古田山自然保护区地下芽植物多样性及其生物量分配策略

地下芽植物能够通过地下储存器官占据生境资源、储存营养物质等策略来获得生态优势,其地下储存器官多样性以及生物量分配策略,对地下芽植物物种组成以及生态系统功能产生重要影响。然而,以往研究多关注草地生态系统的地下芽植物,对森林地下芽植物的了解仍然缺乏。采集了古田山国家级自然保护区不同海拔分布的693个草本植物个体,分析了地下芽植物及其地下储存器官的类型与多样性,比较了地下芽植物与非地下芽植物的地上、地下各器官的绝对、相对生物量。结果显示：（1）地下芽植物的相对丰富度为69.1%,相对多度为88.2%。大多为根状茎植物,主要由禾本科、莎草科、堇菜科和蕨类植物组成。（2）除茎外,地下芽植物各器官的绝对生物量（叶：1.94g,根：0.65g,地上部分：2.0g,地下部分：4.1g）均大于非地下芽植物（叶：0.26g,根：0.13g,地上部分：0.68g,地下部分：0.13g）。（3）地下芽植物叶（0.40）与茎（0.14）的相对生物量小于非地下芽植物（叶：0.48,茎：0.35）,地下部分相对生物量（0.56）大于非地下芽植物（0.17）。本研究表明,以根状茎植物为主的地下芽植物是古田山亚热带森林生态系统草本植物的主要构成者,且个体普遍较大,倾向于将生物量投资于地下器官。这些结果为认识地下芽植物的生态策略与功能以及草本植物群落管理提供了科学依据。     

Nutrient reserves may allow for genome size increase: evidence from comparison of geophytes and their sister non-geophytic relatives

Background and Aims

The genome size of an organism is determined by its capacity to tolerate genome expansion, given the species'' life strategy and the limits of a particular environment, and the ability for retrotransposon suppression and/or removal. In some giant-genomed bulb geophytes, this tolerance is explained by their ability to pre-divide cells in the dormant stages or by the selective advantage of larger cells in the rapid growth of their fleshy body. In this study, a test shows that the tendency for genome size expansion is a more universal feature of geophytes, and is a subject in need of more general consideration.

Methods

Differences in monoploid genome sizes were compared using standardized phylogenetically independent contrasts in 47 sister pairs of geophytic and non-geophytic taxa sampled across all the angiosperms. The genome sizes of 96 species were adopted from the literature and 53 species were newly measured using flow cytometry with propidium iodide staining.

Key Results

The geophytes showed increased genome sizes compared with their non-geophytic relatives, regardless of the storage organ type and regardless of whether or not vernal geophytes, polyploids or annuals were included in the analyses.

Conclusions

The universal tendency of geophytes to possess a higher genome size suggests the presence of a universal mechanism allowing for genome expansion. It is assumed that this is primarily due to the nutrient and energetic independence of geophytes perhaps allowing continuous synthesis of DNA, which is known to proceed in the extreme cases of vernal geophytes even in dormant stages. This independence may also be assumed as a reason for allowing large genomes in some parasitic plants, as well as the nutrient limitation of small genomes of carnivorous plants.     

The scolopidial accessory organs and Nebenorgans in orthopteroid insects: Comparative neuroanatomy,mechanosensory function,and evolutionary origin

Scolopidial sensilla in insects often form large sensory organs involved in proprioception or exteroception. Here the knowledge on Nebenorgans and accessory organs, two organs consisting of scolopidial sensory cells, is summarised. These organs are present in some insects which are model organisms for the physiology of mechanosensory systems (cockroaches and tettigoniids). Recent comparative studies documented the accessory organ in several taxa of Orthoptera (including tettigoniids, cave crickets, Jerusalem crickets) and the Nebenorgan in related insects (Mantophasmatodea). The accessory organ or Nebenorgan is usually a small organ of 8–15 sensilla located in the posterior leg tibia of all leg pairs. The physiological properties of the accessory organs and Nebenorgans are so far largely unknown. Taking together neuroanatomical and electrophysiological data from disparate taxa, there is considerable evidence that the accessory organ and Nebenorgan are vibrosensitive. They thus complement the larger vibrosensitive subgenual organ in the tibia. This review summarises the comparative studies of these sensory organs, in particular the arguments and criteria for the homology of the accessory organ and Nebenorgan among orthopteroid insects. Different scenarios of repeated evolutionary origins or losses of these sensory organs are discussed. Neuroanatomy allows to distinguish individual sensory organs for analysis of sensory physiology, and to infer scenarios of sensory evolution.     

Distribution and morphology of the ampullary organs of the salmontail catfish,Arius graeffei

Whole body staining of Arius graeffei revealed that ampullary pores cover the body with their highest densities occurring on the head and lowest densities on the mid‐ventral surface. Each ampullary organ consists of a long canal (0.2–1.75 mm) passing perpendicular to the basement membrane, through the epidermis into underlying dermal connective tissues, curving thereafter to run roughly parallel to the epidermis. Histochemical staining techniques (Alcian blue and Lillie′s allochrome) indicate that the canals contain a neutral to acidic glycoprotein‐based mucopolysaccharide gel that varies in composition along the length of the canal. Collagen fibers, arranged in a sheath, surround a layer of squamous epithelium that lines each ampullary canal. At the proximal end of the canal, squamous cells are replaced by cuboidal epithelial cells that protrude into the lumen, thus constricting the lumen to form a small pore into the ampulla. The ampulla is lined with receptor and supportive cells. The numerous (60–120) pear‐shaped receptor cells bear microvilli on their luminal surface. Two forms of receptor cells exist in each ampullary organ: basal and equatorial receptor cells. Each receptor cell is connected to an unmyelinated nerve. Each receptor cell is surrounded by supportive cells on all but the apex. Tight junctions and underlying desmosomes occur between adjacent receptor and supportive cells. This form of ampullary organ has not previously been described for teleosts. J. Morphol. 239:97–105, 1999. © 1999 Wiley‐Liss, Inc.     

Preface: Ostracoda and the four pillars of evolutionary wisdom

Morphology, palaeontology, genetics and ecology are the main scientific domains contributing theories, concepts and new data to evolutionary biology. Ostracods are potentially very good model organisms for evolutionary studies because they combine an excellent fossil record with a wide extant distribution and, therefore, allow studies on both patterns and processes leading to extant diversity. This preface provides an overview of the 15 contributions to the present volume and concludes that this set of papers supports the claim that ostracod studies are situated in all main evolutionary domains.     

Functional evolutionary developmental biology (evo-devo) of morphological novelties in plants

The origin of morphological and ecological novelties is a long-standing problem in evolutionary biology.Understanding these processes requires investigation from both the development and evolution standpoints,which promotes a new research field called evolutionary developmental biology (evo-devo).The fundamental mechanism for the origin of a novel structure may involve heterotopy,heterochrony,ectopic expression,or loss of an existing regulatory factor.Accordingly,the morphological and ecological traits cont...     

The isotopic ecology of African mole rats informs hypotheses on the evolution of human diet

The diets of Australopithecus africanus and Paranthropus robustus are hypothesized to have included C4 plants, such as tropical grasses and sedges, or the tissues of animals which themselves consumed C4 plants. Yet inferences based on the craniodental morphology of A. africanus and P. robustus indicate a seasonal diet governed by hard, brittle foods. Such mechanical characteristics are incompatible with a diet of grasses or uncooked meat, which are too tough for efficient mastication by flat, low-cusped molars. This discrepancy, termed the C4 conundrum, has led to the speculation that C4 plant underground storage organs (USOs) were a source of nutrition for hominin species. We test this hypothesis by examining the isotopic ecology of African mole rats, which consume USOs extensively. We measured delta18O and delta13C of enamel and bone apatite from fossil and modern species distributed across a range of habitats. We show that delta18O values vary little and that delta13C values vary along the C3 to C4/CAM-vegetative axis. Relatively high delta13C values exist in modern Cryptomys hottentotus natalensis and Cryptomys spp. recovered from hominin-bearing deposits. These values overlap those reported for A. africanus and P. robustus and we conclude that the USO hypothesis for hominin diets retains certain plausibility.     

Storage ability of overwintering leaves and rhizomes in a semi-evergreen fern, <Emphasis Type="Italic">Dryopteris crassirhizoma</Emphasis> (Dryopteridaceae)

Dryopteris crassirhizoma is a rhizomatous semi-evergreen fern growing in the understory of deciduous forests. Although the top portion of the overwintering leaves began to wither in early winter, intensive senescence occurred in the spring, concurrently with new leaf development. Dry weight comparisons between organs revealed that the rhizome occupied the largest proportion of the total mass, followed by the pinnae. To assess the storage ability of overwintering leaves and the rhizome, seasonal changes in nitrogen content and the dry mass of pinnae and the rhizome were measured. Nitrogen (36.6%) was resorbed from winter-withering pinnae, but not from spring-withering pinnae. In contrast, a similar decrease in dry mass per unit area occurred between winter- and spring-withering pinnae (15%). These results indicate that overwintering leaves serve as a carbohydrate storage organ, but do not serve as a nitrogen storage organ. Nitrogen was not translocated from the rhizome during the early growing season, but translocation did occur in late summer and autumn. The dry mass of the rhizome decreased by 18.4% in spring, at the time of new leaf expansion. The amount of exported dry matter from the rhizome was threefold larger than that from senescent pinnae. Therefore, the rhizome is a major carbohydrate storage organ in this species, although overwintering leaves also act as a carbohydrate storage organ.     

Morphology of locomotor appendages in Arachnida: evolutionary trends and phylogenetic implications

The morphological diversity of locomotor appendages in Arachnida is surveyed lo reconstruct phylogenetic relationships and discover evolutionary trends in form and function. The appendicular skeleton and musculature of representatives from the ten living arachnid orders ate described, and a system of homology is proposed. Character polarities are established through comparison with an outgroup. Limulus polyphemus Xiphosura). Cladistic analysis suggests that Arachnida is monophyletic and that absence of extensor muscles is a primitive condition. Extensors are primitively absent in Araneae. Amblypygi, Uropygi, Palpigradi, Ricinulei and Acari. Most similarities in the appendages of these orders are symplesiomorphic so that phylogenetic relationships among the 'extensorless' groups cannot be resolved solely on the basis of appendicular characters. Extensor muscles appear to have evolved once, and their presence is considered a synapomorphic feature of Opiliones, Scorpiones, Pseudoscorpiones and Solifugae. Solifugae lack extensors, but a parsimonious interpretation of other characters indicates that this is a secondary, derived condition. The phylogenetic relationships among these four orders are clarified by modifications of the patellotibial joint. Cladistic analysis indicates that Opiliones may be the sister group of the other three orders and that Scorpiones is the sister group of Pseudoscorpiones and Solifugae. Conclusions concerning phylogenetic relationships and evolutionary morphology presented here differ substantially from those of earlier studies on the locomotor appendages of Arachnida.     

Morphology of locomotor appendages in Arachnida: evolutionary trends and phylogenetic implications

The morphological diversity of locomotor appendages in Arachnida is surveyed lo reconstruct phylogenetic relationships and discover evolutionary trends in form and function. The appendicular skeleton and musculature of representatives from the ten living arachnid orders ate described, and a system of homology is proposed. Character polarities are established through comparison with an outgroup. Limulus polyphemus Xiphosura). Cladistic analysis suggests that Arachnida is monophyletic and that absence of extensor muscles is a primitive condition. Extensors are primitively absent in Araneae. Amblypygi, Uropygi, Palpigradi, Ricinulei and Acari. Most similarities in the appendages of these orders are symplesiomorphic so that phylogenetic relationships among the ‘extensorless’ groups cannot be resolved solely on the basis of appendicular characters. Extensor muscles appear to have evolved once, and their presence is considered a synapomorphic feature of Opiliones, Scorpiones, Pseudoscorpiones and Solifugae. Solifugae lack extensors, but a parsimonious interpretation of other characters indicates that this is a secondary, derived condition. The phylogenetic relationships among these four orders are clarified by modifications of the patellotibial joint. Cladistic analysis indicates that Opiliones may be the sister group of the other three orders and that Scorpiones is the sister group of Pseudoscorpiones and Solifugae. Conclusions concerning phylogenetic relationships and evolutionary morphology presented here differ substantially from those of earlier studies on the locomotor appendages of Arachnida.     

Phylogenetic trends in the abundance and distribution of pit organs of elasmobranchs

Pit organs (free neuromasts of the mechanosensory lateral line system) are distributed over the skin of elasmobranchs. To investigate phylogenetic trends in the distribution and abundance of pit organs, 12 relevant morphological characters were added to an existing matrix of morphological data (plus two additional end terminals), which was then re-analysed using cladistic parsimony methods ( paup * 4.0b10). Character transformations were traced onto the most parsimonious phylogenetic trees. The results suggest the following interpretations. First, the distinctive overlapping denticles covering the pit organs in many sharks are a derived feature; plesiomorphic elasmobranchs have pit organs in open slits, with widely spaced accessory denticles. Second, the number of pit organs on the ventral surface of rays has been reduced during evolution, and third, spiracular pit organs have changed position or have been lost on several occasions in elasmobranch evolution. The concentrated-changes test in macclade (version 4.05) was used to investigate the association between a pelagic lifestyle and loss of spiracular pit organs (the only character transformation that occurred more than once within pelagic taxa). Depending on the choice of tree, the association was either nonsignificant at P  = 0.06 or significant at P  < 0.05. Future studies, using species within more restricted elasmobranch clades, are needed to resolve this issue.     

Functional evolutionary developmental biology (evo‐devo) of morphological novelties in plants

Abstract The origin of morphological and ecological novelties is a long‐standing problem in evolutionary biology. Understanding these processes requires investigation from both the development and evolution standpoints, which promotes a new research field called “evolutionary developmental biology” (evo‐devo). The fundamental mechanism for the origin of a novel structure may involve heterotopy, heterochrony, ectopic expression, or loss of an existing regulatory factor. Accordingly, the morphological and ecological traits controlled by the regulatory genes may be gained, lost, or regained during evolution. Floral morphological novelties, for example, include homeotic alterations (related to organ identity), symmetric diversity, and changes in the size and morphology of the floral organs. These gains and losses can potentially arise through modification of the existing regulatory networks. Here, we review current knowledge concerning the origin of novel floral structures, such as “evolutionary homeotic mutated flowers”, floral symmetry in various plant species, and inflated calyx syndrome (ICS) within Solanaceae. Functional evo‐devo of the morphological novelties is a central theme of plant evolutionary biology. In addition, the discussion is extended to consider agronomic or domestication‐related traits, including the type, size, and morphology of fruits (berries), within Solanaceae.     

Development of the nasal chemosensory organs in two terrestrial anurans: the directly developing frog, Eleutherodactylus coqui (Anura: Leptodactylidae), and the metamorphosing toad, Bufo americanus (Anura: Bufonidae)

Nearly all vertebrates possess an olfactory organ but the vomeronasal organ is a synapomorphy for tetrapods. Nevertheless, it has been lost in several groups of tetrapods, including aquatic and marine animals. The present study examines the development of the olfactory and vomeronasal organs in two terrestrial anurans that exhibit different developmental modes. This study compares the development of the olfactory and vomeronasal organs in metamorphic anurans that exhibit an aquatic larva (Bufo americanus) and directly developing anurans that have eliminated the tadpole (Eleutherodactylus coqui). The olfactory epithelium in larval B. americanus is divided into dorsal and ventral branches in the rostral and mid-nasal regions. The larval olfactory pattern in E. coqui has been eliminated. Ontogeny of the olfactory system in E. coqui embryos starts to vary substantially from the larval pattern around the time of operculum development, the temporal period when the larval stage is hypothesized to have been eliminated. The nasal anatomy of the two frogs does not appear morphologically similar until the late stages of embryogenesis in E. coqui and the terminal portion of metamorphosis in B. americanus. Both species and their respective developing offspring, aquatic tadpoles and terrestrial egg/embryos, possess a vomeronasal organ. The vomeronasal organ develops at mid-embryogenesis in E. coqui and during the middle of the larval period in B. americanus, which is relatively late for neobatrachians. Development of the vomeronasal organ in both frogs is linked to the developmental pattern of the olfactory system. This study supports the hypothesis that the most recent common ancestor of tetrapods possessed a vomeronasal organ and was aquatic, and that the vomeronasal organ was retained in the Amphibia, but lost in some other groups of tetrapods, including aquatic and marine animals.     

Genets (Carnivora, Viverridae) in Africa: an evolutionary synthesis based on cytochrome b sequences and morphological characters

The taxonomy of the genets (genus Genetta) has long been discussed, thus hampering endeavours towards evolutionary reconstruction. Sequence data from the complete cytochrome b gene (cyt b) were generated for 50 specimens representing 15 morphological species in order to allow the production of the first exhaustive molecular phylogeny of the genets. Second, a revised morphological matrix comprising 50 characters was combined with the cyt b data to estimate the level of morphological homoplasy. Phylogenetic analyses were conducted using parsimony, maximum likelihood and Bayesian procedures. Our results based on cyt b contradict a part of the traditional taxonomy of genus Genetta, the servaline and small‐spotted genets being paraphyletic, but confirmed the species status recently re‐investigated for three genets belonging to the large‐spotted complex, including the newly described G. bourloni. The combined analysis yielded similar results although morphological characters were clearly homoplasic. Partitioned Bremer supports indicated conflicting signals between the two data sets throughout the tree, and species‐diagnostic characters, useful for delimiting species boundaries, were significantly correlated to habitat. However, morphological data supported the monophyly of clades (G. victoriae, other genets) (G. servalina, G. cristata), large‐spotted genet complex and forest forms. Our results suggest a complex evolutionary history of the genets in Africa, with a Poiana‐like ancestor inhabiting rain forest, and then a diversification involving two independent invasions of open habitats and one reversion to rain forest. Divergence estimates based on cyt b revealed that splitting events within genets partly follow a climatic speciation model during the cyclical periods of the Quaternary, although ‘primitive’ rain forest lineages diverged earlier, during the Late Miocene and Early Pliocene. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 81 , 589–610.     

Reconstructing the evolutionary history of the Lorisidae using morphological, molecular, and geological data

Major aspects of lorisid phylogeny and systematics remain unresolved, despite several studies (involving morphology, histology, karyology, immunology, and DNA sequencing) aimed at elucidating them. Our study is the first to investigate the evolution of this enigmatic group using molecular and morphological data for all four well-established genera: Arctocebus, Loris, Nycticebus, and Perodicticus. Data sets consisting of 386 bp of 12S rRNA, 535 bp of 16S rRNA, and 36 craniodental characters were analyzed separately and in combination, using maximum parsimony and maximum likelihood. Outgroups, consisting of two galagid taxa (Otolemur and Galagoides) and a lemuroid (Microcebus), were also varied. The morphological data set yielded a paraphyletic lorisid clade with the robust Nycticebus and Perodicticus grouped as sister taxa, and the galagids allied with Arctocebus. All molecular analyses maximum parsimony (MP) or maximum likelihood (ML) which included Microcebus as an outgroup rendered a paraphyletic lorisid clade, with one exception: the 12S + 16S data set analyzed with ML. The position of the galagids in these paraphyletic topologies was inconsistent, however, and bootstrap values were low. Exclusion of Microcebus generated a monophyletic Lorisidae with Asian and African subclades; bootstrap values for all three clades in the total evidence tree were over 90%. We estimated mean genetic distances for lemuroids vs. lorisoids, lorisids vs. galagids, and Asian vs. African lorisids as a guide to relative divergence times. We present information regarding a temporary land bridge that linked the two now widely separated regions inhabited by lorisids that may explain their distribution. Finally, we make taxonomic recommendations based on our results.     

Anatomy and histology of the brain and sense organs of the Antarctic eel cod Muraenolepis microps (Gadiformes; Muraenolepididae)

Brain regions, cranial nerves, and sense organs in Muraenolepis microps, an Antarctic gadiform fish, were examined to determine which features could be attributed to a gadiform ancestry and which to habitation of Antarctic waters. We found that the central nervous system and sense organs are well developed, showing neither substantial regression nor hypertrophy. A detailed drawing of the brain and cranial nerves is provided. The rostral position of the olfactory bulbs and telencephalic size and lobation are common for the order. The optic tectum and corpus cerebelli are smaller than in most other gadiforms. The shape of the corpus cerebelli is not distinctive among gadiforms. The lateral line region is moderately well-developed, but not hypertrophied to the extent seen in deep-sea gadiforms. As is the case in gadids possessing barbels and elongated pelvic rays, Muraenolepis has well-developed facial lobes, although these are smaller and more laterally positioned. The vagal lobes are deeply placed in the rhombencephalon and project into the fourth ventricle. The brain of Muraenolepis resembles that of a phyletically derived gadoid, especially a phycid, more than it resembles the brain of a phyletically basal macrourid. Two histological features of the diencephalon of Muraenolepis appear to be unique among gadiforms: a well-organized thalamic central medial nucleus and subependymal expansions. Muraenolepis has a pure rod retina like many deep-sea species but lacks the superimposed layers of rod outer segments. The histology of the nonvisual sense organs, especially the olfactory and external taste systems, are well-developed in Muraenolepis but not hypertrophied. We relate our findings to what is known about neural morphology in other gadiforms and in phyletically distant notothenioids and liparids that are sympatric with Muraenolepis on the Antarctic shelf. The only feature that reflects an Antarctic existence is the diencephalic subependymal expansions, which within notothenioids mirror the habitation of cold waters and have been found in every Antarctic species examined to date. Although the waters of the Antarctic shelf are cold, dark, and deep, brain and sense organ morphology in Muraenolepis are remarkably free of extreme specialization.     

Romundina and the evolutionary origin of teeth

Theories on the origin of vertebrate teeth have long focused on chondrichthyans as reflecting a primitive condition—but this is better informed by the extinct placoderms, which constitute a sister clade or grade to the living gnathostomes. Here, we show that ‘supragnathal’ toothplates from the acanthothoracid placoderm Romundina stellina comprise multi-cuspid teeth, each composed of an enameloid cap and core of dentine. These were added sequentially, approximately circumferentially, about a pioneer tooth. Teeth are bound to a bony plate that grew with the addition of marginal teeth. Homologous toothplates in arthrodire placoderms exhibit a more ordered arrangement of teeth that lack enameloid, but their organization into a gnathal, bound by layers of cellular bone associated with the addition of each successional tooth, is the same. The presence of enameloid in the teeth of Romundina suggests that it has been lost in other placoderms. Its covariation in the teeth and dermal skeleton of placoderms suggests a lack of independence early in the evolution of jawed vertebrates. It also appears that the dentition—manifest as discrete gnathal ossifications—was developmentally discrete from the jaws during this formative episode of vertebrate evolution.     

Hindlimb morphology and locomotor performance in waders: an evolutionary approach

Locomotion performance (measured as stride frequency and stride length) was studied in 16 species of waders. Differences in hindlimb morphology (osteology and myology) were analysed among species. Evolutionary changes in both locomotion and morphological variables were analysed using comparative methods revealing the existence of some ecomorphological patterns relating these two sets of characters. Evolutionary changes in stride frequency were correlated with changes in the muscles M. iliotibialis cranialis, M. iliotibiales lateralis and M. gastrocnemius, whereas changes in stride length showed correlated evolution with changes in the length of distal segments of the leg. We identify two different evolutionary strategies in locomotion of waders. One is a change in distal leg segments (skeletal system), an adaptive modification that increases stride length; the second is a change in the skeletal-muscular system, providing an increase in muscular performance (force or speed of contraction) in several muscles, and is an adaptation that increases stride frequency.     

Organ culture of rat carotid artery: Maintenance of morphological characteristics and of pattern of matrix synthesis

Summary Segments of rat carotid artery were maintained in serum-free or serum-supplemented medium for 2 wk, and at intervals of 3, 7, and 14 d the morphology and pattern of matrix synthesis were compared to those in vivo. In serum-free medium and 0.2% serum both the endothelium and the smooth muscle cells (SMC) could be maintained with a minimum of change for 7 d and without substantial change for 14 d. In 2% and 10% serum there was little change for the first 3 d, but subsequently there was a progressive overlapping of the endothelial cells to produce a 3 to 4 layered cell sheet, often separated from the subendothelial matrix; the SMC, however, did not appear to proliferate or migrate and in general retained their typical cellular features for the full time in culture. The synthesis of matrix components was measured by autoradiographic detection of incorporated [³H]glucosamine and³⁵S. At all time periods and serum concentrations the percentage distribution for each label across the arterial wall was found to be similar to that in live animals injected with the same labels. [³H]Glucosamine predominated in the endothelium and the narrow subendothelial layer, which together make up the intima, whereas³⁵S predominated in the media. In vitro more than 50% of the [³H]glucosamine in the intima and 40% in the adjacent first layer of the media was susceptible toStreptomyces hyaluronidase. As the morphology of both cell types and their synthesis of matrix components could be maintained in organ culture without substantial change we believe that the rat carotid artery may be a suitable model for the investigation of factors affecting arterial structure. This work was supported by grants from the Medical Research Council and the National Heart Foundation of New Zealand.     

Functional and evolutionary consequences of cranial fenestration in birds

Ostrich‐like birds (Palaeognathae) show very little taxonomic diversity while their sister taxon (Neognathae) contains roughly 10,000 species. The main anatomical differences between the two taxa are in the crania. Palaeognaths lack an element in the bill called the lateral bar that is present in both ancestral theropods and modern neognaths, and have thin zones in the bones of the bill, and robust bony elements on the ventral surface of their crania. Here we use a combination of modeling and developmental experiments to investigate the processes that might have led to these differences. Engineering‐based finite element analyses indicate that removing the lateral bars from a neognath increases mechanical stress in the upper bill and the ventral elements of the skull, regions that are either more robust or more flexible in palaeognaths. Surgically removing the lateral bar from neognath hatchlings led to similar changes. These results indicate that the lateral bar is load‐bearing and suggest that this function was transferred to other bony elements when it was lost in palaeognaths. It is possible that the loss of the load‐bearing lateral bar might have constrained diversification of skull morphology in palaeognaths and thus limited taxonomic diversity within the group.