The early ontogeny of Jurassic thecideoid brachiopods and its contribution to the understanding of thecideoid ancestry

Abstract: A collection of very early brephic juveniles from the Upper Aalenian of the Cotswolds, England, has provided the first opportunity to study the ontogeny of thecideoids prior to the recognizable development of a median septum. Traced towards their origin, the enduring characters of thecideides appear to be cementation, a delthyrium closed by a pseudodeltidium, an almost circular dorsal valve with a prominent erect bilobed cardinal process, well‐defined inner socket ridges, widely spaced lateral adductor muscle fields, tubercles, fibrous secondary shell lining both valves and probably endopunctae. The subperipheral rim and median septum so characteristic of adult thecideoids are undeveloped in the earliest juveniles. Initiation of the development of the free ventral wall in the ventral valve is identified as an important event in thecideoid ontogeny. Further important discoveries arising from the study are that the crura‐like outgrowths which form the brachial bridge in thecideides are not structurally homologous with the crura of spiriferides or terebratulides and that during their earliest ontogenetic development thecidellinids and thecideids are indistinguishable. Also, the identification of morphological characters correlative with both palaeontological and neontological approaches to thecideoid phylogeny has important implications for thecideide taxonomy. Interpretation of the morphology exhibited by the Cotswold specimens introduces the probability that during their earliest ontogeny moorellinin, thecidein and lacazellin dorsal valves follow the same development pattern and the described ontogenetic sequence has been corroborated by evidence from Jurassic rocks of Argentina, North America and France, from Cretaceous rocks of England, Central Europe and Czechoslovakia, also from extant species from the Atlantic, Indian and Pacific oceans, and to an extent which suggests the pattern of ontogenetic development revealed has been typical of thecideoids throughout their history.     

The function of the brachial valve septa in plectambonitacean brachiopods

Eoplectodonta transversalis (Wahlcnberg) and other sowcrbyellids have a complex arrangement of septa in the brachial valve. It is argued that the function of these is primarily to support the lophophore and to strengthen the shell, and that the septa mirror is almost exactly the shape of the lophophore. In Eoplectodonta transversalis the ontogenetic develop ment of the lophophore varies from a simple to a complcx ptycholophe. Other plectam-bonitaceans have trocholophous and schizolophous lophophores.     

Geometric shell growth and mode of life of a Devonian chonetoidean brachiopod

Morphological and statistical analysis of the chonetoid species Kentronetes variabilis from the Lower Devonian (Lochkovian) of the Argentine Precordillera demonstrate ontogenetic changes and allometric relationships between characters. A special study was made of spine distribution, morphology, and growth, compared to valve growth. The first, inner, developed spines (pairs 1–1'and 2–2') continued to grow after development of the following outer pairs. The spacing of spines, their diameter, and the density of growth rings vary from beak to posterolateral margins following a specific 2ⁿ geometric growth factor, compared to the regular, almost linear growth of the valves, attested by growth lines. The linear growth rate of outer spines (pairs 3–3'and 4–4') can be 6–8 times more rapid than that of the shell on the valve margin. Ontogenetic changes in spine morphology are interpreted as a response to changes in the mode of life.     

The tube-like structures on the juvenile shells of earliest strophomenides and billingsellides as evidence of their life cycles

Possible life cycle of some ancient plectambonitoids (order Strophomenida) from the Middle Ordovician of Russia is reconstructed based on the well-preserved specimens composing the ontogenetic series. Four regions may be distinguished on their shell surface: protegulum, brephic shell, neanic shell and adult shell. The posterior margin of ventral protegulum bears pedicle sheath, which is a tubular outgrowth with a 40-μm-wide aperture at the distal end. The protegulum and brephic shell have common type of microstructure that possibly is spherular; the neanic and adult shells are fibrous. The strophomenide ontogeny possibly was similar to that of recent discinides. The strophomenide life cycle possibly included the planktotrophic juvenile stage; the protegulum and brephic shell were formed in the water column. The aperture of the pedicle sheath was possibly used as an anal opening of the floating juvenile and as an attachment organ during the settlement; at early adult stages, the sheath erased, the anus closed, and the animal started to lie on the ventral valve. The origin of the order Strophomenida and its relative groups is possibly connected with the loss of the pedicle lobe; judging by some strophomenide morphological features, true pedicle was present in the strophomenide ancestors. The tubes on the ventral umbones of strophomenides and billingsellides are not homologous as pedicle sheaths of strophomenides are formed at the planktotrophic swimming stage, and the tubes surrounding the pedicles of billingsellides were formed by deltidial plates of almost adult shell after settling.     

The mesolophe, a new lophophore type for chonetacean brachiopods

Rachebaeuf, P. R. & Copper, P. 1990 10 15: The mesolophe, a new lophophore type for chonetacean brachiopods. Lethaia , Val. 23, pp. 341–346. Oslo. ISSN 0024–1164.
Following a summary of previous lophophore reconstructions for the chonetaceans, we describe an unusual pyritized structure within the calcite infill of an exceptionally preserved shell of Archeochonetes primigenius (Twenhofel) from the Late Ordovician (Ashgill) of Anticosti Island, Quebec, Canada. The brachial valve interior of most Lower Devonian to Permian chonetaceans shows the development of three depressed deepened areas (gutters) in the valve floor. The disposition of these gutters coincides remarkably with the shape of the pyritized structure, which we postulate as a new type of lophophore, the mesolophe. ▭ Brachiopoda, Chonetacea, functional morphology, lophophore .     

Shell disintegration and taphonomic loss in rudist biostromes

Radiolitid biostromes in the Upper Cretaceous of Austria and Italy record a marked taphonomic loss controlled mainly by the composition of the biocoenosis, by the density of rudist colonization, by the style of radiolitid shell disintegration and by early diagenetic processes. Radiolitid shells consisted of a calcitic ostracum and an originally aragonitic hypostracum. The attached valve of most radiolitids was built of (1) an outermost ostracal layer of delicate calcite lamellae, (2) a thick layer of ‘boxwork ostracum’ built of radial funnel plates and cell walls, (3) a thin, inner ‘ostracal layer 3’ of thick-walled boxwork, and (4) the hypostracum that formed the innermost shell layer. The attached valve disintegrated by spalling of radial funnel plates of layer 2, and by selective removal of the boxwork ostracum. In the free valve, the ostracum consisted of two layers: (a) an inner, lid-shaped layer of dense calcite, and (b) an outer layer composed of calcite lamellae. The free valve disintegrated by spalling into ostracal and hypostracal portions, by spalling of the ostracum into layers a and b, and by disintegration of layer b into packages of calcite lamellae and individual lamellae. The specific style of disintegration of the radiolitids was aided or induced by discontinuities in shell structure. Lamellar fragments from the ostracum of the upper valve and from the radial funnel plates of the lower valve locally are abundant in free-valve-funnel-plate floatstones that comprise the matrix of or occur in lenses within radiolitid biostromes. In biostromes with an open parautochthonous fabric, selective removal of the boxwork ostracum of the attached valve occurred by mechanical spalling and, most probably, by early diagenetic dissolution. Complete removal of the boxwork ostracum yielded thin, relict shells composed of the ‘ostracal layer 3’ and the hypostracum. During early diagenesis, the hypostracum was replaced by blocky calcite spar, or was dissolved and became filled by internal sediments. The combination of both selective removal of boxwork ostracum and early diagenetic dissolution of aragonite locally resulted in the formation of ghost biostromes that entirely or largely consist of faint relics of radiolitids. The syndepositional formation of radiolitid shell relics and the presence of radiolitid ghost biostromes produced by bios-tratinomic and early diagenetic processes show that rudist biostromes can undergo marked taphonomic loss during fossilization. The presence of ghost biostromes with a burrowed, open parautochthonous rudist fabric indicates that the final preservation of a rudist biostrome was directly influenced by the characteristics of the biocoenosis, including unpreserved burrowing taxa. Rudist biostromes may be of markedly different taphonomy as a result of the taxonomic composition of the entire assemblage and the density of colonization by the rudists.     

Torsion in Patella caerulea (Mollusca, Patellogastropoda): ontogenetic process, timing, and mechanisms

Abstract. Torsion is a process in gastropod ontogenesis where the visceral body portion rotates 180° relative to the head/foot region. We investigated this process in the limpet Patella caerulea by using light microscopy of living larvae, as well as scanning electron microscopy (SEM) of larvae fixed during the torsion process. The completion of the 180° twist takes considerably less time in larvae of Patella caerulea than previously described for other basal gastropod species. At a rearing temperature of 20–22°C, individuals complete ontogenetic torsion in ?2 h. Furthermore, the whole process is monophasic, i.e., carried out at a constant speed, without any evidence of distinct ‘fast” or ‘slow” phases. Both larval shell muscles—the main and the accessory larval retractor—are already fully contractile before the onset of torsion. During the torsion process both retractors perform cramp‐like contractions at ～30 s intervals, which are followed by hydraulic movements of the foot. However, retraction into the embryonic shell occurs only after torsion is completed. The formation of the larval operculum is entirely in‐dependent from ontogenetic torsion and starts before the onset of rotation, as does the mineralization of the embryonic shell. The reported variability regarding the timing (mono‐ versus biphasic; duration) of torsion in basal gastropod species precludes any attempt to interpret these data phylogenetically. The present findings indicate that the torsion process in Patella caerulea, and probably generally in basal gastropods, is primarily caused by contraction of the larval shell muscles in combination with hydraulic activities. In contrast, the adult shell musculature, which is independently formed after torsion is completed, does not contribute to ontogenetic torsion in any way. Thus, fossil data relying on muscle scars of adult shell muscles alone appear inappropriate to prove torted or untorted conditions in early Paleozoic univalved molluses. Therefore, we argue that paleontological studies dealing with gastropod phylogeny require data other than those based on fossilized attachment sites of adult shell muscles.     

Constraint and the square snail: life at the limits of a covariance set. The normal teratology of Cerion disforme

Two major developmental constraints, identified and assessed by the covariance structure of ontogenetic measures, strongly influence the form and variation of Cerion in every study I have ever undertaken on this protean genus: the jigsaw constraint of large-and few vs small-and-many whorls for shells reaching a similar final size; and the compensatory constraint of later heights balancing juvenile widths to bring adult shells into a limited range of final proportions.
This study explores a set of taxa (the Cerion dimidiatum complex of eastern Cuba) growing at one extreme of the compensatory constraint–maximally flat juvenile shells followed by abrupt allometric transition to orthogonal adult whorls. I show that the most curious of all Cerion taxa, the teratologously uncoiling C. disforme , owes its unique and distinctive form to growth at an extreme of the compensatory constraint: the juvenile cross-section goes beyond mere flatness to actual concavity; and the adult shell, after maximally abrupt allometric transition to downward growth, loses contact with the juvenile whorls and begins to unwind in teratologic fashion. I present several categories of evidence to demonstrate that C. disforme is the extreme in a morphocline of taxa, whose correlated features express increasing discordance (and correlated sequelae of size and shape) under the compensatory constraint: C. geophilus – C. dimidiatum – C. alberli – C.disforme. I provide data for clines in morphology, in amount and character of variation, and in mode of growth. This genre of constraint–Darwin's old "correlations of growth"–must be given equal weight with immediate adaptation in a unified explanation of growth and form.     

ONTOGENETIC NICHE SHIFT IN THE BRACHIOPOD TEREBRATALIA TRANSVERSA: RELATIONSHIP BETWEEN THE LOSS OF ROTATION ABILITY AND ALLOMETRIC GROWTH

Abstract: Many articulated brachiopods experience marked life habit variations during ontogeny because they experience their fluid environment at successively higher Reynolds numbers, and they can change the configuration of their inhalant and exhalant flows as body size increases. We show that the extant brachiopod Terebratalia transversa undergoes a substantial ontogenetic change in reorientation governed by rotation around the pedicle. T. transversa′s reorientation angle (maximum ability to rotate on the pedicle) decreases during ontogeny, from 180 degrees in juveniles to 10–20 degrees in individuals exceeding 5 mm, to complete cessation of rotation in individuals larger than 10 mm. Rotation ability is substantially reduced after T. transversa achieves the adult lophophore configuration and preferred orientation with respect to ambient water currents at a length of 2.5–5 mm. We hypothesize that the rotation angle of T. transversa is determined mainly by the position of ventral and dorsal points of attachment of dorsal pedicle muscles relative to the pedicle. T. transversa shows a close correlation between the ontogenetic change in reorientation angle and ontogeny of morphological traits that are related to points of attachment of dorsal pedicle muscles, although other morphological features can also limit rotation in the adult stage. The major morphological change in cardinalia shape and the observed reduction of rotation affect individuals 2.5–10 mm in length. The position of ventral insertions of dorsal pedicle muscles remains constant, but contraction of dorsal pedicle muscles is functionally handicapped because dorsal insertions shift away from the valve midline, rise above the dorsal valve floor, and become limited by a wide cardinal process early in ontogeny (<5 mm). The rate of increase of cardinal process width and of distance between dorsal pedicle muscle scars substantially decreases in the subadult stage (5–10 mm), and most of the cardinalia shell traits grow nearly isometrically in the adult stage (>10 mm). T. transversa attains smaller shell length in crevices than on exposed substrates. The proportion of small‐sized individuals and population density is lower on exposed substrates than in crevices, indicating higher juvenile mortality on substrates prone to grazing and physical disturbance. The loss of reorientation ability can be a consequence of morphological changes that strengthen substrate attachment and maximize protection against biotic or physical disturbance (1) by minimizing torques around the pedicle axis and/or (2) by shifting energy investments into attachment strength at the expense of the cost involved in reorientation.     

Turtles as hopeful monsters.

A recently published study on the development of the turtle shell highlights the important role that development plays in the origin of evolutionary novelties. The evolution of the highly derived adult anatomy of turtles is a prime example of a macroevolutionary event triggered by changes in early embryonic development. Early ontogenetic deviation may cause patterns of morphological change that are not compatible with scenarios of gradualistic, stepwise transformation.     

A comparative study of embryonic development of Japanese quail selected for different patterns of postnatal growth

Patterns of early embryonic development have traditionally been viewed as invariant within vertebrate taxa. It has been argued that the specific differences which are found arise during the later stages of development. These differences may be a result of allometry, heterochrony or changes in relative growth rates. To test whether early embryonic development is indeed invariant, or whether selection of adult characteristics can alter embryonic growth, we compared embryonic development in birds selected for different patterns of postnatal growth. Using quail lines selected for high and low body mass, we compared somite formation, and muscle and feather development. We obtained data that showed changes in the rate of myotome formation in the brachial somites which contribute to muscle formation in the limbs and thorax. We think these observations are connected with intraspecific changes in adult morphology, ie., breast muscle size. Our findings suggest that selection for late ontogenetic/adult stages affects early embryonic development.     

Larval muscle contraction fails to produce torsion in a trochoidean gastropod.

The causes and effects of ontogenetic torsion in gastropods have been debated intensely for more than a century (1-19). Occurring rapidly and very early in development, torsion figures prominently in shaping both the larval and adult body plans. We show that mechanical explanations of the ontogenetic event that invoke contraction of larval retractor muscles are inadequate to explain the observed consequences in some gastropods. The classic mechanical explanation of Crofts (4, 5) and subsequent refinements of her explanation have been based on species with rigid larval shell properties (18, 19) that cannot be extrapolated to all gastropods. We present visual evidence of the lack of rigidity of the uncalcified larval shell in a basal trochid gastropod, Margarites pupillus (Gould), and provide photographic confirmation of our prediction that larval retractor muscle contraction is insufficient to produce more than local deformation or dimpling at the site of muscle insertion. These findings do not refute muscular contraction as a primary cause of ontogenetic torsion in gastropods that calcify their larval shells prior to the onset of torsion, nor do they refute the monophyly of torsion. They do, however, suggest that torsion may be a loosely constrained developmental process with multiple pathways to the more constrained end result (20, 21).     

First finds of larval shells of Ordovician craniids in the Pskov Region

The inner surface of the dorsal valve at the early developmental stages and the larval shells lacking adult shell are described for the first time for the Ordovician craniids. The presence of a larval calcareous shell in the Early Paleozoic craniids is proposed.     

Earliest ontogeny of Early Palaeozoic Craniiformea: implications for brachiopod phylogeny

Popov, L.E., Bassett, M.G., Holmer, L.E., Skovsted, C.B. & Zuykov, M.A. 2010: Earliest ontogeny of Early Palaeozoic Craniiformea: implications for brachiopod phylogeny. Lethaia, Vol. 43, pp. 323–333. Well preserved specimens of the Early Palaeozoic craniiform brachiopods Orthisocrania and Craniops retain clear evidence of a lecithotrophic larval stage, indicating the loss of planktotrophy early in their phylogeny. The size of the earliest mineralized dorsal shell was <100 μm across, and the well preserved shell structure in these fossil craniiforms allows their earliest ontogeny to be compared directly with that of living Novocrania, in which the first mineralized dorsal shell (metamorphic shell) is secreted only after settlement of the lecithotrophic larvae. Immediately outside this earliest shell (early post‐metamorphic or brephic shell) and in the rest of the dorsal valve the primary layer in both fossil and living craniiforms has characteristic radially arranged laths, which are invariably lacking in the earliest dorsal shell. The ventral valve of the fossil specimens commonly preserves traces of an early attachment scar (cicatrix), which is equal in size to the dorsal metamorphic shell, and the brephic post‐metamorphic ventral valve also has a primary shell with radially arranged laths. However, a primary shell with radial laths is completely lacking in the ventral valve of living Novocrania, indicating that heterochrony may have been involved in the origin of the encrusting mode of life in living craniids; the entire ventral valve of Recent craniids (with the possible exception of Neoancistrocrania) may correspond to the earliest attachment scar of some fossil taxa such as Orthisocrania. It is also probable that the unique absence of an inner mantle lobe as well as the absence of lobate cells in Novocrania could be the result of heterochronic changes. The dorsal valve of both fossil and living craniiforms has a marked outer growth ring, around 500 μm across, marking the transition to the adult, and a significant change in regime of shell secretion. The earliest craniiform attachment is considered to be homologous to the unique attachment structures described recently in polytoechioids (e.g. Antigonambonites) and other members of the strophomenate clade. However, unlike the craniiforms, polytoechioids and strophomenates all have planktotrophic larvae, and planktotrophy is most probably a plesiomorphic character for all Brachiopoda. □Brachiopoda, Craniiformea, Early Palaeozoic, ontogeny, phylogeny.     

Support from early juvenile Jurassic,Cretaceous and Holocene thecideoid species for a postulated common early ontogenetic development pattern in thecideoid brachiopods

Abstract: The analysis of morphological characters exhibited by the earliest ontogenetic stages of Middle Jurassic, Cretaceous and Holocene thecideoid species, together with the application of techniques that reveal the presence of early juvenile morphological characters buried in the shell fabric of adult representatives of the species concerned, reinforce the idea of a development pattern common to thecideoids from the early Middle Jurassic to the present day. Basic and long‐standing differences in the architecture of the brachidial skeleton of thecidellinids and lacazellins may be correlated with the morphology of thecospiroids and indicate that the Thecidellinidae and Thecideidae may have emerged as sister groups as early as the Triassic.     

Xenochironomus canterburyensis (Diptera: Chironomidae) an insectan inquiline commensal of Hyridella menziesi (Mollusca: Lamellibranchia)

Larvae and pupae of the chironomid Xenochironomus canterburyensis (Freeman) are recorded for the first time. They are inquiline commensals of the fresh water mussel Hyridella menziesi (Gray) in Lake Taupo, New Zealand. The chironomid is univoltine and closely dependent on the seasonal growth of the mussel shell for its pupation and adult emergence. The first instar larva is probably a free-swimming, dispersal stage. Second instar larvae appeared in mussels between the outer surface of the mantle and the inner surface of the valve in later summer. They overwintered there as third and fourth instars until spring when they were found at the margin of the valve opposite the inhalent siphon from where they pupated and left the host to emerge at the lake surface. Some mortality of first and second instar larvae occurred as there was an average of 25–5 whole larvae buried in the nacre of each valve of the mussels.     

Variation in growth rates of the zebra mussel, Dreissena polymorpha, within Lake Wawasee

1. Field experiments conducted in Lake Wawasee in 1995 and 1996 measured the response of shell growth of Dreissena polymorpha to environmental gradients.
2. Shell growth decreased with initial shell length in four mussel size classes ranging between 8 and 22 mm, and decreased with depth, with mussels in shallow water (<4 m) having growth rates nearly twice those of mussels in deeper water (4–7 m).
3. Growth occurred early in the spring–summer period (May–June) with relatively little shell added later in the summer (July–September), and varied significantly among sites within Lake Wawasee, but not between the 2 years of this study.
4. Rank order of sites was consistent for both years implying that environmental conditions responsible for variation in shell growth were stable within Lake Wawasee.
5. Cage design did not have a significant effect on mussel shell growth nor did the distance of growth cages above the bottom (0.5–0.75 m above the bottom versus directly on the bottom).
6. This study demonstrates the sensitivity of adult mussel growth to subtle variation in environmental conditions occurring within and among lakes, with potential consequences for mussel population dynamics and community structure and function.     

A morphometric study on morphological plasticity of shell form in crevice-dwelling Pterioida (Bivalvia)

Relative warp analysis and eigenshape analysis were used to explore the patterns of variation of erratic shell shapes in and among six species of crevice-dwelling pterioid bivalves. The results of morphometric analyses revealed that a great deal of the variance of shell shape within each species can be reduced into principal components which display patterns of variation common to all species examined. In the species with striking variability, a few sets of principal components account for most of the variance of shell shape. On the other hand, in the species lacking considerable variability, several components contribute to form a given variety. Comparison of relative warp and eigenshape scores with centroid sizes indicates that the direction of ontogenetic shape change has been modified through evolution to produce adaptations to habitats such as crevices or the undersides of rocks.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 79, 285–297.     

Ontogeny of robusticity of craniofacial traits in modern humans: A study of South American populations

To date, differences in craniofacial robusticity among modern and fossil humans have been primarily addressed by analyzing adult individuals; thus, the developmental basis of such differentiation remains poorly understood. This article aims to analyze the ontogenetic development of craniofacial robusticity in human populations from South America. Geometric morphometric methods were used to describe cranial traits in lateral view by using landmarks and semilandmarks. We compare the patterns of variation among populations obtained with subadults and adults to determine whether population‐specific differences are evident at early postnatal ontogeny, compare ontogenetic allometric trajectories to ascertain whether changes in the ontogeny of shape contribute to the differentiation of adult morphologies, and estimate the amount of size change that occurs during growth along each population‐specific trajectory. The results obtained indicate that the pattern of interpopulation variation in shape and size is already established at the age of 5 years, meaning that processes acting early during ontogeny contribute to the adult variation. The ontogenetic allometric trajectories are not parallel among all samples, suggesting the divergence in the size‐related shape changes. Finally, the extension of ontogenetic trajectories also seems to contribute to shape variation observed among adults. Am J Phys Anthropol 2010. © 2009 Wiley‐Liss, Inc.