Lecture 4, Vertebrate Phylogeny: From Jawless Fish to Jaws
Introduction to Vertebrate Phylogeny
Overview
This lecture provides an extensive examination of vertebrate phylogeny, focusing on the evolutionary trajectory leading to the clade encompassing humans and other vertebrates. Understanding key terminologies that define relationships among various organismal groups is a priority of this discussion.
Key Terminologies
Crown Group: Refers to the most recent common ancestor of a specific group and all of its descendants. It represents the core assemblage of living organisms within a particular lineage.
Stem Group: Denotes the lineage leading to a crown group, capturing the evolutionary history of the group prior to its modern configuration. It includes all extinct taxa that are more closely related to the crown group than to any other group.
Total Group: The total group embodies the complete lineage, summing both the stem and crown groups. This encompasses all organisms, both extant and extinct, belonging to that particular lineage, thereby offering a comprehensive perspective on its evolutionary history.
Evolutionary Transition
The lecture transitions from Crown group Cyclostomata (jawless fish such as lampreys and hagfish) to Crown group Gnathostomata (jawed vertebrates). It is crucial to note that humans are classified as gnathostomes and not as cyclostomes, laying the foundation for exploring the evolutionary history of jawed vertebrates.
Conodonts: An Enigmatic Group of Stem Cyclostomes
Conodonts are introduced as an intriguing group of stem group cyclostomes, known for their tooth-like structures termed elements, prevalent in the fossil record from the Cambrian to Jurassic periods.
These elements, composed of calcium phosphate, exhibit considerable morphological diversity, which aids biostratigraphy — the utilisation of fossils to ascertain the relative age of rock layers.
For an extended period, only the tooth-like structures were known, leaving the actual organism that produced them a mystery until the first intact conodont fossil was discovered in 1983 in Edinburgh, Scotland, by Derek Briggs, a PhD student at the University of Cambridge.
To date, approximately 11 intact specimens have been recorded, significantly clarifying their genetic positioning. Presently, conodonts are acknowledged as stem group cyclostomes, aligning them with the lineage leading to lampreys and hagfish, although they do not represent direct ancestors of the gnathostomes.
Taxonomic Bias in the Fossil Record
The conodonts' narrative exemplifies the principle of taxonomic bias in the fossil record, characterised by:
The preservation likelihood of hard tissues, such as bones and teeth, compared to soft tissues, influencing our understanding of evolutionary history.
Preservation of soft tissues necessitates specific and infrequent environmental conditions. Consequently, the fossil record frequently skews towards organisms with hard parts.
The extended period where only the hard tooth-like structures of conodonts were known elucidates how taxonomic bias influences our grasp of evolutionary relationships.
Exceptional Fossil Preservation and Early Cyclostomes
The fossil site in Edinburgh is notably important for its extraordinary preservation of soft tissues, which facilitated the discovery of intact conodont fossils.
Conodont denticles represent the sole instance of mineralised tissues in the cyclostome evolutionary chronicle, rendering the fossil record of total group cyclostomes quite sparse.
Another significant site of exceptional fossil preservation is the Miguasha UNESCO World Heritage Site in Quebec, Canada, which has yielded notable fossils, including Euphanerops longaevus, a critical stem cyclostome.
Euphanerops features a heterocercal tail (with a longer lower lobe), distinctive from the simplified body forms of contemporary hagfish or lampreys, and exhibits an anal fin along with gut contents indicating a bottom-feeding lifestyle, emphasising the diverse evolutionary history of early cyclostomes.
Shifting Understanding of Phylogeny
Initial hypotheses proposed that conodonts and Euphanerops represented early stem gnathostomes. However, recent phylogenetic analyses have determined that they are, in fact, both stem group cyclostomes, underscoring the evolving nature of our understanding of phylogenetic relationships aided by new evidence and technological advancements.
The ongoing refinement of phylogenetic trees, prompted by novel fossil discoveries and analytical methods, is a defining characteristic of contemporary evolutionary biology.
Gnathostome Evolution: The Rise of Jawed Vertebrates
The lecture shifts focus to the total group Gnathostomata and the evolutionary milestones leading to the Crown group Gnathostomata. This group encompasses all jawed vertebrates, excluding cyclostomes.
The gnathostome stem lineage is a robust and populous branch of the vertebrate tree, with a wealth of available fossil evidence.
An extensive mineralisation of the skeleton is a critical inherent feature of the gnathostome lineage, enhancing fossilisation potential and offering a more thorough record of gnathostome evolution compared to cyclostomes.
The gnathostome stem lineage is not homogeneous; it is diverse and can be classified into multiple monophyletic groups or clades, all of which have become extinct.
The early portion of the gnathostome stem lineage has been colloquially referred to as ostracoderms, though this term lacks strict phylogenetic specifications, indicating an extinct group of armoured jawless fishes.
The terms stem-ward and crown-ward describe the relationship of taxa within a stem lineage, where stem-ward refers to earlier stages and crown-ward to those closer to the crown group.
The placoderms characterise the crown-ward section of the gnathostome stem lineage, an armoured group of extinct fishes that, while once considered a monophyletic clade, are now recognised as a paraphyletic grouping representing progressively more crown-ward stem gnathostomes.
Key Morphological Features of Stem Gnathostomes
Several significant morphological traits arose along the crown-ward path of the gnathostome stem lineage:
Lower Jaw Evolution: The defining characteristic of gnathostomes is the evolution of the lower jaw, contrasting with cyclostomes, which are jawless.
Pelvic Girdle Formation: The initial emergence of pelvic girdles, which support hind limbs, marks the beginning of paired appendage evolution.
Endochondral Bone Discovery: The first appearance of endochondral bone signifies a considerable advancement in skeletal structure; this type of bone develops through cartilage replacement.
Ecological Transition: Early ecological shifts included the rise of large-bodied vertebrates and the formation of macro-predatory ecologies. Dunkleosteus, an early, sizeable predatory placoderm, exemplifies the advent of such ecological roles within the fossil record.
Types of Bone Tissue and Their Evolutionary Origins
Exploring various bone tissues evolving in vertebrates:
Dermal Bone (Intramembranous Bone): This type forms directly within the dermis without a cartilaginous precursor. The bony shields of ostracoderms illustrate the first bone type evolving in vertebrates.
Endochondral Bone: In contrast, this bone type forms via cartilage replacement, constituting the majority of the human skeleton.
Perichondral Bone: This bone outlines a cartilaginous core but does not entirely replace it and is observed in certain bony fishes but is absent in tetrapods.
Dermal bone surfaces first in the fossil record, succeeded by endochondral bone along the stem gnathostome lineage.
Recent discoveries suggest that the understanding of endochondral bone evolution has shifted; it was once thought unique to Osteichthyes (bony fishes), but findings in the placoderm Shuyu indicate earlier origins within the gnathostome lineage, suggesting chondrichthyans (sharks, skates, and rays) may have lost it secondarily.
The patterns of skeletal mineralisation demonstrate that these characteristics were independently acquired and lost throughout early vertebrate evolution.
Osteichthyes are categorised as the most recent common ancestor of both salmon and humans, along with all their descendants, demonstrating the evolutionary connections among vertebrates.
Evolution of Fins and Paired Appendages
Unambiguous pectoral fins have independently emerged across several stem gnathostome clades. Initial fin-like pectoral appendages are seen in stem-ward gnathostomes, disappearing before their eventual reappearance in the crown-ward group, indicating a complex evolutionary narrative.
Fins associated with dorsal and tail structures appeared earlier in gnathostome evolution compared to pelvic and pectoral girdle fins, illustrating the discontinuous evolution of fins and their supporting structures.
Lower Jaw Evolution and the Significance of Entelognathus
The lower jaw represents a key feature distinguishing crown group gnathostomes.
Entelognathus primordialis, a significant stem gnathostome fossil from the Silurian period, exemplifies the evolutionary context surrounding the lower jaw. This fossil demonstrates that the lower jaws, inclusive of dermal bone elements, are homologous with those found in crown gnathostomes.
Advanced imaging technologies such as micro-CT have revolutionised vertebrate fossil studies, permitting the generation of detailed 3D representations of internal structures, yielding enhanced understanding of fossils dating back hundreds of millions of years.
Skull Evolution: The Three Ancestral Components
The vertebrate skull comprises three ancestral components, each emerging from different evolutionary origins:
Neural Cranium (Chondrocranium): A braincase associated with the central nervous system, originating endochondrally.
Dermatocranium: Forms the skull's outer casing, including the roof, arising from dermal bone sourced from the neural crest.
Splanchnocranium (Visceral Cranium): Supports the pharyngeal (gill) arches, leading to jaw and hyoid bone formation, and has both neural crest and endochondral origins.
Notably, the splanchnocranium is crucial in comprehending lower jaw evolution, which is simplified in mammals, comprising solely the dentary bone, while remaining complex in other vertebrates, notably sharks.
In sharks, the upper jaw is supported by the palatoquadrate, with Meckel's cartilage supporting the lower jaw and connected to the skull through various structures.
The prevailing hypothesis posits that the jaw's origin is primarily a modification of the ancestral function of the splanchnocranium, initially serving to support gill arches, with the anterior-most gill arches evolving into palatoquadrate and Meckel's cartilage structures.
Conclusion
This lecture encapsulates the extensive evolutionary transition from stem cyclostomes, exemplified by the enigmatic conodonts, to the crown group gnathostomes, elucidating key evolutionary innovations. These include the development of mineralised skeletons, jaws, paired appendages, endochondral bone, and the evolution of large body sizes. The subsequent lecture will feature a guest speaker focusing on lungfishes and coelacanths, both pivotal groups of osteichthyes, paving the way for deeper insights into bony vertebrate evolution in upcoming discussions.