Complete Ontogenetic Series Research Articles

Ontogenetic variation in the cranium of Mixosaurus cornalianus, with implications for the evolution of ichthyosaurian cranial development

Relatively complete ontogenetic series are comparatively rare in the vertebrate fossil record. This can create biases in our understanding of morphology and evolution, since immaturity can represent a source of unrecognized intraspecific variation in both skeletal anatomy and ecology. In the extinct marine reptile clade Ichthyopterygia, ontogenetic series were widely studied only in some Jurassic genera, while the ontogeny of the oldest and most basal members of the clade is very poorly understood. Here, we investigate cranial ontogeny in Mixosaurus cornalianus, from the Middle Triassic Besano Formation of the Swiss and Italian Alps. This small-bodied taxon is represented by a wealth of material from multiple size classes, including fetal material. This allows us to assess ontogenetic changes in cranial morphology, and identify stages in the ontogenetic trajectory where divergence with more derived ichthyosaurs has occurred. Early ontogenetic stages of Mixosaurus show developmental patterns that are reminiscent of the presumed ancestral (early diverging sauropsid) condition. This is prominently visible in the late fetal stage in both the basioccipital, which shows morphology akin to basal tubera, and in the postorbital, which has a triradiate head. The ontogenetic trajectory of at least some of the cranial elements of Mixosaurus is therefore likely still very akin to the ancestral condition, even though the adult cranium diverges from the standard diapsid morphology.

Bone histology of the Late Pleistocene Prolagus sardus (Lagomorpha: Mammalia) provides further insights into life-history strategy of insular giant small mammals

Abstract Fossils provide an excellent opportunity to study and understand the evolution of insular environments free of human-made perturbations. Here, we evaluated the life-history traits and strategy of the extinct insular giant Prolagus sardus (Mammalia: Lagomorpha) by examining microscopically its fossilized bone tissues (osteohistology, skeletochronology, and quantitative geometry). For this task, a complete ontogenetic series of femora retrieved from the Late Pleistocene Grotta della Medusa (NW Sardinia, Italy) were analysed. Our results reveal that: (i) P. sardus’ pups were weaned at large size; (ii) the species’ maturation was delayed in its life cycle; and (iii) P. sardus lived longer than expected for its size. Hence, the giant P. sardus should have a slow pace of life triggered by the low levels of extrinsic mortality of the insular habitat. On the other hand, bone tissue differences between P. sardus and Ochotona were found in the vascularization, slow- growing bone presence, and cortical and medullary growth trajectories. The results obtained in this study concur with the eco-evolutionary responses described to date in extinct insular lagomorphs, as well as provide new empirical evidence about the phenomenon known as ‘insular gigantism’ as a pattern of evolution of small-sized mammals in genuine insular ecosystems.

A Review of the Asexual Mite Genus Paralycus Womersley, 1944 (Acari: Oribatida: Pediculochelidae), with Description of Three New Species and A Key to Species of the World

Mites of the genus Paralycus (Oribatida: Pediculochelidae) are minute, asexual, paedomorphic oribatids that have been largely overlooked by major biodiversity surveys. Here, we review this genus and describe three new species based on adult females: Paralycus persephone sp. n. and Paralycus daeira sp. n. from deep soil in Colorado, USA and Paralycus pricei sp. n. from South Africa. We provide the first complete ontogenetic series in the family using P. daeira sp. n. as a model, clarify the species boundaries in several species, and compile an annotated checklist and key to species of Paralycus of the World. Our work suggests that the pediculochelid biodiversity is underappreciated as these mites may be common in subterranean habitats/deep soil. Further discoveries of the Paralycus diversity in these habitats are anticipated.

Intraspecific craniomandibular and dental analysis of Pseudotypotherium exiguum (Mesotheriidae, Notoungulata) from Monte Hermoso, late Neogene, Buenos Aires Province, Argentina

ABSTRACT Mesotheriid taxonomy for Monte Hermoso, Buenos Aires province, Argentina is extremely complex, with much of the controversy centered on the Late Miocene–Pliocene mesotheriine notoungulate type species Pseudotypotherium pulchrum, which is a subjective junior synonym of P. exiguum. Confusion around P. exiguum intraspecific variation is exacerbated by specimens of the Pleistocene taxon Mesotherium cristatum originally defined in Toscas del Río de La Plata. As a result, remains of Monte Hermoso mesotheres have been referred to six species and four genera (Typotherium, Pseudotypotherium, Xenotherium, and Typotheriodon) during the past 100 + years. The current consensus is that Pseudotypotherium is the correct genus for Monte Hermoso mesotheres, but detailed analyses of variation, especially ontogenetic, have been lacking. We rectify that deficiency in this work through bibliographic and firsthand study of type materials and comparison of molar size (PCA of linear dimensions) and i1 ontogenetic variation (via CT) using a broad ontogenetic series. We conclude that the variation observed in Monte Hermoso mesotheres is consistent with ontogenetic and individual variation within a single species. As previously suggested, Pseudotypotherium (=Typotherium) exiguum is the valid senior synonym for Monte Hermoso mesotheres. We provide a detailed diagnosis and description of P. exiguum, including a rather complete ontogenetic series, the first for any species of Pseudotypotherium.

An exceptionally preserved Sphenodon-like sphenodontian reveals deep time conservation of the tuatara skeleton and ontogeny

Sphenodontian reptiles are an extremely old evolutionary lineage forming the closest relatives to squamates (lizards and snakes) and were globally distributed and more diverse than squamates during the first half of their evolutionary history. However, the majority of their fossils are highly fragmentary, especially within sphenodontines—the group including its single surviving species, Sphenodon punctatus (the tuatara of New Zealand)—thus severely hampering our understanding on the origins of the tuatara. Here, we present a new sphenodontian species from the Early Jurassic of North America (Arizona, USA) represented by a nearly complete articulated skeleton and dozens of upper and lower jaws forming the most complete ontogenetic series in the sphenodontian fossil record. CT-scanning provides plentitude of data that unambiguously place this new taxon as one of the earliest evolving and oldest known sphenodontines. Comparisons with Sphenodon reveal that fundamental patterns of mandibular ontogeny and skeletal architecture in Sphenodon may have originated at least ~190Mya. In combination with recent findings, our results suggest strong morphological stability and an ancient origin of the modern tuatara morphotype.

Continuous and Regular Expansion of a Distributed Visual System in the Eyed Chiton Tonicia lebruni.

AbstractChitons have a distinctive armature of eight articulating dorsal shells. In all living species, the shell valves are covered by a dense array of sensory pores called aesthetes; but in some taxa, a subset of these are elaborated into lensed eyes, which are capable of spatial vision. We collected a complete ontogenetic series of the eyed chiton Tonicia lebruni de Rochebrune, 1884 to examine the growth of this visual network and found that it expands continuously as eyes are added at the margin during shell growth. Our dataset ranged from a 2.58-mm juvenile with only 16 eyes to adults of 25-31 mm with up to 557 eyes each. This allowed us to investigate the organization (and potential constraints therein) of these sensory structures and their development. Chiton eyes are constrained to a narrowly defined region of the shell, and data from T. lebruni indicate that they are arranged roughly bilaterally symmetrically. We found deviations from symmetry of up to 10%, similar to irregularity reported in some other animals with multiplied eyes. Distances separating successive eyes indicate that, while shell growth slows during the life of an individual chiton, eyes are generated at regular time intervals. Although we could not identify a specific eye-producing tissue or organ, we propose that the generation of new eyes is controlled by a clock-like mechanism with a stable periodicity. The apparent regularity and organization of the chiton visual system are far greater than previously appreciated. This does not imply the integration of shell eyes to form composite images, but symmetry and regular organization could be equally beneficial to a highly duplicated system by ensuring even and comprehensive sampling of the total field of view.

Development of inflorescences and flowers in Fabaceae subfamily Dialioideae: an evolutionary overview and complete ontogenetic series for Apuleia and Martiodendron

AbstractThe goal of this study was to better understand the origin and development of flowers and inflorescences in the newly established subfamily Dialioideae, an unusual and morphologically variable clade of Fabaceae due to its varied levels of floral reduction. We present here the complete ontogenetic series for two species characterizing different levels of floral reduction: Apuleia leiocarpa, an andromonoecious species with trimerous flowers; and Martiodendron fluminense, a species lacking the inner whorl of stamens. We also performed a literature review and herbarium specimen survey of the inflorescence and floral morphology of the other 15 genera in Dialioideae. Among the exclusive traits of Apuleia found here are the absence of two sepals and petals from initiation, the simultaneous initiation of the sepals (never before documented for Dialioideae), the absence of carpel initiation in staminate flowers and the formation of the carpel in the staminal whorl of monoclinous flowers, with the presence of a nectariferous hypanthium in both flower types. In Martiodendron the two exclusive traits are the heteromorphic development of stamens of the outer whorl, with the abaxial one being the last to elongate, and the possible initiation of an inner staminal whorl, which stops developing immediately thereafter and is no longer visible at anthesis. Among the potential synapomorphies for the subfamily are the absence of bracteoles and a pair of bracts subtending a triad of flowers or inflorescence axes, the distichous anthotaxy of the thyrsoid inflorescences, the bidirectional initiation of the sepals and the simultaneous initiation of the stamens.

Ontogenetic changes in the body plan of the sauropodomorph dinosaur Mussaurus patagonicus reveal shifts of locomotor stance during growth

Ontogenetic information is crucial to understand life histories and represents a true challenge in dinosaurs due to the scarcity of growth series available. Mussaurus patagonicus was a sauropodomorph dinosaur close to the origin of Sauropoda known from hatchling, juvenile and mature specimens, providing a sufficiently complete ontogenetic series to reconstruct general patterns of ontogeny. Here, in order to quantify how body shape and its relationship with locomotor stance (quadruped/biped) changed in ontogeny, hatchling, juvenile (~1 year old) and adult (8+ years old) individuals were studied using digital models. Our results show that Mussaurus rapidly grew from about 60 g at hatching to ~7 kg at one year old, reaching >1000 kg at adulthood. During this time, the body’s centre of mass moved from a position in the mid-thorax to a more caudal position nearer to the pelvis. We infer that these changes of body shape and centre of mass reflect a shift from quadrupedalism to bipedalism occurred early in ontogeny in Mussaurus. Our study indicates that relative development of the tail and neck was more influential in determining the locomotor stance in Sauropodomorpha during ontogeny, challenging previous studies, which have emphasized the influence of hindlimb vs. forelimb lengths on sauropodomorph stance.

A new species of Eutrachytes (Acari: Uropodina: Eutrachytidae) associated with the indian mangrove (Avicennia officinalis)

A new species of Eutrachytes (Eutrachytes flagellatus) is described based on a complete ontogenetic series, starting from larva and including the adult female and male. This Uropodina mite was isolated from the pneumatophore surface of Avicennia officinalis having algal (Bostryschia sp.) growth in Goa, India. Notable morphological attributes peculiar to immature instars of this species include a flagellate tubular dorsolateral respiratory structure extending from the peritreme, nude pygidial shields in the adult male and female and a deep concave formation at the posterolateral margins of the dorsal shield. A taxonomic discussion with salient diagnostic features of the genus is given and a key to genera of the family is presented. We present two nomenclature modifications: Deraiophoridae syn. nov. as the junior synonym of Eutrachytidae and Dentibaiulus Hirschmann, 1979 syn. nov. as a junior synonym of Eutrachytes Berlese, 1914. A compiled list of all new species discovered to date from mangrove roots in different parts of the world is given.

New ontogenetic information on Duyunaspis duyunensis Zhang & Qian in Zhou et al., 1977 (Trilobita, Corynexochida) from the Cambrian and its possible sexual dimorphism

Lei, Q.P., September .2015. New ontogenetic information on Duyunaspis duyunensis Zhang & Qian in Zhou et al., 1977 (Trilobita, Corynexochida) from the Cambrian and its possible sexual dimorphism. Alcheringa 40, XXX–XXX. ISSN 0311-5518.The hypostomal condition and the complete ontogenetic development of Duyunaspis duyunensis Zhang & Qian in Zhou et al., 1977 are restudied on the basis of abundant specimens from the Balang Formation (Cambrian, Qiandongian) in Zila Village, Paiwu Township, Huayuan County, Hunan Province. The relatively complete ontogenetic series (degree 0 to 9) provides new evidence that the holaspis of this species has nine rather than seven thoracic segments as proposed by McNamara et al. (2006). Comparisons between Duyunaspis duyunensis and two associated species, Arthricocephalus chauveaui and Changaspis elongata, support its placement in the Subfamily Oryctocarinae. Moreover, based on observations of a large number of specimens, the presence of an anterior indentation on the pygidial end is possibly representative of sexual dimorphism.Qianping Lei [cicelyapple@126.com], Natural Department, Changzhou Museum, Changzhou, Jiangsu Province, 213022, PR China; State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, 210008, PR China.

Ontogenetic development and sexual dimorphism of franciscana dolphin skull: A 3D geometric morphometric approach.

The aim of this work was to study the postnatal ontogenetic development of Pontoporia blainvillei skull, identifying major changes on shape, and relating them to relevant factors in the life history of the species. We analyzed a complete ontogenetic series (73♂, 83♀) with three-dimensional geometric morphometric techniques. Immature dolphins showed a very well-developed braincase and a poorly developed rostrum, and the principal postnatal changes affected the rostrum and the temporal fossa, both structures implied functionally to the feeding apparatus, thus suggesting a specialized mode for catch fast prey in P. blainvillei. Osseous elements associated with sound production were already well developed on immature dolphins, suggesting the importance of this apparatus since the beginning of postnatal life. Sexual dimorphism was detected on both shape and size variables. Females were bigger than males, in accordance with previous studies. Shape differences between sexes were found on the posterior part of premaxillaries and external bony nares (P < 0.01), suggesting that this sexual dimorphism is related to differences on vocalization capabilities.

Ontogeny of the redlichiid trilobiteEoredlichia intermediatafrom the Chengjiang Lagerstätte, lower Cambrian, southwest China

A detailed investigation of the morphology and ontogeny of the redlichioid trilobite Eoredlichia intermediata (Bulletin of the Geological Society of China, 3–4, 1940, 333) from the lower Cambrian Yu'anshan Member of the Heilinpu Formation, in Kunming, Yunnan Province, southwest China, is presented. The new material comprises a relatively complete ontogenetic series ranging from the early meraspid to the holaspid period, which reveals more details on morphological variation such as the appearance of bacculae in some holaspid specimens, contraction and disappearance of fixigenal spines, and macropleural spines of the first and second thoracic segments, which are all documented for the first time and can also be used as developmental markers defining ontogenetic phases. Two distinct morphotypes, possibly an expression of intraspecific variation or sexual dimorphism, are distinguished by the morphology of pleural spines of the second thoracic segment in meraspid degree 14 and holaspides. The trunk segmentation schedule of E. intermediata is also discussed and conforms to the protarthrous developmental mode. The distinction of the thoracic region into two portions can be observed during late meraspid and early holaspid periods, which might be considered as a reference for a better understanding on the relationship of tagmosis and growth segmentation among the Cambrian redlichiid trilobites.

Ossification sequences and associated ontogenetic changes in the bone histology of pachypleurosaurids from Monte San Giorgio (Switzerland/Italy)

ABSTRACT Evolutionary changes in lifestyle (e.g., terrestrial vs. aquatic habits) influence tetrapod limb morphology. Similar evolutionary trends in osteogenetic sequences (order of bone ossification) and bone microstructure are often shared in extant tetrapods with similar habit preferences: such data are sometimes available from extinct taxa. The pachypleurosaurids from Monte San Giorgio (Switzerland, Italy) are Triassic marine reptiles with high quality, complete ontogenetic series. We studied osteogenesis and bone histology in the four species from this locality, comparing these with data from Recent terrestrial lizards and secondarily aquatic reptiles in order to determine if the osteogenetic sequences of pachypleurosaurids were similar to either the hypothesized plesiomorphic condition for terrestrial eureptilians or those of Recent aquatic reptiles. Pachypleurosaurian limb osteogenesis occurs in two steps: (1) developmental sequences of ossification during embryology and/or in early neonates, (2a) additional primary periosteal compaction processes, and (2b) additional primary and secondary endosteal compaction processes during neonate ontogeny. Taphonomic patterns reveal information on the order of the initiation and termination of these steps, which are (A) onset of ossification, (B) onset of additional compaction processes (early phase), and (C) termination of additional compaction processes (final phase). An event pairing analysis found that ossification of forelimb elements in pachypleurosaurids precedes that of the hind limb elements in all osteogenetic stages except for initiation of ossification (A). The order of their early phase compaction processes is similar to the hypothesized plesiomorphic eureptilian condition, whereas their final phase compaction processes varies among pachypleurosaurids, with S. mirigiolensis showing minor heterochronic shifts and N. edwardsii showing many heterochronic shifts relative to the hypothesized ancestral condition. Pachypleurosaurids from Monte San Giorgio increase the number of heterochronic shifts with decreasing stratigraphic age, showing a transition from more ‘terrestrial’ to more ‘aquatic’ osteogenetic sequences in comparison to data on ossification sequences of Recent aquatic reptiles.

Anatomy of the Early Cretaceous bird Rapaxavis pani, a new species from Liaoning Province, China

ABSTRACT The lacustrine deposits of the Jiufotang Formation, Liaoning Province, China have yielded a diverse avifauna representing a wide spectrum of basal lineages. Here we add to this diversity by describing the longirostrine enantiornithean taxon Rapaxavis pani gen. et sp. nov. The specimen possesses the following autapomorphies: (1) caudoateral processes of the sternum with two prominent branches and a smaller third branch, (2) paired triangular thoracic elements of unknown homology, and (3) the combination of six sacral vertebrae and six free caudal vertebrae. Otherwise, this genus is closely allied with the Yixian Formation species Longirostravis hani, sharing elongate, curving rostrum with teeth restricted to the premaxilla and rostral-most dentary, and a derived manus lacking unguals. The holotype of Rapaxavis is exquisitely preserved, with such details as distal tarsals, keratinous sheaths of the pedal unguals and some three-dimensional features of the bones intact. The holotype appears to lack a fused tibiotarsus and a fused carpometa-carpus. These features may indicate that the specimen is not yet fully grown, though the life stages of members of the Enantiornithes will not be clear until a complete ontogenetic series can be constructed. A comparison of the proportions of the pedal phalanges of Rapaxavis to modern birds shows Rapaxavis to be highly adapted to a grasping, arboreal lifestyle. The combination of strongly arboreal pedal adaptations and the probing adaptations in the rostrum is here confirmed in a member of the Enantiornithes for the first time in the literature, adding to their already impressive known functional diversity.

Developmental growth patterns of the filter-feeder pterosaur,Pterodaustro guiñazui

Life-history parameters of pterosaurs such as growth and ontogenetic development represent an enigma. This aspect of pterosaur biology has remained perplexing because few pterosaur taxa are represented by complete ontogenetic series. Of these, Pterodaustro is unique in that besides being represented by hundreds of individuals with wing spans ranging from 0.3 to 2.5m, it includes an embryo within an egg. Here we present a comprehensive osteohistological assessment of multiple skeletal elements of a range of ontogenetic sizes of Pterodaustro, and we provide unparalleled insight into its growth dynamics. We show that, upon hatching, Pterodaustro juveniles grew rapidly for approximately 2 years until they reached approximately 53% of their mature body size, whereupon they attained sexual maturity. Thereafter, growth continued for at least another 3-4 years at comparatively slower rates until larger adult body sizes were attained. Our analysis further provides definitive evidence that Pterodaustro had a determinate growth strategy.

A new study of Marrella splendens (Arthropoda, Marrellomorpha) from the Middle Cambrian Burgess Shale, British Columbia, Canada

Study of over 1000 specimens of Marrella splendens Walcott, 1912, out of the more than 9000 collected by the Royal Ontario Museum (ROM) since 1975, has produced new information on the anatomy, functional morphology, and behaviour of this most common arthropod in the Burgess Shale fauna. Among the new features recognized is the distinction between the alimentary canal and circulatory system; where the former is generally three-dimensional and slightly reflective, the latter never presents any relief and is very reflective. A larger range of size is now known, from 2.4 to 24.5 mm in length, with small individuals possessing 17 body segments to large specimens with more than 26 body segments, representing an almost complete ontogenetic series. The second pair of "antennae" is now interpreted as swimming appendages, since the five distal segments are dorsoventrally compressed, fringed with setae and with a considerable blood supply, providing a paddlelike appendage capable of producing a considerable propelling force. The ROM collections extend the geographical distribution of Marrella 13 km to the southeast and the stratigraphical range through the lowest five members of the Burgess Shale Formation.

COMPARISON OF CRANIAL ANATOMY AND PROPORTIONS OF SIMILARLY SIZED SEYMOURIA SANJUANENSIS AND DISCOSAURISCUS AUSTRIACUS

Similarly sized skulls of juvenile specimens of the seymouriid and discosauriscid seymouriamorph tetrapods Seymouria and Discosauriscus are compared for the first time. Comparisons are based on a single juvenile skull of Seymouria sanjuanensis from the Lower Permian Bromacker locality, Germany, measuring 56 mm in length and several skulls of Discosauriscus austriacus from the Lower Permian Boskovice Furrow, Czech Republic, reaching a maximum length of 62 mm. Establishment of their stage of development as juvenile is based on the small size of S. sanjuanensis compared to adults of the same species and previous comparisons of various-sized specimens of D. austriacus to complete ontogenetic series of several temnospondyls. In addition, a similar degree of cranial ossification, although slightly more advanced in the S. sanjuanensis skull, is also cited as evidence of very similar ontogenetic stages of development. However, D. austriacus is remarkable in retaining remnants of the lateral line canals, suggesting that a longer premetamorphic period of its life history was aquatic. Despite representing similar, juvenile stages of development, the S. sanjuanensis and D. austriacus specimens can be distinguished clearly from one another on structural and proportional cranial features, thus dispelling suggestions that Seymouria may represent the adult stage of development of Discosauriscus, which is known only by larval and juvenile stages of development.

Shell and hinge ontogeny of the Antarctic bivalve Lissarca notorcadensis

Lissarca notorcadensis, a small pallial-brooding mollusc with a circum-Antarctic distribution, is frequently found attached by stout byssal threads to the spines of sea urchins. Here, complete ontogenetic series can be found. Using standard, structural definitions, the prodissoconch I of L. notorcadensis is small compared to the large prodissoconch II, a feature atypical of brooded young and possibly reflective of a planktotrophic past. Fine commarginal striae are found on prodissoconch II of this subtidal clam, which also possesses more pronounced commarginal striae on the dissoconch prior to brood release. The provinculum is retained and remains functional well into adulthood. A small central ligament pit is present through the postlarval stages. This develops into a large, triangular pit in adulthood that shows obvious growth lines. Calcareous, presumably aragonitic fibres develop in the ligament of specimens still retained in the adult brood. Five to six adult hinge teeth are formed on either side of the ligament. Alignment of shell valves by these well-developed teeth is augmented by retention of larval denticles. The functional and structural demise of the provinculum occurs between 4 and 7 mm shell height and results from denticle erosion and subumbonal shell overgrowth. Many canals permeate larval and adult valves, but external pores in the adult shell differ from those of the larval shell and could indicate mode of formation. Prodissoconch form, provinculum retention, and shell canals are considered primitive traits.

The tadpole of Bufo brongersmai Hoogmoed 1972

Abstract The tadpole of the north african toad Bufo brongersmai is described for the first time in chronological developmental stages from hatching to the onset of metamorphosis based on a complete ontogenetic series. The structure of the oral disk and of the buccopharyngeal cavity are shown. Fifteen metric parameters are described in eight tadpole size classes by means of standard statistical methods and they are compared with those of other circummediterranean Bufo species. The eidonomic characteristics of the tadpole of B. brongersmai turn out to be intermediate between B. bufo and B. viridis.