Lecture 5, Sarcopterygii: Lobe-Finned Fishes and the Fish-Tetrapod Transition

Comprehensive Overview of Sarcopterygii (Lobe-Finned Fishes)

Introduction

• Sarcopterygii, known as lobe-finned fishes, serve as a key group in the study of vertebrate evolution, notably as a crucial link between aquatic life and the terrestrial adaptations exemplified by tetrapods.

• Classified as a crown group within vertebrates, Sarcopterygii comprises all extant species and their nearest evolutionary ancestors, underscoring their significance in vertebrate phylogeny.

• Their evolutionary history commenced in the Silurian period, reaching a peak of diversification during the Devonian period, frequently referred to as the “Age of Fishes.” Presently, extant sarcopterygians represent only a small fraction of the diversity that existed in these earlier geological epochs.

Early Sarcopterygians: Key Examples

Guiyu oneiros

• Chronology: Silurian; provenance in China.

• Morphological Characteristics:

  • Possesses ganoid scales, which incorporate an outer enamel layer and an underlying bony layer, thereby providing both protective and hydrodynamic advantages in aquatic habitats.

  • Contains dermal bones in the skull, critical for structural support and protection of sensory organs.

  • Exhibits median fin spines, indicative of structural similarities to primitive chondrichthyans and acanthodians.

  • Noteworthy for its enlarged cleithrum, which serves as a vital structural support for the pectoral fin.

Psarolepis romeri

• Chronology: Lower Devonian; provenance in China.

• Morphological Attributes:

  • Exhibits a combination of sarcopterygian and actinopterygian characteristics:

    • Sarcopterygian Features:

      • Employs infolded teeth coated with cosmine, a unique mineral that strengthens and maintains tooth durability.

      • Displays an intracranial joint which enhances skull mobility, crucial for various feeding strategies.

      • Contains a parasymphysial tooth whorl and a denticulate anterior cleithrum, important for effective jaw mechanics.

    • Actinopterygian Features:

      • Displays median fin spines and a pectoral spine, along with preopercular canals that enhance sensory perception.

Defining Characteristics of Sarcopterygii

• Dorsal Fin Composition: Sarcopterygii typically possess two dorsal fins, a distinction that contrasts with many actinopterygians which generally have one dorsal fin.

• Fin Attachment: Characterised by monobasal articulation, where a single skeletal element connects the fin to the shoulder girdle, in contrast to the multiple-element connections seen in actinopterygians.

• Skeletal Features:

  • Displays a robust humerus, indicative of strong muscle attachment and robust locomotion capabilities.

  • Cranial Composition:

    • Incorporates a squamosal bone within the skull, essential for jaw articulation.

    • Presence of a splenial bone in the lower jaw aids in its mechanical function.

  • Typically includes more than four sclerotic plates within the eye orbit, enhancing visual acuity and providing additional protection for the eye.

  • The presence of cosmine covering the outer surfaces of dermal bones serves a sensory function, enhancing mechanosensory capabilities in aquatic environments.

Phylogenetic Relationships

Sarcopterygii encompasses several clades, showcasing a complex and multifaceted evolutionary history:

• Actinistia (Coelacanths)

• Dipnomorpha (Lungfish)

• Porolepiformes

• Rhizodontiformes

• Tetrapodamorpha

Phylogenetic Debates

• Ongoing research examines the phylogenetic relationships of living fishes relative to tetrapods, with various leading hypotheses:

  1. Suggests that tetrapods are most closely allied with lungfish.

  2. Proposes that coelacanths may exhibit a closer evolutionary path to tetrapods than lungfish do.

  3. Postulates that lungfish and coelacanths share a closer relationship with each other compared to tetrapods.

  4. Asserts that these relationships remain unresolved due to limitations inherent in comparative osteological studies.

Genomic and Molecular Evidence

• Recent genomic studies support the assertion that lungfish constitute the closest extant relatives of tetrapods. Molecular data provide clarity where traditional osteological data falls short or appears ambiguous.

Coelacanths (Actinistia): The "Living Fossils"

• Historical Context: Coelacanths boast a fossil lineage that traces back to the Devonian and extends through the Cretaceous periods, marked by the dramatic narrative surrounding their presumed extinction until the rediscovery of a living specimen in 1938.

Discovery

• The first living coelacanth, named Latimeria chalumnae, was identified by Marjorie Courtenay-Latimer in South Africa, honouring both the discoverer and the location of the find.

• A subsequent species, Latimeria menadoensis, was later discovered in Indonesia, thereby expanding the understanding of coelacanth diversity.

Physical Characteristics of Latimeria

• Notochord: Characterised by a constricted notochord, as opposed to possessing a fully developed vertebral column, this structure extends from the braincase to the tail, reflecting evolutionary retention of primitive features.

• Tail Structure: Features a bilateral diphycercal tail, wherein the dorsal and ventral lobes are of equal size, facilitating diverse swimming abilities.

• Skeletal Structure: The internal skeletons of the second dorsal and anal fins resemble those of the paired fins, indicating evolutionary continuity and specialization.

• Cranial Anatomy: Comprises a divided braincase which consists of an anterior ethmosphenoid portion and a posterior otoccipital portion, with associated musculature allowing for downward movement of the snout.

• Reproductive Strategy: Demonstrates internal fertilisation and ovoviviparity, with females capable of retaining a clutch of embryos—up to 24 reported—thereby exemplifying adaptability in reproductive strategies.

Habitat and Behaviour

• Typically found in deep oceanic troughs, coelacanths exhibit nocturnal feeding behaviours that require them to venture into shallower depths for foraging.

• Utilising both their fins and tail for locomotion, coelacanths are capable of swimming in multiple orientations, enhancing their adaptability within diverse marine habitats.

Fossil Record

• The fossil record of coelacanths exhibits substantial variation in body sizes. Recent discoveries, such as Ngamugawi, were delineated from the Upper Devonian of Australia.

• The greatest diversity of coelacanths is recorded during the Triassic period, signifying their evolutionary success, although there exists a notable hiatus in the fossil record between the Cretaceous extinction and the rediscovery of extant coelacanths, likely linked to their deep-sea habitat preferences.

Lungfish (Dipnomorpha/Dipnoi): Adaptations for Freshwater Life

• Lungfish epitomise a specialised group of freshwater sarcopterygians, showcasing distinct morphologies and adaptations crucial for survival in their specific ecological niches.

Classification

• Lungfish are classified into three primary genera:

  • Lepidosiren (South America)

  • Protopterus (Africa)

  • Neoceratodus (Australia), regarded as the most primitive living lungfish.

Physical Characteristics

• Tail Morphology: Lungfish generally possess a protocercal tail, wherein the dorsal, caudal, and anal fins coalesce into a singular fin structure, fostering efficient locomotion and navigation in freshwater environments.

• Vertebral Structure: Many extant lungfish demonstrate an unconstructed notochordal vertebral column, while certain fossilised species exhibit ossified vertebral centra, indicating an evolutionary divergence.

• Fin Structure: Characterised by leaf-like pectoral fins, which facilitate effective movement through slow-moving or stagnant waters.

• Respiratory Adaptations: Possess functional lungs—Neoceratodus has a single lung, while Protopterus and Lepidosiren possess paired lungs, aiding in survival in oxygen-depleted freshwater.

• Aestivation: Exhibits the ability to aestivate, allowing lungfish to burrow into mud during arid seasons, thus conserving energy and moisture.

Cranial Anatomy and Jaw Structure

• Jaw Configuration: Features an autostylic jaw structure in which the upper jaw is fused directly to the braincase, advantageous for durophagy (the consumption of hard materials).

• Jaw Musculature: An exclusivity of jaw-closing musculature enables powerful biting capabilities, diverging significantly from the highly mobile jaw structures observed in actinopterygians.

• Dentition: Absence of marginal teeth, instead relying on tooth plates for grinding and feeding; these dentition elements reveal extensive adaptations to their ecological niches and predominant feeding strategies.

Evolutionary Trends in Lungfish

• The evolutionary trajectory indicates the amalgamation of dorsal and anal fins into a cohesive protocercal tail structure, enhancing their swimming capabilities.

• A marked reduction in endochondral ossification appears evident post-Devonian, especially in fin skeletons and braincase structures, suggesting adaptations to their freshwater environments.

• Adult dentition consists solely of tooth plates, while early lungfish, such as Diabolepis and Uranolophus, lacked these structures, indicating significant evolutionary shifts in feeding adaptations over time.

Mass Extinctions and the Devonian Extinction Event

• This lecture highlights five significant mass extinction events in Earth's biological history, with particular focus on the pivotal extinction at the conclusion of the Devonian period.

• The Devonian extinction transpired in two primary phases:

  • The Kellwasser event marking the end of the Frasnian,

  • The Hangenberg event occurring at the end of the Famennian.

Causes of the Devonian Extinction

• Contributing factors for this extinction event include:

  • Meteorite impacts

  • Climatic alterations transitioning from greenhouse states to icehouse conditions

  • Volcanic activity resulting in significant atmospheric shifts

  • Eutrophication, predominantly instigated by increased terrestrial vegetation runoff into marine systems, leading to hypoxic conditions in shallow marine ecosystems.

• Eutrophication is viewed as a central contributor to the extinction, drastically impacting fish diversity—particularly among sarcopterygians—by driving down oxygen levels in aquatic ecosystems.

Recovery and Diversification

• Post-Devonian extinction, lungfish appear to have recovered and diversified, likely due to their exploitation of newly available freshwater ecological niches.

• By the end of the Permian period, some groups that initially survived the Devonian extinction event eventually succumbed to further extinctions, shaping the contemporary distributions of lungfishes, coelacanths, and tetrapods.

Summary of Key Points

• Sarcopterygians are primarily distinguished by their monobasal articulation of paired fins, a vital feature in their evolutionary journey towards terrestrial adaptations.

• Coelacanths are identified by a transverse joint in the skull and intricate adaptations within their fin and tail morphology, emphasising their unique evolutionary lineage.

• Lungfish are characterised by their specialised autostyly, unique jaw mechanics for efficient feeding, and adaptations critical for survival in freshwater habitats.

• Among all extant vertebrates, lungfish are recognised as the closest living relatives of tetrapods, a conclusion reinforced through molecular studies.

• The end-Devonian extinction event delineates a significant evolutionary milestone that profoundly reshaped aquatic biodiversity, setting the stage for the subsequent rapid radiation of tooth-plated lungfish in the aftermath of this extinction.