Journal of Morphology最新文献

The axolotl (Ambystoma mexicanum) is an endemic amphibian of the order Urodela, which has the ability to reproduce while retaining its larval characteristics. The primary objective of this study was to outline the stages of gonadal development leading to sexual maturity. Gonads from both male and female larvae, aged 4-12 months, were histologically processed and characterized according to their developmental stage. Histological analysis of the ovaries revealed that larvae aged 4-8 months were immature. At 9 and 10 months, the oocytes showed early signs of maturity, characterized by an increase in volume, yolk accumulation, and the presence of cortical pigment at one cellular pole. At 12 months, mature oocytes are predominant, as indicated by a significant increase in size and the migration of the germinal vesicle toward the pigmented animal pole. Male larvae have immature gonads at 4-5 months of age. By 7 to 8 months, the beginning of the spermatogenic process is evident, regionalizing the testis in a cephalocaudal and lateromedial direction. In 9-month-old males, the spermiation process begins, becoming dominant by 10 months. In 12-month-old males, a clear regionalization of the testis is evident; while spermiation continues in the caudal and medial regions, spermatogenesis restarts in the more cephalic and dorsal areas. This early and accelerated gametogenesis in males signifies a gonadal maturity that occurs sooner than in females, which reflects a process of heterochrony. The relatively early attainment of sexual maturity for both sexes, a direct result of their paedomorphic condition, is an advantageous adaptive strategy.

Flamingos (Phoenicopteridae) feed using a uniquely adapted bill that extracts small particles from the water and sediments. This study provides a detailed osteological description of the skull of the Chilean Flamingo (Phoenicopterus chilensis), with particular emphasis on feeding-related features. The skull exhibits a broad, flattened frontal region that articulates with an elongated upper jaw, which is deflected ventrally at approximately 45° and aligns seamlessly with the laterorostrally curved mandible, forming a configuration well suited for filter feeding. The mandible exhibits a low mechanical advantage, indicating an adaptation for rapid and repetitive movements rather than forceful biting. Jaw muscle attachment sites, including the fossa subtemporalis, fossa temporalis, and fossa ventralis palatini, are reduced, suggesting the presence of relatively small muscles that favor speed and precision.

Traveling on arboreal substrates requires behavioral and morphological adaptations to reduce the likelihood of falls. Many arboreal specialists have greater distal limb mass compared with closely related terrestrial taxa. In this study, we test the hypothesis that augmenting the distal limb mass will increase stability during locomotion, allowing a quadrupedal mammal to rely less on static methods of stability when moving on a cylindrical support. Thus, we predict that weighted bracelets will result in less crouching, greater vertical oscillation, lower duty factor, and higher limb phase. We trained four laboratory rats to walk on a "rope-mill," the arboreal equivalent of a treadmill. We marked the fur over the glenohumeral joint and the greater trochanter, and then encouraged the animals to run either unmodified, wearing sham bracelets, or weighted bracelets (1.8 g each) on the wrists and ankles. From two video cameras, we extracted sequences of 15 strides for each experimental setup and digitized the right shoulder, hand, hip, and foot. When the rats walked with weighted distal limbs, the limb phase was lower, but duty factor was unchanged. The shoulder height was somewhat greater, whereas vertical accelerations were lower. The hip height and vertical accelerations were unchanged. Each individual adjusted to increased distal limb mass in a variety of ways. We conclude that adding mass to the distal parts of limbs did not enhance stability. We suggest that neuromuscular and developmental differences among individuals contribute to the variability among animals in responding to wearing weighted bracelets.

The dermal skeleton is the most primitive component of the vertebrate mineralized skeleton, and features of its structure and development are key to resolving the evolutionary relationships of early vertebrates and bony fishes. In particular, the nature and phylogenetic distribution of cosmine, a dermal complex of hard tissue and vascular systems, have been the focus of debate over the nature of the ancestral osteichthyan and the timing of actinopterygian-sarcopterygian divergence. In large part, this controversy occurs because of a paucity of knowledge of the nature of the dermal skeleton in stem-actinopterygians. Here, we describe the dermal skeletal histology of stem-actinopterygian Moythomasia durgaringa using Scanning Electron Microscopy and Synchrotron Radiation X-ray Tomographic Microscopy with a reconstruction of its topological variation and development. The dermal skeleton of Moythomasia consists of a superficial layer of stacked odontodes that undergo extensive odontogenic resorption and a basal layer of lamellar bone. A middle vascular bone layer is variably developed in cranial dermal bones but is completely absent in postcranial dermal elements. Additional histological variation among dermal elements includes the number of odontode generations, odontode growth patterns and the relative thickness of osteogenic and odontogenic tissues. A comparison of the histological condition in Moythomasia and stem- and early crown-osteichthyans reveals numerous similarities, including the presence of a three-layered dermal skeleton, stacked odontodes and odontogenic resorption. Phylogenetic comparative analyses on early jawed vertebrates indicate that features associated with cosmine evolved in groups outside Rhipidistia, whereas true cosmine remains restricted to this group comprising Dipnomorpha and Tetrapodomorpha. The concept of cosmine is phylogenetically uninformative because of the multiplicity of its definitions and usage. These findings suggest that fossil taxa currently classified as stem-sarcopterygians may instead be stem-actinopterygians, or even stem-osteichthyans, with implications for the nature of the ancestral bony fish and the timing of osteichthyan diversification.

This study investigated the spermatogenesis of the freshwater stingray Potamotrygon motoro using microscopic analyses. The testes of this species were described as being composed of germinal zones, a degenerated zone, and the epigonal organ, consisting of connective tissue and lymphomyeloid cells. A cystic pattern of spermatogenesis was observed, in which each cyst is formed and maintained by Sertoli cells that undergo morphological and positional changes throughout the process. After the release of spermatozoa into the duct, the peripheral cysts formed a degenerating layer. Spermiogenesis, the final phase of sperm development and differentiation, was identified as a complex process in P. motoro, involving nuclear compaction and structural modifications. This study presents, for the first time, a detailed description of the germ cell development process in P. motoro, contributing to the understanding of spermatogenesis in freshwater stingrays.

Hermit crabs rely on external shells for protection due to their non-calcified pleons. This study focuses on the anatomical features and functional roles of various appendages in Calcinus tibicen to understand their mechanisms for shell anchoring. Using scanning electron microscopy (SEM), histological analyses and micro-computed tomography (µCT), we examined the morphology and internal structure of the fourth and fifth pereopods, telson, and uropods. SEM revealed that the pereopods are equipped with scale setae and teeth, which facilitate a firm grip on the shell's internal surface. µCT imaging showed that the uropods play a critical role in gripping the shell, with the left uropod exhibiting more developed musculature. Histological analysis showed that the muscles of the tailfan are striated and also revealed the presence of connective, hemolymphatic and, epithelial tissues. These findings enhance our understanding of the morphological adaptations that facilitate shell use in hermit crabs, emphasizing the importance of both external and internal structures in maintaining grip and stability. This study fills gaps in the literature regarding the role of the tailfan and pereopods in hermit crab shell anchoring, suggesting that uropods function as hooks, and the fourth and fifth pairs of pereopods act as supporting structures.

Understanding the ecological adaptations of extinct species is a central goal in vertebrate palaeontology, but is often limited by the incomplete nature of the fossil record. While skulls and limb bones have traditionally been emphasised in functional and ecological reconstructions, vertebrae are frequently overlooked. While isolated vertebrae are among the most commonly preserved postcranial elements, they are rarely found as complete vertebral columns, raising the question of whether isolated elements alone can yield meaningful ecological information. In this study, we assess the potential of vertebral morphology to predict two key ecological traits, running speed and hunting mode, using three-dimensional geometric morphometrics across 10 presacral vertebrae from a broad sample of extant carnivorans. We evaluate the predictive power of individual vertebrae, regional groupings (cervical, thoracic, lumbar), and multi-element combinations. Our results show that certain vertebrae retain strong ecological signals on their own, especially the first thoracic and lumbar elements. However, combining multiple vertebrae often dilutes ecological signal, likely due to their differing functional roles along the axial column. This highlights the importance of treating vertebral regions independently and suggests that single, strategically informative vertebrae may outperform multi-element approaches in some contexts. We apply this framework to the extinct dire wolf (Aenocyon dirus) and find contrasting signals along the spine, the first thoracic and lumbar vertebrae suggest adaptations for faster locomotion, while some cervical vertebrae indicate an intermediate running speed. This mosaic supports the idea that A. dirus occupied a complex ecological niche involving both active predation and scavenging. These findings underscore the power of vertebral morphology for ecological inference in fossil taxa, particularly when remains are fragmentary, and argue for a more nuanced use of isolated axial elements in reconstructing extinct carnivoran behaviour.