Early tetrapod trackways

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There are several cases of reported trackways of the earliest land-going vertebrates, also known as tetrapods. These trackways provide crucial insights to the study of the transition of aquatic to terrestrial lifestyles in vertebrate evolution. Such fossils help to illuminate not only the timing of this keystone transition of evolutionary history but also what the earliest forms of tetrapod locomotion may have entailed.

Contents

Occurrences

Victoria, Australia

A set of three trackways was found within a single sandstone layer of the Genoa River Formation of Victoria, Australia. [1] The Genoa River Formation is a very fine grained purplish-brown non-marine sandstone considered Late Devonian in age based on fern-like plant fossils within the bed and stratigraphic correlation. [1] The tracks preserve four or more impressions each. Each trackway has a roughly parallel structure to the left and right tracks. None of the impressions are reported to include evidence of body dragging, though one includes evidence of tail drag. This is interpreted to illustrate that the movement shown here demonstrates the animal fully suspending its body with its limbs. Additionally, one of the tracks is interpreted to preserve digitation of the limbs, which are reported to possess 5 digits. These trackways then imply that by the Late Devonian a "typical" tetrapod condition had fully evolved.

Valentia Island, Ireland

Nine tetrapod trackways from three sites have been reported from the Valentia Slate Formation of Valentia Island, Ireland. [2] [3] The Valentia Slate Formation is composed mostly of purple coloured fine-grained sandstones and siltstones interpreted to represent a fluvial setting. The trackways are late Middle Devonian in age based on a palynological assemblage from the Valentia Slate Formation and the U-Pb radioisotopic dating of an interstratified air-fall tuff bed to ca. 385 Ma, [3] making these tetrapod trackways some of the earliest recorded, along with traces of early Middle Devonian (Eifelian) age from Poland. [4] The most extensive of the Valentia Island trackways is preserved in a fine-grained sandstone and records some 145 imprints in a parallel orientation of the left and right impressions. The systematic variation in size of the impressions affords distinction between tracks left by the manus and pes of the animal, but the trackway does not preserve any finer details. Other trackways at the same site preserve tail and body drag impressions; the nature of the impressions and that of the sandstone led to the interpretation that the setting was not saturated in water. Consequently, these tracks are interpreted as evidence of fully terrestrial locomotion.

Holy Cross Mountains, Poland

A collection of trackways and impressions is reported from the Wojciechowice Formation of the Holy Cross Mountains located in south-eastern Poland. [4] The Wojciechowice Formation is a shallow marine-fed tidal or lagoonal unit that dates to the Eifelian Stage of the Middle Devonian, approximately 395 million years ago based on conodont fossils and previous biostratigraphy on bounding units. The preservation of the track assemblage varies with some clearer tracks preserving finer morphology such as digitation while others are more vague, preserving only an outline. Showing consistency with the aforementioned tracks, these fall into two parallel rows of impressions and show no evidence of body or tail drag.

Interpretation

In addition to these early trackways providing additional evidence of tetrapod activity on land as early as the Devonian, recent work has also been aimed at gleaning biomechanical interpretations from these occurrences.

Typically, it is assumed that the earliest tetrapods had a movement pattern very similar to modern amphibians where the entirety of the pectoral and pelvic girdles would swing as the animal moved forward causing the angular pattern seen in these trackways. [1] [2] [4] Although this movement is quite common in animals such as salamanders, recent work has also been done showing similar patterns created by terrestrially locomoting actinopterygian and sarcopterygian fish. In animals such as the actinopterygian cavefish, the alternating footfalls and general layout of the ancient trackways was readily reproduced. [5] Similarly, modern lungfish, a sarcopterygian fish, were shown to be able to produce somewhat similar trackways through axial flexing rather than limb driven locomotion. [6] These studies do not necessarily disprove that these trackways were in fact produced by tetrapods but do at least muddy the interpretation of these trackways.

Work by Niedźwiedzki et al., particularly analyzing the trackways from Poland, interprets some of the tracks as being dominated by only two limbs. [4] This "ladder-like" pattern produces symmetric trackways that imply that one set of limbs is dominating the motion with the other limbs either not supporting as much weight or not being used at all. [4] Recent biomechanical work on Ichthyostega shows support for this showing that the range of motion in the animal's limbs was not capable of the rotation necessary to support an alternating footfall in the gait. [7] Instead, Icthyostega appears to have moved in a manner more akin to the modern mudskipper, lunging forward by pushing its limbs back and then rotating them back into position. [7] The fact that the alternating footfall gait interpreted from the majority of other tracks is not feasible for Icthyostega implies that there are undiscovered early tetrapods with a different limb and girdle configuration than those of the earliest body fossil remains.

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 buds (lobes) within the fins. This is in contrast to the other clade of bony fish, the Actinopterygii, which have only bony spines 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> Subclass of early amphibious tetrapods

"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>Panderichthys</i> Genus of fishes (fossil)

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 species of amphibian

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> Genus of extinct lobe-finned fish

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.

<span class="mw-page-title-main">Fossil track</span> Fossilized footprint (ichnite)

A fossil track or ichnite is a fossilized footprint. This is a type of trace fossil. A fossil trackway is a sequence of fossil tracks left by a single organism. Over the years, many ichnites have been found, around the world, giving important clues about the behaviour of the animals that made them. For instance, multiple ichnites of a single species, close together, suggest 'herd' or 'pack' behaviour of that species.

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

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.

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

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.

<span class="mw-page-title-main">Stegocephali</span> Clade of tetrapodomorphs

Stegocephali is a group containing all four-limbed vertebrates. It is equivalent to a broad definition of Tetrapoda: under this broad definition, the term "tetrapod" applies to any animal descended from the first vertebrate with limbs and toes, rather than fins. This includes both the modern lineage of limbed vertebrates as well as a portion of the stem group, limbed vertebrates that evolved prior to the origin of the crown group. Members of the tetrapod stem group include the earliest limbed tetrapodomorphs such as Ichthyostega and Acanthostega, which evolved in the Devonian Period long before any modern form of tetrapod.

<span class="mw-page-title-main">Elpistostegalia</span> Clade of tetrapodomorphs

Elpistostegalia or Panderichthyida is an order of prehistoric lobe-finned fishes which lived during the Middle Devonian to Late Devonian period. They represent the advanced tetrapodomorph stock, the fishes more closely related to tetrapods than the osteolepiform fishes. The earliest elpistostegalians, combining fishlike and tetrapod-like characters, are sometimes called fishapods, a phrase coined for the advanced elpistostegalian Tiktaalik. Through a strict cladistic view, the order includes the terrestrial tetrapods.

<span class="mw-page-title-main">Ichthyostegalia</span> Extinct order of amphibians

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.

<i>Ymeria</i> Extinct genus of tetrapods

Ymeria is an extinct genus of early stem tetrapod from the Devonian of Greenland. Of the two other genera of stem tetrapods from Greenland, Acanthostega and Ichthyostega, Ymeria is most closely related to Ichthyostega, though the single known specimen is smaller, the skull about 10 cm in length. A single interclavicle resembles that of Ichthyostega, an indication Ymeria may have resembled this genus in the post-cranial skeleton.

<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.

<i>Thinopus</i> Ichnogenus of marine reptile

Thinopus is the name given to a trace fossil (ichnotaxon) found in late Devonian rocks in Pennsylvania. The only specimen was described by paleontologist Othniel Charles Marsh in a brief note published in 1896, with the only ichnospecies Thinopus antiquus. Marsh interpreted it as the fossil footprints of an early amphibian, making it the oldest evidence for tetrapods known at the time. Later research, however, argued that the fossil is better interpreted as the impressions of coprolites of fish. This would make Thinopus the earliest published name of a fish coprolite.

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.

References

  1. 1 2 3 WARREN, JAMES W.; WAKEFIELD, NORMAN A. (August 1972). "Trackways of Tetrapod Vertebrates from the Upper Devonian of Victoria, Australia". Nature. 238 (5365): 469–470. doi:10.1038/238469a0. ISSN   0028-0836. S2CID   4208007.
  2. 1 2 Stössel, I. (1995). "The discovery of a new Devonian tetrapod trackway in SW Ireland". Journal of the Geological Society. 152 (2): 407–413. doi:10.1144/gsjgs.152.2.0407. S2CID   131359994.
  3. 1 2 Stössel, Iwan; Williams, Edward A.; Higgs, Kenneth T. (2016). "Ichnology and depositional environment of the Middle Devonian Valentia Island tetrapod trackways, south-west Ireland". Palaeogeography, Palaeoclimatology, Palaeoecology. 462: 16–40. doi:10.1016/j.palaeo.2016.08.033.
  4. 1 2 3 4 5 Niedźwiedzki, Grzegorz; Szrek, Piotr; Narkiewicz, Katarzyna; Narkiewicz, Marek; Ahlberg, Per E. (7 January 2010). "Tetrapod trackways from the early Middle Devonian period of Poland". Nature. 463 (7277): 43–48. Bibcode:2010Natur.463...43N. doi:10.1038/nature08623. ISSN   1476-4687. PMID   20054388. S2CID   4428903. Archived from the original on 12 February 2018. Supposed trackways of very early tetrapods have been recorded from a number of localities in Europe and Australia. [...]Figure 5 | Phylogenetic implications of tracks. [...]The bars are approximate measures of the uncertainty of dating. These are not statistical error bars but an attempt to reflect ongoing debate. Alt URL
  5. ↑ Flammang, Brooke E.; Suvarnaraksha, Apinun; Markiewicz, Julie; Soares, Daphne (24 March 2016). "Tetrapod-like pelvic girdle in a walking cavefish". Scientific Reports. 6 (1): 23711. doi:10.1038/srep23711. PMC   4806330 . PMID   27010864.
  6. ↑ Falkingham, Peter L.; Horner, Angela M. (27 September 2016). "Trackways Produced by Lungfish During Terrestrial Locomotion". Scientific Reports. 6 (1): 33734. doi:10.1038/srep33734. PMC   5037403 . PMID   27670758.
  7. 1 2 Pierce, Stephanie E.; Clack, Jennifer A.; Hutchinson, John R. (23 May 2012). "Three-dimensional limb joint mobility in the early tetrapod Ichthyostega". Nature. 486 (7404): 523–526. doi:10.1038/nature11124. ISSN   0028-0836. PMID   22722854. S2CID   3127857.
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