22 Evolution and the History of Life: From Early Chordates to Bony Fishes

The Chronological History of Life and Early Evolution

The history of life on Earth is categorized into vast eons and periods beginning approximately $4.5\text{ billion}$ years ago during the Precambrian. According to the geological timeline, the earliest organic structures appeared roughly $3.5\text{ billion}$ years ago. The Paleozoic Era began with the Cambrian Period approximately $542\text{ million}$ years ago ($\text{MYA}$), followed by the Ordovician, Silurian, Devonian, and the Carboniferous (divided into the Mississippian and Pennsylvanian periods), concluding with the Permian. The Mesozoic Era, spanning from approximately $250\text{ MYA}$ to $65\text{ MYA}$, consists of the Triassic, Jurassic, and Cretaceous periods. The Cenozoic Era follows, encompassing the Tertiary and Quaternary periods. The Tertiary period is further subdivided into epochs including the Paleocene, Miocene, and Pliocene, while the Quaternary includes the Pleistocene and the current Holocene epoch. Fossil evidence within this framework tracks the macroevolutionary transitions from early chordates to the complex diversification of fish.

Anatomy and Defining Characteristics of Chordata

The Phylum Chordata is defined by four primary anatomical structures that appear at some stage in the organism's development. The first is the Notochord, an elastic rod-like structure; in modern vertebrates, this structure is reduced and becomes part of the intervertebral discs (pathologically relevant in cases like a "slipped disc"). The second is the Dorsal hollow nerve chord, which serves as the precursor to the central nervous system, giving rise to the brain and spinal cord. Third are the Pharyngeal slits, which were originally filter-feeding organs consisting of repeated openings behind the mouth. In some hemichordates, there may be as many as $200$ slits. In mammalian evolution, the first pharyngeal arch derived from these slits becomes the mandible (jawbone), while the second becomes the hyoid bone. Finally, the Post-anal tail is defined literally as a tail extending posterior to the anus. Representative groups of chordates include vertebrates, sea squirts, tunicates, and lancelets. Notable early specimens include Yunnanozoon lividum, formerly classified as Haikouella lanceolata.

Early Fossil Evidence: Burgess Shale and Chengjiang Fauna

Key fossil sites provide high-resolution data on early chordate evolution. Pikaia, discovered in the Burgess Shale lagerstaetten, represents a significant early chordate specimen. The Chengjiang Fauna lagerstatten has yielded even more primitive vertebrate-like forms. Haikouichthys is recognized as the earliest known vertebrate. Another critical specimen from the Chengjiang fauna is Myllokunmingia. This organism measures approximately $5\,\text{mm}$ in length and exhibits several definitive chordate and vertebrate features, including a notochord, myotomes (muscle segments), a dorsal fin, a gill pouch, and a mouth.

Cambrian to Silurian Vertebrate Records and Conodonts

Vertebrate fossils from the Cambrian and Ordovician periods are generally poorly known and often found as isolated bony plates. Much of our understanding is tied to Conodonts. While Conodonts are definitely chordates, they are probably vertebrates, though their exact origins were initially mysterious because they were primarily known from microscopic tooth-like elements. Recent discoveries of soft-tissue preservation have revealed the whole body of the animal. Conodonts are biostratigraphically invaluable, ranging from the Cambrian to the Triassic periods.

The Conodont Color Alteration Index (CAI)

Conodont elements are highly useful for determining the thermal history of sedimentary rocks through the Color Alteration Index (CAI). As conodonts are buried and subjected to heat, their color changes predictably. The CAI scale correlates with temperature, fixed carbon percentages, and Vitrinite Reflectance ($\% R_o$):

$\bullet$ CAI 1: Temperature $50^{\circ}\text{--}80^{\circ}\text{C}$; Vitrinite Reflectance < 0.80; Fixed Carbon < 60\%.

$\bullet$ CAI 1.5: Temperature $50^{\circ}\text{--}90^{\circ}\text{C}$; Vitrinite Reflectance $0.70\text{--}0.85$; Fixed Carbon $55\%\text{--}70\%$.

$\bullet$ CAI 2: Temperature $60^{\circ}\text{--}140^{\circ}\text{C}$; Vitrinite Reflectance $0.85\text{--}1.30$; Fixed Carbon $70\%\text{--}80\%$.

$\bullet$ CAI 3: Temperature $110^{\circ}\text{--}200^{\circ}\text{C}$; Vitrinite Reflectance $1.40\text{--}1.95$; Fixed Carbon $80\%\text{--}95\%$.

$\bullet$ CAI 4: Temperature $190^{\circ}\text{--}300^{\circ}\text{C}$; Vitrinite Reflectance $1.95\text{--}3.60$; Fixed Carbon $+95\%$

$\bullet$ CAI 5: Temperature $+300^{\circ}\text{C}$; Vitrinite Reflectance > 3.60.

Jawless Fish: Agnathans and Ostracoderms

Jawless fishes, or Agnathans, represent a complex evolutionary stage. In these organisms, the notochord had begun to integrate into the vertebral column, and pharyngeal slits transitioned into functional gills. A major group within the Agnathans is the Ostracoderms, which were armored jawless fishes prevalent from the Ordovician (?) to the Devonian. Their armor was primarily centered on the head shield, while much of the rest of the body remained un-ossified. The Ostracoderms are considered paraphyletic and include the ancestors of jawed fishes. They are divided into three main groups: Thelodonts, Galeaspids, and Osteostracans.

Anaspids were relatively small, unarmored Agnathans. They were prevalent in the Silurian and may have persisted into the Devonian. While they were once linked to lampreys, current scientific thought suggests they are not in the same lineage. Modern remnants of jawless fish include Hagfishes and Lampreys, though it is debated whether these are sister taxa or if Agnathans as a whole are paraphyletic.

Subgroups of Ostracoderms

Thelodonts were small jawless fishes (Ordovician to Devonian) characterized by distinctive scales that did not overlap, rather than solid plated armor. They possessed paired pectoral fins, a single dorsal fin, paired anal fins, and a distinctive heterocercal tail. Galeaspids (Silurian to Devonian) featured a large opening on the top of the head shield connected to the pharynx and gill chamber, with a scalloped pattern of sensory lines and a mouth located on the bottom of the skull. Osteostracans are viewed as the most advanced Agnathans. They featured typical "ostracoderm" anatomy but included paired fins, complicated cranial anatomy with paired canals in the ear, and a dermal skeleton composed of three layers. While most had massive head shields, later lineages showed a reduction in dermal skeletons.

Evolution of Jawed Fish: Placodermi and Acanthodians

The first group of jawed fish (Gnathostomes) was the Placoderms, which appeared in the Silurian and became the dominant fish group during the Devonian. Placoderms featured heavy armored plates on the head and thorax. Some species possessed true teeth, while others had bony shears for cutting. They were the first group to develop pelvic fins and exhibited significant diversity, comprising at least $10$ families. The Placoderms were largely decimated or driven to extinction during the Devonian mass extinction event.

Acanthodians, often referred to as "spiny sharks," represent a paraphyletic grade of fish lineages leading toward extant Chondrichthyans (cartilaginous fish). They existed from the Silurian to the Permian and displayed features of both bony and cartilaginous fish. Some had shoulder armor and spines, with teeth either fused to the jaw or absent in the case of filter feeders.

Cartilaginous Fishes: Chondrichthyes

Chondrichthyes, ranging from the Devonian to the present, possess skeletons made of cartilage instead of bone. Their skin is covered in tooth-like scales called dermal denticles. This group is divided into Holocephali and Elasmobranchii. Holocephali include three extant families of chimeras, though they were far more diverse in the geological past, with fossils dating back to the Middle Devonian. Elasmobranchii include sharks (Selachii) and rays (Batomorphi). These fish lack a swim bladder and maintain buoyancy using large, oil-rich livers. In Elasmobranchs, the upper jaw is not fused to the cranium, and the lower jaw articulates with the upper. Notably, the earliest members of Selachii appeared in the Early Jurassic, making them less ancient than commonly perceived.

Bony Fishes and the Transition to Land: Osteichthyes

Bony fishes, or Osteichthyes, have existed from the Silurian to the recent and are characterized by skeletons made of bone and fins formed from bony rays. They are divided into two main groups: Actinopterygians and Sarcopterygians. Actinopterygians (ray-finned fish) have paired fins supported by bone or keratin spines (rays) and currently account for more than half of all modern vertebrate species.

Sarcopterygians (lobe-finned fishes) are critical to evolutionary history as they include the ancestors of tetrapods. This group includes Lungfish, Coelacanths, and Panderichthyids. These organisms are distinguished by labyrinthodont dentition and share the anatomical character of a frontal bone with tetrapods, marking the lineage that eventually moved onto land.