Vertebrate land invasion

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The vertebrate land invasion refers to the transition of vertebrate animals from being aquatic/semiaquatic to predominantly terrestrial during the Late Devonian period. This transition allowed some vertebrates to escape competitive pressure from other aquatic animals and explore niches on land, [1] which eventually established the vertebrates as the dominant terrestrial phylum. Fossils from this period have allowed scientists to identify some of the species that existed during this transition, such as Tiktaalik [2] and Acanthostega . [3] Many of these species were also the first to develop adaptations suited to terrestrial over aquatic life, such as neck mobility, more robust lungs and hindlimb locomotion.

Contents

The late Devonian vertebrate transition was preceded by terrestrial invasion by fungi, land plants and invertebrates such as arthropods. These land colonization allowed for the development of appropriate terrestrial ecosystems that would be available to accommodate vertebrate habitation. While the late Devonian event was the first land invasion by vertebrate organisms, newer aquatic species have continued to develop adaptations suited to terrestrial life (and vice versa) from the late Devonian to the Holocene. [4]

Overview of transition

The vertebrate species that were important to the initial water to land transition can be sorted into five phases: sarcopterygians, prototetrapods, aquatic stem tetrapods (or "fishapods"), amphibian true tetrapods, and amniote tetrapods. Many morphological changes occurred throughout this transition. Mechanical support structures changed from fins to limbs, the method of locomotion changed from swimming to walking, respiratory structures changed from gills to lungs, feeding mechanisms changed from suction feeding to biting, and mode of reproduction changed from larval development to metamorphosis. [5]

Evolutionary timeline

Lungfish appeared approximately 400 million years ago, enduring rapid evolution during the Devonian era, which became known as the dipnoan renaissance. The Acanthostega species, known as the fish with legs, is considered a tetrapod by structural findings but is postulated to have perhaps never left the aquatic environment. Its legs are not well-suited to support its weight. The bones of its forearm, the radius and ulna, are very thin at the wrist and also unable to support it on land. It also lacks a sacrum and strong ligaments at the hip, which would be integral to supporting the animal against gravity. In this sense, the species is considered a tetrapod but not one that has adapted well enough to walk on land. Furthermore, its gill bars have a supportive brace characterized for use as an underwater ear because it can pick up noise vibrations through the water. Tetrapods that adapted to terrestrial living adapted these gill bones to pick up sounds through air, and they later became the middle ear bones seen in mammalian tetrapods.

Ichthyostega, on the other hand, is considered to be a fully terrestrial tetrapod that perhaps depended on water for its aquatic young. Comparisons between the skeletal features of Acanthostega and Ichthyostega reveal that they had different habits. Acanthostega is likely exclusive to an aquatic environment, while Ichthyostega is progressed in the aquatic to terrestrial transition by living dominantly on the shores. [3]

An evolutionary timeline of the late Devonian vertebrate terrestrial invasion demonstrates the changes that took place. A group of fish from the Givetian stage began developing limbs, and eventually evolved into aquatic tetrapods in the Famennian stage. Pederpes, Westlothiana, Protogyrinus, and Crassigyrinus descended from these species into the Early Carboniferous period and were the first land vertebrates, indicating the crown group originated and split in that time, around 350 Ma. [6] [5]

A particularly important transitional species is one known as Tiktaalik. It has a fin, but the fin has bones within it that are similar to mammalian tetrapods. It has an upper arm bone, a lower arm bone, forearm bones, a wrist, and fingerlike projections. Essentially, it is a fin that can support the animal. Similarly, it also has a neck that allows independent head movement from the body. Its ribs are also able to support the body in gravity. Its skeletal features exhibit its ability as a fish that can live in shallow water and also venture onto land. [2]

Driving factors

It took many millions of years for vertebrates to transition out of water onto land. During this time, perhaps competitive pressures pushed species out of the water and certainly niche occupation incentives pulled species onto land. [1] The culmination of these driving factors are what ultimately facilitated the vertebrate transition.

Evolutionary pushes

Scientists believe that a long period of time where biotic and abiotic factors in the aquatic environment were unfavourable to certain aquatic organisms is what pushed their transition to shallower waters. Some of these push factors are environmental hypoxia, unsuitable aquatic temperatures, and increased salinity. Other constantly present factors such as predation, competition, waterborne diseases and parasites also contributed to the transition. [5]

A theory put forth by Joseph Barrell possibly helps explain what may have initiated these push factors to become relevant in the late Devonian. The extensive oxidized sediments that were present in Europe and North America (since they lived in Euramerica) during the late Devonian are evidence of severe droughts during this time. These droughts would cause small ponds and lakes to dry out, forcing certain aquatic organisms to move on land to find other bodies of water. Natural selection on these organisms eventually led to the evolution of the first terrestrial vertebrates. [3]

Evolutionary pulls

The pull factors were secondary to the push factors, and only became significant once the pressures to leave the aquatic environment became significant. These were largely the niches and opportunities that were available for exploitation in the terrestrial environment, and include higher environmental oxygen partial pressures, favourable temperatures, and the lack of competitors and predators on land. The plants and invertebrates that had preceded the vertebrate invasion also provided opportunities in the form of abundant prey and lack of predators. [5]

Barriers to transition

There were many challenges that the first land vertebrates faced. These challenges allowed for rapid natural selection and niche domination, resulting in an adaptive radiation that produced many different vertebrate land species in a relatively short period of time. [1]

Anatomical

The primary anatomical barrier is the development of lungs for proper gas exchange (although rudimentary lungs are ancestral to bony fish), however other anatomical barriers also exist. The stressors of the musculoskeletal system are different in air than they are in water, and the muscles and bones must be strong enough to withstand the increased effects of gravity on land. [4] The tongue and the three chambered heart evolved similarly for efficient digestion and blood circulation on land respectively. The vomeronasal organ is found in many living tetrapods but not any fish, suggesting it originated in tetrapods only. Similarly, all tetrapods have parathyroid glands which other animals don't.

Depending on the water depth at which a species lives, the visual perception of many aquatic species is better suited to darker environments than those on land. Similarly, hearing in aquatic organisms is better optimized for sounds underwater, where the speed and amplitude of sound is greater than in air. [4] The spiracle of their ancestors, once used for breathing, was repurposed to hold a eardrum inside, connected to the pharynx by the auditory tube (from spiracle) and to the otic vesicle by the columella (from hyomandibula), for hearing.

Physiological

Homeostasis was almost definitely a challenge for land invading vertebrates. Gas exchange and water balance are highly different in water and in air. Homeostasis mechanisms suitable for a terrestrial environment may have been necessary to develop before these organisms invaded land. [4]

Behaviourial

Many behaviours, such as reproduction, are specifically optimized to a wet environment. Navigation and locomotion are also highly different in aquatic environments compared to terrestrial environments. [4]

Notable adaptations

Placement of eyes on head

The ancestral species of tetrapods that lived entirely in water had tall and narrow skulls with eyes facing sideways and forwards to maximize visibility for predators and prey in the aquatic environment. As the ancestors of early tetrapods started inhabiting shallower waters, these species had flatter skulls with eyes at the tops of their heads, which made it possible to spot food above them. Once the tetrapods transitioned onto land, the lineages evolved to have tall and narrow skulls with eyes facing sideways and forwards again. This allowed them to navigate through the terrestrial environment and look for predators and prey. [7]

Head and neck mobility

Fish do not have necks, so the head is directly connected to the shoulders. In contrast, land animals use necks to move their heads so they can look down to see the food on the ground. The greater the mobility of the neck, the more visibility the land animal has. As lineages moved from completely aquatic environments to shallower waters and land, they gradually evolved vertebral columns that increased neck mobility. The first neck vertebra that evolved permitted the animals to have flexion and extension of the head so that they can see up and down. The second neck vertebra evolved to allow rotation of the neck for moving the head left and right. As tetrapod species continued to evolve on land, adaptations included seven or more vertebrae, allowing increasing neck mobility. [7]

Fused sacrum

The sacrum connects the pelvis and hindlimbs and is useful for motion on land. The aquatic ancestors of tetrapods did not have a sacrum, so it was speculated to have evolved for locomotive function exclusive to terrestrial environments. However, the Acanthostega species is one of the earliest lineages to have a sacrum, even though it is a fully aquatic species. Once species moved onto land, the trait was adapted for terrestrial locomotion support, which is evidenced by additional vertebrae fusing similarly to permit additional support. This is an example of exaptation, where a trait performs a function that did not arise through natural selection for its current use. [7]

Lost adaptations

As the lineages evolved to adapt to terrestrial environments, many lost traits that were better suited for the aquatic environment. Many lost their gills, which were only useful for obtaining oxygen in water. Their caudal, dorsal, and anal fins reduced in size before completely disappearing. They lost the lateral line system, a network of canals along the skull and jaw that are sensitive to vibration, which does not work outside of an aquatic environment. [7]

Future invasions

For successful land invasion, the species had several pre-adaptations like air-breathing and limb-based locomotion. Aspects such as reproduction and swallowing, however, have bound these species to the aquatic environment. These pre-adaptations have allowed vertebrates to venture onto land hundreds of times, but were not able to accomplish the same degree of prolific radiation into diverse terrestrial species. [4] To understand the potential of future invasions, studies must evaluate the models of evolutionary steps taken in past invasions. The commonalities to current and future invasions may then be elucidated to predict the effects of environmental changes.

See also

Related Research Articles

<span class="mw-page-title-main">Tetrapod</span> Superclass of the first four-limbed vertebrates and their descendants

A tetrapod is any four-limbed vertebrate animal of the superclass Tetrapoda. Tetrapods include all extant and extinct amphibians and amniotes, with the latter in turn evolving into two major clades, the sauropsids and synapsids. Some tetrapods such as snakes, legless lizards, and caecilians had evolved to become limbless via mutations of the Hox gene, although some do still have a pair of vestigial spurs that are remnants of the hindlimbs.

<span class="mw-page-title-main">Sarcopterygii</span> Class of fishes

Sarcopterygii — sometimes considered synonymous with Crossopterygii — is a taxon of the bony fish known as the lobe-finned fish or sarcopterygians, characterised by prominent muscular limb buds (lobes) within the fins, which are supported by articulated appendicular skeletons. This is in contrast to the other clade of bony fish, the Actinopterygii, which have only skin-covered bony spines (lepidotrichia) supporting the fins.

<i>Ichthyostega</i> Extinct genus of tetrapodomorphs

Ichthyostega is an extinct genus of limbed tetrapodomorphs from the Late Devonian of what is now Greenland. It was among the earliest four-limbed vertebrates ever in the fossil record and was one of the first with weight-bearing adaptations for terrestrial locomotion. Ichthyostega possessed lungs and limbs that helped it navigate through shallow water in swamps. Although Ichthyostega is often labelled a 'tetrapod' because of its limbs and fingers, it evolved long before true crown group tetrapods and could more accurately be referred to as a stegocephalian or stem tetrapod. Likewise, while undoubtedly of amphibian build and habit, it is not a true member of the group in the narrow sense, as the first modern amphibians appeared in the Triassic Period. Until finds of other early stegocephalians and closely related fishes in the late 20th century, Ichthyostega stood alone as a transitional fossil between fish and tetrapods, combining fish and tetrapod features. Newer research has shown that it had an unusual anatomy, functioning more akin to a seal than a salamander, as previously assumed.

<span class="mw-page-title-main">Labyrinthodontia</span> Paraphyletic group of tetrapodomorphs

"Labyrinthodontia" is an informal grouping of extinct predatory amphibians which were major components of ecosystems in the late Paleozoic and early Mesozoic eras. Traditionally considered a subclass of the class Amphibia, modern classification systems recognize that labyrinthodonts are not a formal natural group (clade) exclusive of other tetrapods. Instead, they consistute an evolutionary grade, ancestral to living tetrapods such as lissamphibians and amniotes. "Labyrinthodont"-grade vertebrates evolved from lobe-finned fishes in the Devonian, though a formal boundary between fish and amphibian is difficult to define at this point in time.

<i>Acanthostega</i> Extinct genus of tetrapodomorphs

Acanthostega is an extinct genus of stem-tetrapod, among the first vertebrate animals to have recognizable limbs. It appeared in the late Devonian period about 365 million years ago, and was anatomically intermediate between lobe-finned fishes and those that were fully capable of coming onto land.

<i>Eusthenopteron</i> Extinct genus of tetrapodomorphs

Eusthenopteron is a genus of prehistoric sarcopterygian which has attained an iconic status from its close relationships to tetrapods. Early depictions of this animal show it emerging onto land; however, paleontologists now widely agree that it was a strictly aquatic animal. The genus Eusthenopteron is known from several species that lived during the Late Devonian period, about 385 million years ago. Eusthenopteron was first described by J. F. Whiteaves in 1881, as part of a large collection of fishes from Miguasha, Quebec. Some 2,000 Eusthenopteron specimens have been collected from Miguasha, one of which was the object of intensely detailed study and several papers from the 1940s to the 1990s by paleoichthyologist Erik Jarvik.

<i>Panderichthys</i> Extinct genus of tetrapodomorphs

Panderichthys is a genus of extinct sarcopterygian from the late Devonian period, about 380 Mya. Panderichthys, which was recovered from Frasnian deposits in Latvia, is represented by two species. P. stolbovi is known only from some snout fragments and an incomplete lower jaw. P. rhombolepis is known from several more complete specimens. Although it probably belongs to a sister group of the earliest tetrapods, Panderichthys exhibits a range of features transitional between tristichopterid lobe-fin fishes and early tetrapods. It is named after the German-Baltic paleontologist Christian Heinrich Pander. Possible tetrapod tracks dating back to before the appearance of Panderichthys in the fossil record were reported in 2010, which suggests that Panderichthys is not a direct ancestor of tetrapods, but nonetheless shows the traits that evolved during the fish-tetrapod evolution

<i>Hynerpeton</i> Extinct genus of tetrapodomorphs

Hynerpeton is an extinct genus of early four-limbed vertebrate that lived in the rivers and ponds of Pennsylvania during the Late Devonian period, around 365 to 363 million years ago. The only known species of Hynerpeton is H. bassetti, named after the describer's grandfather, city planner Edward Bassett. Hynerpeton is known for being the first Devonian four-limbed vertebrate discovered in the United States, as well as possibly being one of the first to have lost internal (fish-like) gills.

<i>Tiktaalik</i> Extinct genus of tetrapodomorphs

Tiktaalik is a monospecific genus of extinct sarcopterygian from the Late Devonian Period, about 375 Mya, having many features akin to those of tetrapods. Tiktaalik is estimated to have had a total length of 1.25–2.75 metres (4.1–9.0 ft) based on various specimens.

<i>Ventastega</i> Extinct genus of tetrapodomorphs

Ventastega is an extinct genus of stem tetrapod that lived during the Upper Fammenian of the Late Devonian, approximately 372.2 to 358.9 million years ago. Only one species is known that belongs in the genus, Ventastega curonica, which was described in 1996 after fossils were discovered in 1933 and mistakenly associated with a fish called Polyplocodus wenjukovi. ‘Curonica’ in the species name refers to Curonia, the Latin name for Kurzeme, a region in western Latvia. Ventastega curonica was discovered in two localities in Latvia, and was the first stem tetrapod described in Latvia along with being only the 4th Devonian tetrapodomorph known at the time of description. Based on the morphology of both cranial and post-cranial elements discovered, Ventastega is more primitive than other Devonian tetrapodomorphs including Acanthostega and Ichthyostega, and helps further understanding of the fish-tetrapod transition.

<span class="mw-page-title-main">Terrestrial animal</span> Animals living on land

Terrestrial animals are animals that live predominantly or entirely on land, as compared with aquatic animals, which live predominantly or entirely in the water, and amphibians, which rely on aquatic and terrestrial habitats. Some groups of insects are terrestrial, such as ants, butterflies, earwigs, cockroaches, grasshoppers and many others, while other groups are partially aquatic, such as mosquitoes and dragonflies, which pass their larval stages in water.

<i>Tulerpeton</i> Extinct genus of tetrapodomorphs

Tulerpeton is an extinct genus of Devonian four-limbed vertebrate, known from a fossil that was found in the Tula Region of Russia at a site named Andreyevka. This genus and the closely related Acanthostega and Ichthyostega represent the earliest tetrapods.

Livoniana is a genus of prehistoric tetrapodomorphs which lived during the Devonian period.

<span class="mw-page-title-main">Skull roof</span> Roofing bones of the skull

The skull roof or the roofing bones of the skull are a set of bones covering the brain, eyes and nostrils in bony fishes and all land-living vertebrates. The bones are derived from dermal bone and are part of the dermatocranium.

<span class="mw-page-title-main">Ichthyostegalia</span> Paraphyletic group of tetrapodomorphs

Ichthyostegalia is an order of extinct amphibians, representing the earliest landliving vertebrates. The group is thus an evolutionary grade rather than a clade. While the group are recognized as having feet rather than fins, most, if not all, had internal gills in adulthood and lived primarily as shallow water fish and spent minimal time on land.

Antlerpeton is an extinct genus of early tetrapod from the Early Carboniferous of Nevada. It is known from a single poorly preserved skeleton from the Diamond Peak Formation of Eureka County. A mix of features in its compound vertebrae suggest that Antlerpeton is a primitive stem tetrapod that has affinities with later, more advanced forms. Its robust pelvis and hind limbs allowed for effective locomotion on land, but the animal was likely still tied to a semiaquatic lifestyle near the coast.

<span class="mw-page-title-main">Evolution of tetrapods</span> Evolution of four legged vertebrates and their derivatives

The evolution of tetrapods began about 400 million years ago in the Devonian Period with the earliest tetrapods evolved from lobe-finned fishes. Tetrapods are categorized as animals in the biological superclass Tetrapoda, which includes all living and extinct amphibians, reptiles, birds, and mammals. While most species today are terrestrial, little evidence supports the idea that any of the earliest tetrapods could move about on land, as their limbs could not have held their midsections off the ground and the known trackways do not indicate they dragged their bellies around. Presumably, the tracks were made by animals walking along the bottoms of shallow bodies of water. The specific aquatic ancestors of the tetrapods, and the process by which land colonization occurred, remain unclear. They are areas of active research and debate among palaeontologists at present.


Innovations conventionally associated with terrestrially first appeared in aquatic elpistostegalians such as Panderichthys rhombolepis, Elpistostege watsoni, and Tiktaalik roseae. Phylogenetic analyses distribute the features that developed along the tetrapod stem and display a stepwise process of character acquisition, rather than abrupt. The complete transition occurred over a period of 30 million years beginning with the tetrapodomorph diversification in the Middle Devonian.

The Zachelmie trackways are a series of Middle Devonian-age trace fossils in Poland, purportedly the oldest evidence of terrestrial vertebrates (tetrapods) in the fossil record. These trackways were discovered in the Wojciechowice Formation, an Eifelian-age carbonate unit exposed in the Zachełmie Quarry of the Świętokrzyskie Mountains (Holy Cross Mountains]. The discovery of these tracks has complicated the study of tetrapod evolution. Morphological studies suggest that four-limbed vertebrates are descended from a specialized type of tetrapodomorph fish, the epistostegalians. This hypothesis was supported further by the discovery and 2006 description of Tiktaalik, a well-preserved epistostegalian from the Frasnian of Nunavut. Crucial to this idea is the assumption that tetrapods originated in the Late Devonian, after elpistostegalians appear in the fossil record near the start of the Frasnian. The Zachelmie trackways, however, appear to demonstrate that tetrapods were present prior to the Late Devonian. The implications of this find has led to several different perspectives on the sequence of events involved in tetrapod evolution.

The evolution of fishes took place over a timeline which spans the Cambrian to the Cenozoic, including during that time in particular the Devonian, which has been dubbed the "age of fishes" for the many changes during that period.

References

  1. 1 2 3 Sahney, Sarda; Benton, Michael J.; Ferry, Paul A. (2010-08-23). "Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land". Biology Letters. 6 (4): 544–547. doi:10.1098/rsbl.2009.1024. PMC   2936204 . PMID   20106856.
  2. 1 2 "Tiktaalik roseae: Q&A Session with Neil Shubin". Tiktaalik.uchicago.edu. April 2006. Retrieved 7 April 2015.
  3. 1 2 3 Clack, Jennifer A. (2012). Gaining ground : the origin and evolution of tetrapods (2nd ed.). Bloomington: Indiana University Press. ISBN   978-0-253-35675-8.
  4. 1 2 3 4 5 6 Ashley-Ross, M. A.; Hsieh, S. T.; Gibb, A. C.; Blob, R. W. (9 May 2013). "Vertebrate Land Invasions-Past, Present, and Future: An Introduction to the Symposium". Integrative and Comparative Biology. 53 (2): 192–196. doi: 10.1093/icb/ict048 . PMID   23660589.
  5. 1 2 3 4 Long, JA; Gordon, MS (September–October 2004). "The Greatest Step in Vertebrate History: A Paleobiological Review of the Fish‐Tetrapod Transition". Physiological and Biochemical Zoology. 77 (5): 700–719. doi:10.1086/425183. PMID   15547790. S2CID   1260442.
  6. ↑ Moreira, M.O., Qu, Y.-F. and Wiens, J.J. (2021), Large-scale evolution of body temperatures in land vertebrates. Evolution Letters, 5: 484-494. https://doi.org/10.1002/evl3.249
  7. 1 2 3 4 "The origin of tetrapods". Evolution.berkeley.edu. Retrieved 7 April 2015.
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