10.2: Phanerozoic Diversity of Life (According to the Hard Parts)

Before and after: the exoskeleton appears

Figure \(\PageIndex{1}\): Eukarya is the Domain that includes most familiar organisms, including humans.

One of the most fascinating aspects of Earth history is the diversification of life through time. There are many ways to graphically represent this diversification. The “tree of life” diagram is one way to broadly represent phylogenetic relationships of the three major Domains of life, including the Archaea, Bacteria, and Eukarya. Within each of these main stems, further distinctions can be made as organisms are grouped with others that share similar physical traits, as discussed in the cladistics section of this textbook. However, one of the factors that is missing from the tree of life diagram is the aspect of time. As we have seen in the prior sections, it took a long time for life to separate into the three Domains, and even longer for Eukarya to split into the four Kingdoms: Protista, Fungi, Plantae, and Animalia. The next major innovation of life is so revolutionary that geoscientists literally draw a line in the sand(stone)–the Precambrian and everything after. During the Cambrian Period, which kicks off the Phanerozoic Eon, shelled organisms with an exoskeleton proliferate in the fossil record. This significant uptick in the diversity of marine life is known as the Cambrian Explosion. (Note there are small, shelled animals (metazoans) of uncertain affinities found in rocks of the Late Ediacaran known as the Tommotian fauna, including the genus Cloudina.)

Figure \(\PageIndex{2}\): Trilobites, like modern arthropods, molted their exoskeletons as they grew. (Creative Commons Attribution 2.0 Generic license.)

Why did animals develop hard, mineralized exoskeletons in abundance during the Cambrian? It was likely that shells were the evolutionary answer to a biological need. Increased animal diversity at this time would lead to greater competition for resources, and an external shell offers several evolutionary advantages: it offers protection from physical and chemical variations in the environment, such as changes in water temperature and salinity; it offers protection from biological predators; and, it allows muscles additional mechanical leverage during locomotion, making it easier to burrow, for example. Being able to withstand seasonal shifts in currents, ward off predators, or exploit food sources below the seafloor would increase chances of survival. Interestingly, the boundary between the Proterozoic and Phanerozoic is marked by a shift in seawater chemistry that impacted carbonate mineral deposition in the oceans (Wood et al., 2017). During the Ediacaran (Neoproterozoic), dolomite (\(\ce{CaMg(CO3)2}\)) was the dominant carbonate deposited in the oceans, but increased erosion of continental crustal rocks and chemical weathering led to an influx of calcium ions to the seas during the early stages of the Cambrian. Higher calcium levels allowed for easier precipitation of aragonite and calcite, the two polymorphs of \(\ce{CaCO3}\) that most marine organisms use to create their shells. As the Cambrian and Ordovician progressed, a wide array of body plans developed among representatives of most of today’s major invertebrate phyla, including sponges, corals, brachiopods, bryozoans, arthropods, annelids, mollusks, and echinoderms.

Figure \(\PageIndex{3}\): Modern lancelet showing position of notochord along dorsal side of body. (Creative Commons Attribution 4.0 International license.)

Internal skeletons, or endoskeletons, also have their origins in the Cambrian Period. The first endoskeletons were not mineralized bone, but rather, strong, flexible tissue known as cartilage. This is the same material that makes up shark skeletons and your own ears and nose today. Some of the first vertebrates are fossils from the early Cambrian Chengjiang locale in China. These vertebrates were marine organisms, eel-like in shape, with elongate, tapered bodies that possess a notochord and simple vertebrae. A notochord is a skeletal structure shared by all members of the phylum Chordata at some stage in their life cycle. The notochord helps to support muscles and define characteristic, mirroring, bilateral symmetry on either side of the chordate body’s midline. For many chordates, additional strength, flexibility and support are provided with the bones of the vertebral column. Note the spinal cord found in vertebrates is part of the nervous system, which is a completely different set of tissues from the notochord and vertebrae, which are part of the skeletal system.

Figure \(\PageIndex{4}\): Spindle diagram of fish diversity through time. Vertical length of shapes indicate geologic range of species while the width represents diversity. (Creative Commons Attribution-Share Alike 3.0 Unported license.)

Throughout the Cambrian and into the Silurian, jawless fish (agnathans) and armored fish (ostracoderms) diversified. Interestingly, while the development of body external plates of the ostracoderms has traditionally been thought to be a classic response of protection to predation, there is also the added benefit of ion storage. In particular, bone stores phosphorous which is used by muscles for quick bursts of energy, like when swimming quickly away from predators as well. Jawed fish, including cartilaginous fish like sharks and rays (chondrichthyes), and bony fish (osteichthyes) appeared during the Silurian and radiated during the Devonian.

Here come the plants: landward, ho!

As human animals, we tend to focus on the parts of Earth history that tell our story; however, animals are not the only life form that moved to land during the Paleozoic, and in fact, plants beat our ancestors to the punch.

Figure \(\PageIndex{5}\): Moss (a bryophyte) with spore releasing structures. (Creative Commons Attribution-Share Alike 4.0 International license.)

The first multicellular plants are marine green and red algaes that were likely derived from photosynthetic cyanobacteria during the Neoproterozoic. The first land plants evolved around the early Ordovician, when fossil evidence of the first bryophytes are found. Bryophytes include plants like mosses, hornworts, and liverworts, that are commonly found in moist areas. They do not rely on a root system for uptake of water and nutrients, rather taking these in via their leaves, and reproduce via spores. Later in the Ordovician, the first true vascular plants (trachaeophytes) evolved, and by the Devonian, this group looked like what you think of when you think of a plant–leafy and green, possessing a lignin-reinforced stem and roots that conduct water and nutrients through their tissues, and using seeds for reproduction.

Figure \(\PageIndex{6}\): Diagram showing the phylogeny of major plant groups. (Creative Commons Attribution-Share Alike 4.0 International license.)

Imagine land on Earth before plants. It would have been lots of bare rock, very much like deserts or areas after volcanic eruptions today. The first liverworts and mosses to colonize land would have begun to significantly impact physical and chemical weathering of terrestrial landscapes. Plants can assist in the development of soils from bedrock as they extract nutrients, retain water, and produce organic matter. Later, root systems of vascular plants could wedge into cracks in rock in search of water, physically creating more surface area for physical and chemical weathering to occur. However, given enough time and spread of populations, plants and their roots can also add stability to slopes, lead to the development of new ecological environments (forests, swamps), and provide new resources (leaves, seeds) for other life forms to put to evolutionary use.

Next up: invertebrate animals invade terrestrial environments

Invertebrate groups diversified significantly throughout the Paleozoic oceans (see diversity section below), increasing the competition for resources. As time went by, there was an increase in ecological tiering above and below the seafloor, as animals evolved forms that allow for deeper burrowing and higher extension away from the sediment-water interface. It was only a matter of time until life found a way to exploit the vast expanse of geographic space on land. This transition to land is thought to have occurred during the Devonian period, when global changes in oxygen levels in the ocean were in decline. This dysoxia was in part driven by excess runoff of organic matter from land due to dying plants, which triggered large scale algal and other plankton blooms in the oceans. An abundance of plankton uses up dissolved oxygen in the oceans, leaving less behind for the larger ocean dwellers. Today, “dead zones” develop at the mouths of modern rivers where concentrations of nutrients from over-fertilization of farms and other industrial processes create plankton blooms. This phenomenon ultimately may have been connected to the mass extinctions at the end of the Devonian.

Figure \(\PageIndex{7}\): Modern horseshoe crab showing specialized appendages and book gills. (Creative Commons CC0 1.0 Universal Public Domain Dedication)

It is important to recognize the transition to land by complex organisms was not instantaneous. Aquatic invertebrates would have had an advantage due to their exoskeletons, which allow for protection and moisture retention as these organisms adapted to freshwater ecosystems, like rivers and lakes, and eventually land. It is not surprising then that arthropods are the first aquatic invertebrate group to venture into the terrestrial realm. Out of all of the invertebrate phyla existing at the time, arthropods had the body plan with the greatest evolutionary flexibility–in particular, lots of segmented appendages that were highly specialized for different types of movement such as walking, burrowing, swimming, and feeding. Aquatic arthropods take in oxygen via book gills (a structure that looks similar to folded or overlapping sheets of paper) which are easily visible on horseshoe crabs you might find along the East Coast of the United States today. The first land arthropods would have developed internal book lungs from these structures, which we see in modern arachnids, like spiders and scorpions. Around the Late Silurian to Early Devonian, terrestrial fossils of tiny spider-like arthropods often found in association with plant fossils, scorpions, and myriapods (clade including millipedes, centipedes, eventually insects) have been found. In time, some groups of arthropods, including insects developed tracheae, a system of tube-like structures that directly deliver oxygen from external pores in the exoskeleton to tissues of the body. And so, the rest of us animals have come to live in a bug’s world.

Figure \(\PageIndex{8}\): Large fossilized Meganeura dragonfly. (Creative Commons Attribution-Share Alike 3.0 Unported, 2.5 Generic, 2.0 Generic and 1.0 Generic license.)

Coevolution among insects and with other animals, coevolution with plants, adaptation to freshwater, and variable predation in larval versus adult forms are just some of the means by which insects have spread across the terrestrial realm. Perhaps most importantly though was the adaptation to the sky–arthropods are the only phyla of invertebrates to develop wings and flight. The first flying insects show up in the fossil record of the Carboniferous. While there is still debate in scientific circles, it is generally assumed wings represent another modification of appendages that were given a molecular and mechanical boost from high oxygen levels in muscles achieved via tracheae. The development of flight also corresponds to increases in atmospheric oxygen on a global level during the Carboniferous as vascular plants diversified, grew into large populations, and pumped out oxygen as a byproduct of photosynthesis. This rapid diversification of insects into all available ecological niches is known as an adaptive radiation, and this pattern is repeated by other terrestrial animal groups later in geologic time. Today, there are about one million described insect species, representing almost 75% of all known animals.

Vertebrates take their first steps

While arthropods are crawling and scurrying their way onto land, vertebrates were keeping an eye on their progress–literally! An interesting avenue of recent research suggests that vertebrates begin to show critical increases in eye size and a shift in eye position from the side to the top of the head during the interval leading up to their transition to land (MacIver et al., 2017). These researchers hypothesize that these changes in eyesight would have yielded better vision over longer distances, allowing these animals a better sightline to prey on land.

Figure \(\PageIndex{9}\): Comparison between the fins of lobe-finned fishes (Sarcopterygii) and legs of early tetrapods.
1. Tiktaalik. 2. Panderichthys. 3. Eusthenopteron. 4. Acanthostega.5. Ichthyostega (hindleg). (Public domain image.)

Other structural changes were also required to the vertebrate body plan to produce success on land. The first land-based animals are known collectively as tetrapods, referring to those animals moving about on four feet. Without the buoyancy provided by water, the bones of tetrapods would need to be more robust to provide more structural support to the body and allow for larger muscles to develop to aid in locomotion. Bony, lobe-finned fish (those in the sarcopterygian class) possess the basic vertebrate body plan in their appendages, and this same

Figure \(\PageIndex{10}\): Comparison between the fins of A., Crossopterygiian fishes and the legs of B., tetrapods. Bones which correspond to each other have the same color. (Public domain image.)

musculoskeletal template in their ancestors would have allowed for the evolution of muscular arms and legs. Specifically, their fins exhibit the same sequence of bones as all other tetrapod limbs: one bone at the point of body attachment, followed by a joint, two bones, another joint, then lots of little bones. Other changes are not particularly well-fossilized, but would also have been key to success on land: development of a three-chambered heart and more complex lungs to extract oxygen from air, continual modification to auditory systems to allow for better hearing in air, and development of more waterproof skin to retain tissue moisture.

Paleontologists who study the colonization of land are commonly looking for what are known as transitional fossils that show these gradual changes over time. One of the most famous transitional fossils of the Late Devonian is Tiktaalik, a fish discovered by a team of researchers headed by Neil Shubin and Ted Daeschler.

This fossil shows clear traits of being fish-like while also showing some of the necessary adaptations to being able to move about without the buoyancy of water. Tiktaalik possessed stronger limbs including hip-like structures on the hind limbs, a neck that allowed for swiveling motions, and large eye sockets on the top of the head. Interestingly, an increase in size and shift in the position of spiracles (skull openings that allow air to pass across a fish’s gills) toward the back and side of the skull in several transitional fossils (Eusthenopteron, Icthyostega, and Tiktaalik) suggest a positioning more in line with the location of lungs.

Tetrapods and plants: conquering land, sea, and air

Figure \(\PageIndex{11}\): Skeleton of Ichthyostega, one of the earliest amphibians. (Creative Commons Attribution-Share Alike 3.0 Unported license)

The first tetrapods on land during the Devonian quickly evolve characteristics that are reminiscent of today’s amphibians, although modern amphibian groups generally evolved during the Triassic. While these animals began to spread into various environments on the continents as adults, there would be some limitations due to key aspects of their life cycle–amphibians, like fish, require water for reproduction. The eggs of these animals must be laid in water because they do not have any sort of protective outer coating to protect the egg from drying out. Additionally, early life stages after hatching (for example, a tadpole stage in frogs) require water until the organism develops enough to survive on land.

Figure \(\PageIndex{12}\): Modern chicken egg showing structures of amniotic egg. (Creative Commons Attribution-Share Alike 4.0 International license.)

During the Carboniferous, a major innovation arose in tetrapod groups–the amniotic egg. An amniotic egg protects a developing embryo and its required nourishment (yolk sac) within several protective layers. These layers provide protection from environmental elements, and also allow for the transmission of gases needed for respiration and a place to store metabolic wastes. Whether the outermost shell layer is leathery or more hardened, this change to eggs allowed tetrapods to spread out into different inland environments, further away from permanent water sources. Amniotes, as a clade, are represented by reptiles (including dinosaurs and birds) and mammals.

This ability to exploit more terrestrial environments allowed amniote groups to diversify rapidly and persist for millions of years, with natural selection aided by the constant shift of tectonic plates and resulting climate patterns. During the Late Carboniferous, two distinct lineages of tetrapods arose–the sauropsids and synapsids. Within the sauropsid clade, an enormous diversity of body forms and life habits arose, particularly after the end-Permian extinction cleared out a variety of ecological niches and allowed reptiles to undergo an adaptive radiation. This lineage leads to reptilian groups like crocodilians, turtles, snakes, dinosaurs, and dinosaurs’ cousins, birds. The end-Cretaceous mass extinction wiped out a significant number of the reptilian groups, effectively wiping the slate clean for emerging mammalian groups to repopulate ecological niches on land, air, and sea.

Figure \(\PageIndex{13}\): Cynognathus, a cynodont from the Triassic of South Africa, pencil drawing by N. Tamura. (Creative Commons Attribution-Share Alike 3.0 Unported license.)

The Late Carboniferous synapsids began with a reptilian-like, low-slung body plan, and a distinctive single opening in the skull behind the eye socket. Many of the larger early members of the synapsid group such as Dimetrodon and Cynognathus were greatly affected by the end-Permian and end-Triassic mass extinctions. Eventually, changes within this clade led to the various groups of mammals we see today, including monotremes (egg-laying mammals, like platypus), marsupials (pouched mammals, like koalas and kangaroos) and placental mammals (most mammals, including such diverse groups as cows, whales, cats, and humans).

While vertebrate groups on land were evolving throughout the Mesozoic, so were vascular plants. The ability to reproduce via seeds had developed during the Devonian among gymnosperms (seed ferns, and eventually conifers, cycads, and ginkgos), but it was millions of years later, during the Cretaceous, when flowering plants (angiosperms) evolved. Angiosperms are the most common plants on Earth today and includes grasses, common trees like oaks and maples, herbs, and all of our domesticated crops, such as corn, rice, and soybeans.

Gymnosperms primarily reproduce via wind-blown pollen released from cones. Flowers on angiosperms are reproductive structures that allow for pollen distribution by organisms (e.g., insects like bees) and are often colorful, showy features that attract organisms for gamete dispersal. Gymnosperm seeds are literally “naked seeds”–the seed is not encased in an ovary. However, on angiosperms, (“vessel seeds”) the plant’s seeds are encased in ovaries we know as fruit. So the more fruit that is produced and eaten, the more seeds are dispersed via animal dung, continuing the plant population, and potentially altering the plant’s geographic distribution. These tightly woven relationships between plants, pollinators, and dispersers represent some of the most elegant examples of coevolution throughout geologic time.

The interactions of living organisms clearly play a key role in how Earth has changed during its 4.6 billion year history. Likewise, interactions with the atmosphere, hydrosphere, and geosphere have shaped and pruned the tree of life. As humans, we are part of the unique, weird, beautiful, fascinating,opportunistic, innovative, and simply wonderful story of life on Earth.