation of apoptosis during this critical period. To clarify this early expression, two antibodies to EAT were generated by use of immunizing oligopeptide (aa 37-55) and recombinant protein (aa 31-229) for use in immunohistochemistry and immunoblotting, respectively. With these antibodies, we then determined EAT expression during murine embryogenesis and in human development, using human fetal tissue of 6 to 23 gestational weeks. During murine embryogenesis, the EAT protein was found to be rapidly induced after fertilization, to peak at the 2-cell stage, to remain constant until the 8-cell stage, and then to decrease to below unfertilized egg levels in blastocysts. EAT expression patterns in early human development were found to essentially overlap those observed in adult tissues which suggest that EAT expression continues until adulthood in terminally differentiated tissues. Among tissues distinct to fetal development, EAT was detected in the mesonephric (Wolffian) duct and paramesonephric (Müllerian) duct. It is also noteworthy that prominent EAT immunoreactivity was also observed in large primary oocytes in 21-week fetal ovary, but was not detected in primordial germ cells in 23-week fetal testis. In summary, EAT expression was detected in hematopoietic, epithelial, neural, endocrine, and urogenital cells; this provides evidence that EAT, as an anti-apoptotic molecule, possibly functions to regulate apoptosis during development in these systems.     

Variabilité architecturale du test chez le genre Collyrites (Echinoidea, Disasteroidea) au Jurassique moyen

The aim of this paper is to study the evolution of the architecture of the « apical system–periproct complex » of the genus Collyrites (Echinoidea, Disasteroidea) for species from the Bathonian and Callovian stages (Middle Jurassic). The studied material comes from several localities of the Paris basin. The apical system of the genus Collyrites is subdivided in two parts: (i) an anterior part composed by four genital (1, 2, 3, 4) and three ocular (II, III, IV) plates, called trivium; (ii) a posterior part composed by two ocular plates (I, V) and one genital plate (5), without gonopor, which circle the periproct, called bivium. The genital plate 5, which may collapse in the periproct, is not always visible. The Bathonian–Callovian transition is marked by a subdivision of the bivium in two parts: the periproct breaks up from the posterior ocular plates (I, V). These morphological changes are associated with architectural modifications. The trivium stays relatively stable during the Bathonian and the Callovian, but a supplementary plate may be inserted into the trivium. The bivium shows important modifications linked to the separation of the periproct and the ocular plates (I and V). Such a separation is marked by a strong development of supplementary plates, these ones keeping in connection with the periproct and the ocular plates I and V. The supplementary plates are more and more developed whereas the distance between the periproct and ocular plates increases. The connection between the trivium and the bivium is similarly provided by supplementary plates. The size of these plates seems to significantly increase between the Bathonian and the Callovian. Moreover, some specimens from the Bathonian and the Callovian may have an atypical architecture with a supplementary genital plate or a genital plate with two pores. The “extraxial axial theory” allows to recognize two types of skeleton: (i) an “axial skeleton” corresponding to ocular plates and plates of the ambulacra and interambulacra; (ii) an “extraxial skeleton” corresponding to the genital and supplementary plates, and the periproct. Architectural modifications between the Bathonian and the Callovian is a result of a more important development of the “extraxial skeleton” while the “axial skeleton” shows few modifications during this time interval.