Axial allometry in a neutrally buoyant environment: effects of the terrestrial‐aquatic transition on vertebral scaling

Ecological diversification into new environments presents new mechanical challenges for locomotion. An extreme example of this is the transition from a terrestrial to an aquatic lifestyle. Here, we examine the implications of life in a neutrally buoyant environment on adaptations of the axial skeleton to evolutionary increases in body size. On land, mammals must use their thoracolumbar vertebral column for body support against gravity and thus exhibit increasing stabilization of the trunk as body size increases. Conversely, in water, the role of the axial skeleton in body support is reduced, and, in aquatic mammals, the vertebral column functions primarily in locomotion. Therefore, we hypothesize that the allometric stabilization associated with increasing body size in terrestrial mammals will be minimized in secondarily aquatic mammals. We test this by comparing the scaling exponent (slope) of vertebral measures from 57 terrestrial species (23 felids, 34 bovids) to 23 semi‐aquatic species (pinnipeds), using phylogenetically corrected regressions. Terrestrial taxa meet predictions of allometric stabilization, with posterior vertebral column (lumbar region) shortening, increased vertebral height compared to width, and shorter, more disc‐shaped centra. In contrast, pinniped vertebral proportions (e.g. length, width, height) scale with isometry, and in some cases, centra even become more spool‐shaped with increasing size, suggesting increased flexibility. Our results demonstrate that evolution of a secondarily aquatic lifestyle has modified the mechanical constraints associated with evolutionary increases in body size, relative to terrestrial taxa.     

Amphibious adaptations in a newly recognized amphibious fish: Terrestrial locomotion and the influences of body size and temperature

Amphibious animals are adapted for both aquatic and terrestrial habitats. The conflicting requirements for dual habitats are perhaps most pronounced in the air‐breathing fishes, which represent an intermediate stage between the totally aquatic habitat and terrestrial colonization. A key requirement for amphibious fishes is terrestrial locomotion. The different densities and compositions of air and water impose constraints for efficient terrestrial locomotion that differ from those required for aquatic locomotion. I investigated terrestrial locomotion in a small South African fish, Galaxias ‘nebula’, by exposing 60 individual fish to air in specially designed raceways and quantifying movement type and occurrence as a function of availability of water, fish size and environmental temperature. Nebula showed a sustained undulating form of terrestrial locomotion characteristic of amphibious fishes and also a transient ballistic locomotion (jumps) typical of fully aquatic species. Terrestrial movement was influenced by fish size, with medium‐sized fish undertaking more jumps towards water, and fewer jumps away from water, than their smaller or larger conspecifics. In contrast, axial undulation was mainly influenced by temperature. However, there was no consistent pattern in temperature effects presumably because temperature is just one of a suit of environmental factors that may affect terrestrial locomotion. Nebula's amphibious adaptations allow it to cope with the unpredictability inherent in its natural environment.     

Neither season nor sex affects the cost of terrestrial locomotion in a circumpolar diving duck: the common eider (Somateria mollissima)

Locomotion accounts for a significant proportion of the energy budget in birds, and selection is likely to act on its economy, particularly where energy conservation is essential for survival. Birds are capable of different forms of locomotion, such as walking/running, swimming, diving and flying, and adaptations for these affect the energetic cost [cost of locomotion (CoL)] and kinematics of terrestrial locomotion. Furthermore, seasonal changes in climate and photoperiod elicit physiological and behavioural adaptations for survival and reproduction, which also influence energy budget. However, little is understood about how this might affect the CoL. Birds are also known to exhibit sex differences in size, behaviour and physiology; however, sex differences in terrestrial locomotion have only been studied in two cursorially adapted galliform species in which males achieved higher maximum speeds, and in one case had a lower mass-specific CoL than females. Here, using respirometry and high-speed video recordings, we sought to determine whether season and sex would affect the CoL and kinematics of a principally aquatic diving bird: the circumpolar common eider (Somateria mollissima). We demonstrate that eiders are only capable of a walking gait and exhibit no seasonal or sex differences in mass-specific CoL or maximum speed. Despite sharing identical limb morphometrics, the birds exhibited subtle sex differences in kinematic parameters linked to the greater body mass of the males. We suggest that their principally aquatic lifestyle accounts for the observed patterns in their locomotor performance. Furthermore, sex differences in the CoL may only be found in birds in which terrestrial locomotion directly influences male reproductive success.     

The effects of locomotion on the structural characteristics of avian limb bones

Despite the wide range of locomotor adaptations in birds, little detailed attention has been given to the relationships between the quantitative structural characteristics of avian limb bones and bird behaviour. Possible differences in forelimb relative to hindlimb strength across species have been especially neglected. We generated cross‐sectional, geometric data from peripheral quantitative computed tomography scans of the humerus and femur of 127 avian skeletons, representing 15 species of extant birds in 13 families. The sample includes terrestrial runners, arboreal perchers, hindlimb‐propelled divers, forelimb‐propelled divers and dynamic soarers. The hindlimb‐propelled diving class includes a recently flightless island form. Our results demonstrate that locomotor dynamics can be differentiated in most cases based on cross‐sectional properties, and that structural proportions are often more informative than bone length proportions for determining behaviour and locomotion. Recently flightless forms, for example, are more easily distinguished using structural ratios than using length ratios. A proper phylogenetic context is important for correctly interpreting structural characteristics, especially for recently flightless forms. Some of the most extreme adaptations to mechanical loading are seen in aquatic forms. Penguins have forelimbs adapted to very high loads. Aquatic species differ from non‐aquatic species on the basis of relative cortical thickness. The combination of bone structural strength and relative cortical area of the humerus successfully differentiates all of our locomotor groups. The methods used in this study are highly applicable to fossil taxa, for which morphology is known but behaviour is not. The use of bone structural characteristics is particularly useful in palaeontology not only because it generates strong signals for many locomotor guilds, but also because analysing such traits does not require knowledge of body mass, which can be difficult to estimate reliably for fossil taxa. © 2008 The Linnean Society of London, Zoological Journal of the Linnean Society, 2008, 153 , 601–624.     

Evolution and function of anterior cervical vertebral fusion in tetrapods

The evolution of vertebral fusion is a poorly understood phenomenon that results in the loss of mobility between sequential vertebrae. Non‐pathological fusion of the anterior cervical vertebrae has evolved independently in numerous extant and extinct mammals and reptiles, suggesting that the formation of a ‘syncervical’ is an adaptation that arose to confer biomechanical advantage(s) in these lineages. We review syncervical anatomy and evolution in a broad phylogenetic context for the first time and provide a comprehensive summary of proposed adaptive hypotheses. The syncervical generally consists of two vertebrae (e.g. hornbills, porcupines, dolphins) but can include fusion of seven cervical vertebrae in some cetaceans. Based on the ecologies of taxa with this trait, cervical fusion most often occurs in fossorial and pelagic taxa. In fossorial taxa, the syncervical likely increases the out‐lever force during head‐lift digging. In cetaceans and ricochetal rodents, the syncervical may stabilize the head and neck during locomotion, although considerable variation exists in its composition without apparent variability in locomotion. Alternatively, the highly reduced cervical vertebral centra may require fusion to prevent mechanical failure of the vertebrae. In birds, the syncervical of hornbills may have evolved in response to their unique casque‐butting behaviour, or due to increased head mass. The general correlation between ecological traits and the presence of a syncervical in extant taxa allows more accurate interpretation of extinct animals that also exhibit this unique trait. For example, syncervicals evolved independently in several groups of marine reptiles and may have functioned to stabilize the head at the craniocervical joint during pelagic locomotion, as in cetaceans. Overall, the origin and function of fused cervical vertebrae is poorly understood, emphasizing the need for future comparative biomechanical studies interpreted in an evolutionary context.     

The evolution of metamorphosis in temnospondyls

Temnospondyls, possible relatives of extant amphibians and crudely similar to recent salamanders, are known from larval, neotenic and metamorphosed stages. Here, ontogenetic data of various temnospondyl taxa are analysed in order to recognize metamorphosis. Here, metamorphosis is strictly defined as a shift from an aquatic to a terrestrial existence. Following a check-list of criteria, the most likely metamorphosis-induced changes are proved in three temnospondyl lineages: eryopids, zatrachydids and dissorophoids. In a few other, unrelated taxa, terrestrial adults are known but no larval or metamorphosing forms. The distribution of metamorphosis among the Temnospondyli does not strictly correlate with phylogeny, which highlights the widespread occurrence of neoteny. In each group, characteristic patterns of metamorphosis are described and compared. Among temnospondyls, dissorophoids had the most intensive type of metamorphosis, characterized by a condensed ontogeny and a relatively small body size. The result was a distinct transformed morphotype with far-reaching terrestrial adaptations.     

Trabecular architecture in the humeral metaphyses of non-avian reptiles (Crocodylia,Squamata and Testudines): Lifestyle,allometry and phylogeny

The lifestyle of extinct tetrapods is often difficult to assess when clear morphological adaptations such as swimming paddles are absent. According to the hypothesis of bone functional adaptation, the architecture of trabecular bone adapts sensitively to physiological loadings. Previous studies have already shown a clear relation between trabecular architecture and locomotor behavior, mainly in mammals and birds. However, a link between trabecular architecture and lifestyle has rarely been examined. Here, we analyzed trabecular architecture of different clades of reptiles characterized by a wide range of lifestyles (aquatic, amphibious, generalist terrestrial, fossorial, and climbing). Humeri of squamates, turtles, and crocodylians have been scanned with microcomputed tomography. We selected spherical volumes of interest centered in the proximal metaphyses and measured trabecular spacing, thickness and number, degree of anisotropy, average branch length, bone volume fraction, bone surface density, and connectivity density. Only bone volume fraction showed a significant phylogenetic signal and its significant difference between squamates and other reptiles could be linked to their physiologies. We found negative allometric relationships for trabecular thickness and spacing, positive allometries for connectivity density and trabecular number and no dependence with size for degree of anisotropy and bone volume fraction. The different lifestyles are well separated in the morphological space using linear discriminant analyses, but a cross-validation procedure indicated a limited predictive ability of the model. The trabecular bone anisotropy has shown a gradient in turtles and in squamates: higher values in amphibious than terrestrial taxa. These allometric scalings, previously emphasized in mammals and birds, seem to be valid for all amniotes. Discriminant analysis has offered, to some extent, a distinction of lifestyles, which however remains difficult to strictly discriminate. Trabecular architecture seems to be a promising tool to infer lifestyle of extinct tetrapods, especially those involved in the terrestrialization.     

From clear lakes to murky waters – tracing the functional response of high‐latitude lake communities to concurrent ‘greening’ and ‘browning’

Climate change and the intensification of land use practices are causing widespread eutrophication of subarctic lakes. The implications of this rapid change for lake ecosystem function remain poorly understood. To assess how freshwater communities respond to such profound changes in their habitat and resource availability, we conducted a space‐for‐time analysis of food‐web structure in 30 lakes situated across a temperature‐productivity gradient equivalent to the predicted future climate of subarctic Europe (temperature +3°C, precipitation +30% and nutrient +45 μg L^?1 total phosphorus). Along this gradient, we observed an increase in the assimilation of pelagic‐derived carbon from 25 to 75% throughout primary, secondary and tertiary consumers. This shift was overwhelmingly driven by the consumption of pelagic detritus by benthic primary consumers and was not accompanied by increased pelagic foraging by higher trophic level consumers. Our data also revealed a convergence of the carbon isotope ratios of pelagic and benthic food web endmembers in the warmest, most productive lakes indicating that the incorporation of terrestrial derived carbon into aquatic food webs increases as land use intensifies. These results, reflecting changes along a gradient characteristic of the predicted future environment throughout the subarctic, indicate that climate and land use driven eutrophication and browning are radically altering the function and fuelling of aquatic food webs in this biome.     

Is sexual body shape dimorphism consistent in aquatic and terrestrial chelonians?

Comparisons between aquatic and terrestrial species provide an opportunity to examine how sex-specific adaptations interact with the environment to influence body shape. In terrestrial female tortoises, selection for fecundity favors the development of a large internal abdominal cavity to accommodate the clutch; in conspecific males, sexual selection favors mobility with large openings in the shell. To examine to what extent such trends apply in aquatic chelonians we compared the body shape of males and females of two aquatic turtles (Chelodina colliei and Mauremys leprosa). In both species, females were larger than males. When controlled for body size, females exhibited a greater relative internal volume and a higher body condition index than males; both traits potentially correlate positively with fecundity. Males were more streamlined (hydrodynamic), and exhibited larger openings in the shell providing more space to move their longer limbs; such traits probably improve mobility and copulation ability (the males chase and grab the female for copulation). Overall, although the specific constraints imposed by terrestrial and aquatic locomotion shape the morphology of chelonians differently (aquatic turtles were flatter, hence more hydrodynamic than terrestrial tortoises), the direction for sexual shape dimorphism remained unaffected. Our main conclusion is that the direction of sexual shape dimorphism is probably more consistent than sexual size dimorphism in the animal kingdom.     

Functional morphology of cercopithecoid primate metacarpals

The primate fossil record suggests that terrestriality was more common in the past than it is today, particularly among cercopithecoid primates. Whether or not a fossil primate habitually preferred terrestrial substrates has typically been inferred from its forelimb anatomy. Because extant large-bodied terrestrial cercopithecine monkeys utilize digitigrade hand postures during locomotion, being able to identify if a fossil primate habitually adopted digitigrade postures would be particularly revealing of terrestriality in this group. This paper examines the functional morphology of metacarpals in order to identify osteological correlates of digitigrade versus palmigrade hand postures. Linear measurements were obtained from 324 individuals belonging to digitigrade and palmigrade cercopithecoid species and comparisons were made between hand posture groups. Digitigrade taxa have shorter metacarpals, relative to both body mass and humerus length, than palmigrade taxa. Also, digitigrade taxa tend to have metacarpals with smaller dorsoventral diameters, relative to the product of body mass and metacarpal length, compared to palmigrade taxa. The size and shape of the metacarpal heads do not significantly differ between hand posture groups. Multivariate analyses suggest that metacarpal shape can only weakly discriminate between hand posture groups. In general, while there are some morphological differences in the metacarpals between hand posture groups, similarities also exist that are likely related to the fact that even digitigrade cercopithecoids can adopt palmigrade hand postures in different situations (e.g., terrestrial running, arboreal locomotion), and/or that the functional demands of different hand postures are not reflected in all aspects of metacarpal morphology. Therefore, the lack of identifiable adaptations for specific hand postures in extant cercopithecoids makes it difficult to determine a preference for specific habitats from fossil primate hand bones.     

Feeding biomechanics in Acanthostega and across the fish–tetrapod transition

Acanthostega is one of the earliest and most primitive limbed vertebrates. Its numerous fish-like features indicate a primarily aquatic lifestyle, yet cranial suture morphology suggests that its skull is more similar to those of terrestrial taxa. Here, we apply geometric morphometrics and two-dimensional finite-element analysis to the lower jaws of Acanthostega and 22 other tetrapodomorph taxa in order to quantify morphological and functional changes across the fish–tetrapod transition. The jaw of Acanthostega is similar to that of certain tetrapodomorph fish and transitional Devonian taxa both morphologically (as indicated by its proximity to those taxa in morphospace) and functionally (as indicated by the distribution of stress values and relative magnitude of bite force). Our results suggest a slow tempo of morphological and biomechanical changes in the transition from Devonian tetrapod jaws to aquatic/semi-aquatic Carboniferous tetrapod jaws. We conclude that Acanthostega retained a primitively aquatic lifestyle and did not possess cranial adaptations for terrestrial feeding.     

Microanatomical diversity of the humerus and lifestyle in lissamphibians

A study of body size and the compactness profile parameters of the humerus of 37 species of lissamphibians demonstrates a relationship between lifestyle (aquatic, amphibious or terrestrial) and bone microstructure. Multiple linear regressions and variance partitioning with Phylogenetic eigenVector Regressions reveal an ecological and a phylogenetic signal in some body size and compactness profile parameters. Linear discriminant analyses segregate the various lifestyles (aquatic vs. amphibious or terrestrial) with a success rate of up to 89.2%. The models built from data on the humerus discriminate aquatic taxa relatively well from the other taxa. However, like previous models built from data on the radius of amniotes or on the femur of lissamphibians, the new models do not discriminate amphibious taxa from terrestrial taxa on the basis of body size or compactness profile data. To make our inference method accessible, spreadsheets (see supplementary material on the website), which allow anyone to infer a lissamphibian lifestyle solely from body size and bone compactness parameters, were produced. No such easy implementation of habitat inference models is found in earlier papers on this topic.     

Head‐turning morphologies: Evolution of shape diversity in the mammalian atlas–axis complex

Mammals flex, extend, and rotate their spines as they perform behaviors critical for survival, such as foraging, consuming prey, locomoting, and interacting with conspecifics or predators. The atlas–axis complex is a mammalian innovation that allows precise head movements during these behaviors. Although morphological variation in other vertebral regions has been linked to ecological differences in mammals, less is known about morphological specialization in the cervical vertebrae, which are developmentally constrained in number but highly variable in size and shape. Here, we present the first phylogenetic comparative study of the atlas–axis complex across mammals. We used spherical harmonics to quantify 3D shape variation of the atlas and axis across a diverse sample of species, and performed phylogenetic analyses to investigate if vertebral shape is associated with body size, locomotion, and diet. We found that differences in atlas and axis shape are partly explained by phylogeny, and that mammalian subclades differ in morphological disparity. Atlas and axis shape diversity is associated with differences in body size and locomotion; large terrestrial mammals have craniocaudally elongated vertebrae, whereas smaller mammals and aquatic mammals have more compressed vertebrae. These results provide a foundation for investigating functional hypotheses underlying the evolution of neck morphologies across mammals.     

Osteology and relationships of the temnospondyl genus Sclerocephalus

The temnospondyl Sclerocephalus from the Permo‐Carboniferous of Germany is one of the most completely preserved and most abundant Palaeozoic tetrapods. Here, we review the complete osteology of the genus, based on a range of fully grown specimens housed in public collections. Among the four valid species, Sclerocephalus haeuseri and Sclerocephalus nobilis reached an adult size of well beyond 1 m in length, and had robust postcranial skeletons. In the skull, the exoccipital and sphenethmoid bones were ossified, completing the well‐known ossification sequence in S. haeuseri. Large adults had an elongate trunk and a laterally compressed tail, and some individuals also retained lateral line sulci: features that taken together suggest an aquatic life. The coracoid, pubis, carpals, tarsals, and the bony tail are fully ossified in the largest specimens. The genus Sclerocephalus forms a weakly supported clade nesting firmly at the base of the Stereospondylomorpha, and the close resemblance between Sclerocephalus, Onchiodon, and Eryops is found to be partially based on shared derived states, but is mostly based on symplesiomorphies. Cladistic analysis of 54 characters and 18 taxa finds more support for the Eryopoidea hypothesis (Eryopidae + Stereospondylomorpha) than for the Euskelia hypothesis (Eryopidae + Zatracheidae + Dissorophoidea). This indicates that the large temnospondyls of the Permian and Mesozoic probably formed a natural group, and that the terrestrial adaptations of Eryops and the dissorophoids probably evolved by convergence.     

How muscles accommodate movement in different physical environments: aquatic vs. terrestrial locomotion in vertebrates.

Representatives of nearly all vertebrate classes are capable of coordinated movement through aquatic and terrestrial environments. Though there are good data from a variety of species on basic patterns of muscle recruitment during locomotion in a single environment, we know much less about how vertebrates use the same musculoskeletal structures to accommodate locomotion in physically distinct environments. To address this issue, we have gathered data from a broad range of vertebrates that move successfully through water and across land, including eels, toads, turtles and rats. Using high-speed video in combination with electromyography and sonomicrometry, we have quantified and compared the activity and strain of individual muscles and the movements they generate during aquatic vs. terrestrial locomotion. In each focal species, transitions in environment consistently elicit alterations in motor output by major locomotor muscles, including changes in the intensity and duration of muscle activity and shifts in the timing of activity with respect to muscle length change. In many cases, these alterations likely change the functional roles played by muscles between aquatic and terrestrial locomotion. Thus, a variety of forms of motor plasticity appear to underlie the ability of many species to move successfully through different physical environments and produce diverse behaviors in nature.     

Turning Semicircular Canal Function on Its Head: Dinosaurs and a Novel Vestibular Analysis

Previous investigations have correlated vestibular function to locomotion in vertebrates by scaling semicircular duct radius of curvature to body mass. However, this method fails to discriminate bipedal from quadrupedal non-avian dinosaurs. Because they exhibit a broad range of relative head sizes, we use dinosaurs to test the hypothesis that semicircular ducts scale more closely with head size. Comparing the area enclosed by each semicircular canal to estimated body mass and to two different measures of head size, skull length and estimated head mass, reveals significant patterns that corroborate a connection between physical parameters of the head and semicircular canal morphology. Head mass more strongly correlates with anterior semicircular canal size than does body mass and statistically separates bipedal from quadrupedal taxa, with bipeds exhibiting relatively larger canals. This morphologic dichotomy likely reflects adaptations of the vestibular system to stability demands associated with terrestrial locomotion on two, versus four, feet. This new method has implications for reinterpreting previous studies and informing future studies on the connection between locomotion type and vestibular function.     

Trabecular bone structure in the primate wrist

Trabecular (or cancellous) bone has been shown to respond to mechanical loading throughout ontogeny and thus can provide unique insight into skeletal function and locomotion in comparative studies of living and fossil mammalian morphology. Trabecular bone of the hand may be particularly functionally informative because the hand has more direct contact with the substrate compared with the remainder of the forelimb during locomotion in quadrupedal mammals. This study investigates the trabecular structure within the wrist across a sample of haplorhine primates that vary in locomotor behaviour (and thus hand use) and body size. High‐resolution microtomographic scans were collected of the lunate, scaphoid, and capitate in 41 individuals and eight genera (Homo, Gorilla, Pan, Papio, Pongo, Symphalangus, Hylobates, and Ateles). We predicted that particular trabecular parameters would 1) vary across suspensory, quadrupedal, and bipedal primates based on differences in hand use and load, and 2) scale with carpal size following similar allometric patterns found previously in other skeletal elements across a larger sample of mammals and primates. Analyses of variance (trabecular parameters analysed separately) and principal component analyses (trabecular parameters analysed together) revealed no clear functional signal in the trabecular structure of any of the three wrist bones. Instead, there was a large degree of variation within suspensory and quadrupedal locomotor groups, as well as high intrageneric variation within some taxa, particularly Pongo and Gorilla. However, as predicted, Homo sapiens, which rarely use their hands for locomotion and weight support, were unique in showing lower relative bone volume (BV/TV) compared with all other taxa. Furthermore, parameters used to quantify trabecular structure within the wrist scale with size generally following similar allometric patterns found in trabeculae of other mammalian skeletal elements. We discuss the challenges associated with quantifying and interpreting trabecular bone within the wrist. J. Morphol. 275:572–585, 2014. © 2013 Wiley Periodicals, Inc.     

Rib and vertebral micro‐anatomical characteristics of hydropelvic mosasauroids

Houssaye, A. & Bardet, N. 2011: Rib and vertebral micro‐anatomical characteristics of hydropelvic mosasauroids. Lethaia, Vol. 45, pp. 200–209. Mosasauroids are squamates secondarily adapted to an aquatic life that dominated the sea during the Late Cretaceous. Mosasauroids display distinct types of morphologies illustrating steps in the adaptation of this lineage to increasing obligatory habits. Hydropelvic mosasauroids (sensu Caldwell & Palci) were the most highly adapted to an open‐sea environment. Contrary to plesiopelvic forms, they are considered to have relied on a hydrodynamic, rather than hydrostatic buoyancy and body trim control strategies. This led previous authors to consider that these taxa would favour bone lightening (osteoporotic‐like condition) rather than bone mass increase. Although an osteoporotic‐like state was indeed described in Clidastes and Tylosaurus, bone mass increase was reported in Platecarpus. As a matter of fact, the new analysis of vertebral thin sections of various taxa combined with the reanalysis of the rib sections available in Sheldon’s PhD thesis in a micro‐anatomical perspective reveals the absence of both bone mass increase and bone lightening in these organisms. These taxa in fact display a vertebral micro‐anatomy much peculiar within squamates. It characteristically corresponds to a true network of thin trabeculae whose tightness varies between taxa, probably as a result of both species and individual size differences, particularly the latter. In addition, analysis of the pattern of vascularization as observed in hydropelvic mosasauroids, which is unique amongst squamates, suggests that large size in hydropelvic mosasauroids would mainly rely on protracted rather than faster growth rates. □Histology, hydropelvic mosasauroids, micro‐anatomy, ribs, vascularization, vertebrae.     

Stem biomechanics of the giant moss Dendroligotrichum dendroides s.l. and its significance for growth form diversity in mosses

Abstract

The giant moss Dendroligotrichum dendroides s.l. grows as self-supporting plants up to 40 cm in height in forest habitats in Chile and New Zealand. This moss represents one of the tallest self-supporting bryophytes. Biomechanical tests indicate that the stems can develop a high degree of stiffness (Young’s modulus) via a dense hypodermal sterome that is comparable with that of woody stems of vascular plants. A comparison with mechanical properties of other terrestrial and aquatic mosses indicates that different moss growth and life forms can produce very different mechanical architectures. Values of stem stiffness can vary between different growth forms of mosses to a comparable extent to that observed among diverse growth forms of vascular plants. Plants varying profoundly in overall size, development, and phylogenetic position nevertheless appear to develop comparable mechanical adaptations and growth forms in response to certain environmental conditions.