The methods of comparative anatomy and its contribution to the study of evolution

Of mice (dogs, horses, sheep) and men: A novel comparative anatomy dissection course in a United Kingdom university

At the University of Bristol, we established a novel dissection course to complement our anatomy degree. Students enrolled in this undergraduate course are trained as comparative anatomists, with equal time given to both human and veterinary anatomy. Historically, students opted to dissect either human or veterinary donors as part of the course. To fully reflect the comparative nature of the degree, the dissection course was redesigned so students could dissect both human and veterinary specimens as part of the same course. This facilitated a wide-ranging experience of anatomy, encouraging detailed knowledge of a multitude of species and allowing for multifaceted anatomy graduates to be ready for employment in a wide and competitive job market. Across three iterations of the amended version of the course, median marks ranged from 58.7% to 62.0%, with between 22 and 39 students enrolled. In comparison to the course prior to the introduction of the change, median marks ranged from 59.8% to 62.8%, with between 16 and 24 students enrolled. There was no significant difference between marks before or after the introduction of the concurrently comparative aspect. This paper describes the course, with learning materials and assessments considered, along with some reflection on its value. The course offers benefits to students by widening their perspective on anatomical knowledge and making them more equipped for the job market. It also broadens their understanding of form-function relationships. However, student feedback implied that having the choice between human or veterinary dissection was preferable, and this may outweigh the perceived benefits of the course.

Against unifying homology concepts: Redirecting the debate

The term "homology" is persistently polysemous, defying the expectation that extensive scientific research should yield semantic stability. A common response has been to seek a unification of various prominent definitions. This paper proposes an alternative strategy, based on the insight that scientific concepts function as tools for research: When analyzing various conceptualizations of homology, we should preserve those distinguishing features that support particular research goals. We illustrate the fruitfulness of our strategy by application to two cases. First, we revisit Lankester's celebrated evolutionary reappraisal of homology and argue that his analysis has been distorted by assimilation to modern agendas. His "homogeny" does not mean the same thing as modern evolutionary "homology," and his "homoplasy" is no mere antonym. Instead, Lankester uses both new terms to pose a question that remains strikingly relevant-how do mechanistic and historical causes of morphological resemblance interact? Second, we examine the puzzle of avian digit homology, which exemplifies disciplinary differences in homology conceptualization and assessment. Recent progress has been fueled by the development of new tools within the relevant disciplines (paleontology and developmental biology) and especially by increasing interdisciplinary cooperation. Conceptual unification has played very little role in this work, which instead seeks concrete evolutionary scenarios that integrate all the available evidence. Together these cases indicate the complex relationship between concepts and other tools in homology research.

Deciphering regeneration through non-model animals: A century of experiments on cephalopod mollusks and an outlook at the future

The advent of marine stations in the last quarter of the 19th Century has given biologists the possibility of observing and experimenting upon myriad marine organisms. Among them, cephalopod mollusks have attracted great attention from the onset, thanks to their remarkable adaptability to captivity and a great number of biologically unique features including a sophisticate behavioral repertoire, remarkable body patterning capacities under direct neural control and the complexity of nervous system rivalling vertebrates. Surprisingly, the capacity to regenerate tissues and complex structures, such as appendages, albeit been known for centuries, has been understudied over the decades. Here, we will first review the limited in number, but fundamental studies on the subject published between 1920 and 1970 and discuss what they added to our knowledge of regeneration as a biological phenomenon. We will also speculate on how these relate to their epistemic and disciplinary context, setting the base for the study of regeneration in the taxon. We will then frame the peripherality of cephalopods in regeneration studies in relation with their experimental accessibility, and in comparison, with established models, either simpler (such as planarians), or more promising in terms of translation (urodeles). Last, we will explore the potential and growing relevance of cephalopods as prospective models of regeneration today, in the light of the novel opportunities provided by technological and methodological advances, to reconsider old problems and explore new ones. The recent development of cutting-edge technologies made available for cephalopods, like genome editing, is allowing for a number of important findings and opening the way toward new promising avenues. The contribution offered by cephalopods will increase our knowledge on regenerative mechanisms through cross-species comparison and will lead to a better understanding of the complex cellular and molecular machinery involved, shedding a light on the common pathways but also on the novel strategies different taxa evolved to promote regeneration of tissues and organs. Through the dialogue between biological/experimental and historical/contextual perspectives, this article will stimulate a discussion around the changing relations between availability of animal models and their specificity, technical and methodological developments and scientific trends in contemporary biology and medicine.

Reduction, reorganization and stasis in the evolution of turtle shell elements

Novel phenotypic configurations can profoundly alter the evolutionary trajectories of species. Although innovation can precede lengthy periods of evolutionary stasis, the potential for species to diversify further can be realized via modular changes across distinct levels of hierarchical organization. To test this expectation, we undertook anatomical network analyses to model the organization and composition of the turtle’s shell. Our results suggest that stem turtles featured the greatest diversity in the number of skeletal (bones) and epidermal (scutes) shell elements. The shell subsequently underwent numerical simplification. Thus, the sum of potential connections (links) in shell networks has diminished in modern turtles. Some network system descriptors of complexity, integration and modularity covaried with the number of network components (nodes), which has remained evolutionarily stable since the Jurassic. We also demonstrated that shell reorganization might be feasible within modular subdivisions, particularly in modern turtles with simplified and less integrated network structures. We discuss how these findings align with previous studies on numerical simplification with enhanced skeletal specialization in the tetrapod skull. Altogether, our analyses expose the evolvability of the turtle’s shell and bolster the foundation for further macroevolutionary comparisons of ancient and modern species.

On the “cartilaginous rider” in the endocasts of turtle brain cavities

Abstract

In recent years, paleoneurology became a very popular research field and hundreds of brain-endocasts were described. The interpretation of a dorsal protuberance of the brain-endocast puzzled researchers for a long time, the so-called (cartilaginous) rider. This is mainly because of technical limitations in the past and due to non-accessibility of comparative material. Using turtles as a case-study, we conducted a literature review and studied embryological data in addition to fossil and extant species’ endocasts. We assessed three hypotheses on the origin of the rider as relating to 1) the pineal gland, to 2) the blood vessel system, and to 3) skull roof elements. Based on our integrated anatomical observations, we refute the pineal gland hypothesis (1) and an exclusive blood vessel explanation (2). However, we show that, in most cases, the cartilaginous origin applies (3). The related cartilages, mainly the anterior process of the chondrocranial tectum synoticum, can persist until adulthood. Its diversity is interpreted in regard to the mechanical support for the temporal skull region, the shape of which has been shown to be in turn related to neck retraction and jaw mechanics. Finally, we highlight the value of embryological data to provide profound hypotheses for evolutionary research despite its low quantitative evaluability. We argue that it should be studied in conjunction with modern computer-aided data acquisition whenever possible.

A Primitive Trait in Two Breeds of Equus Caballus Revealed by Comparative Anatomy of the Distal Limb

Simple Summary

Understanding the complexities and evolutionary links between extinct and extant equids has been vital to genetic conservation and preservation of primitive traits. As domestication of the equid expanded, the loss of primitive traits that ensured survival in a wild environment has not been documented. In this study, the presence of functional interosseous muscle II and IV in the distal limb has been reported, and yet its existence could only be confirmed in relatives and two closely bred descendants of the extinct Tarpan. The morphology described was ligamentous in structure displaying clear longitudinal fibres with a skeletal origin and soft tissue insertion into the medial and lateral branches of the interosseous muscle III (suspensory ligament) dorsal to the sesamoids, similar in orientation to the flexor digitorum profundus ligamentum accessorium (inferior check ligament). Hence, providing a functional medial and lateral stability to the metacarpophalangeal joint (fetlock joint), which equates to one of the functions of the medial and lateral digits in the Mesohippus and Merychippus. The comparable anatomic links between species of the same family that experienced geographical isolation yet display structural conformity appears to be in response to a specific environment. Surmising this potential remnant of functional evolution is a primitive trait and not a breed anomaly.

Abstract

The 55-million-year history of equine phylogeny has been well-documented from the skeletal record; however, this is less true for the soft tissue structures that are now vestigial in modern horse. A recent study reported that two ligamentous structures resembling functional interosseous muscle II and IV were evident in Dutch Konik horses. The current study investigates this finding and compares it to members of the genus Equus to identify either a breed anomaly or functional primitive trait. Distal limbs (n = 574) were dissected from four species of Equus; E. caballus, E. asinus, E. przewalskii and E. quagga boehmi. E. caballus is represented by 18 breeds of horse, including the primitive Dutch Konik’. The interosseous muscle II and IV were evident in all four species, but only two breeds of E. caballus expressed this trait-the Dutch Konik and Bosnian Mountain Horse. These two breeds were the only close descendants of the extinct Equus ferus ferus (Tarpan) represented in this study. In conclusion, the interosseous muscle II and IV originated from the distal nodule of metacarpal II and IV, respectively, and inserted into the corresponding branches of interosseous muscle III proximal to the sesamoids. This suggests a functional role in medial and lateral joint stability and a primitive trait in modern equids.

The head and neck anatomy of sea turtles (Cryptodira: Chelonioidea) and skull shape in Testudines

BACKGROUND

Sea turtles (Chelonoidea) are a charismatic group of marine reptiles that occupy a range of important ecological roles. However, the diversity and evolution of their feeding anatomy remain incompletely known.

METHODOLOGY/PRINCIPAL FINDINGS

Using computed tomography and classical comparative anatomy we describe the cranial anatomy in two sea turtles, the loggerhead (Caretta caretta) and Kemp's ridley (Lepidochelys kempii), for a better understanding of sea turtle functional anatomy and morphological variation. In both taxa the temporal region of the skull is enclosed by bone and the jaw joint structure and muscle arrangement indicate that palinal jaw movement is possible. The tongue is relatively small, and the hyoid apparatus is not as conspicuous as in some freshwater aquatic turtles. We find several similarities between the muscles of C. caretta and L. kempii, but comparison with other turtles suggests only one of these characters may be derived: connection of the m. adductor mandibulae internus into the Pars intramandibularis via the Zwischensehne. The large fleshy origin of the m. adductor mandibulae externus Pars superficialis from the jugal seems to be a characteristic feature of sea turtles.

CONCLUSIONS/SIGNIFICANCE

In C. caretta and L. kempii the ability to suction feed does not seem to be as well developed as that found in some freshwater aquatic turtles. Instead both have skulls suited to forceful biting. This is consistent with the observation that both taxa tend to feed on relatively slow moving but sometimes armoured prey. The broad fleshy origin of the m. adductor mandibulae externus Pars superficialis may be linked to thecheek region being almost fully enclosed in bone but the relationship is complex.

What, exactly, is cladistics? Re-writing the history of systematics and biogeography

The development of comparative biology (systematics) has been of interest to philosophers and historians. Particular attention has been placed on the 'war' of the 1970s and 1980s, the apparent dispute among those who preferred this or that methodology. In this contribution we examine the history of comparative biology from the perspective of fundamentals rather than methodologies. Our examination is framed within the artificial-natural classification dichotomy, a viewpoint currently lost from view but worth resurrecting.

‘Type’ in morphology and phylogeny

Comparative morphologists, developmental biologists, as well as paleontologists, recognize the existence of 'types,' or 'Baupläne,' in nature, marked out by a certain 'sameness' of structure that prevails through all variation in shape and function. The 'sameness' that marks out 'types' is one of structural correspondence (topology and connectivity), which is believed to be causally rooted in ontogeny (developmental constraints). In an evolutionary context, the structural relations that mark out 'types' are explained as relations of homology. The use of concepts such as 'type' or 'Bauplan' has been criticized from a 'populational thinking' point of view as being incompatible with current evolutionary theory. The present article explores the contrasting viewpoints, and concludes that current evolutionary theory can accommodate the concept of a 'type' in the sense of a 'homeostatic property cluster natural kind.' The 'homeostatic property cluster natural kind' is a nonessentialistic concept that allows the kind to be historically delimited.

Type-concept, higher classification and evolution

A study is made of the history of the type and related concepts, from Greek Antiquity up to the present. It is demonstrated that the type-concept of eighteenth century biology was based on Leibniz's concept of substantial form, and was not related to a Platonic Idea, whilst it is now generally understood in the sense of model or norm. In the present paper, a type-concept is developed which includes ontogenetic and phylogenetic time and various evolutionary mechanisms. This type (an archetype) can serve as a model of the evolutionary potentialities of a taxon, and as a standard of higher classification. All classifications based on the same archetype, whether typological, numerical or phylogenetic, will be comparable, although not necessarily identical.