Palaeospondylus

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Palaeospondylus
Temporal range: Middle Devonian
Palaeospondylus CMC.jpg
Fossil on display at the Cincinnati Museum Center
Scientific classification
Kingdom:
Phylum:
Subphylum:
Infraphylum:
Genus:
Palaeospondylus

Traquair, 1890
Species:
P. gunni
Binomial name
Palaeospondylus gunni
Traquair, 1890

Palaeospondylus ("early vertebrae") is a prehistoric fish, a fossil vertebrate. Its fossils are described from the Achanarras slate quarry in Caithness, Scotland.

The fossil as preserved is carbonized, and indicates an eel-shaped animal up to 6 centimetres (2 in) in length. The skull, which must have consisted of hardened cartilage, exhibits pairs of nasal and auditory capsules, with a gill apparatus below its hinder part, and ambiguous indications of ordinary jaws.[ citation needed ]

Artist's reconstruction of Palaeospondylus as an agnathan. Palaeospondylus.jpg
Artist's reconstruction of Palaeospondylus as an agnathan.

The phylogeny of this fossil has puzzled scientists since its discovery in 1890, and many taxonomies have been suggested. In 2004, researchers proposed that Palaeospondylus was a larval lungfish. [1] Previously, it had been classified as a larval tetrapod, unarmored placoderm, an agnathan, an early stem hagfish, and a chimera. [2] [3] A 2017 study suggested that it was a stem chondrichthyan. [4]

In 2022, researchers reported, based on studies using synchrotron radiation X-ray micro-computed tomography, that the neurocranium of Palaeospondylus was similar to those of the stem-tetrapods Eusthenopteron and Panderichthys , and concluded that Palaeospondylus was between those two phylogenetically. [5] Questions were raised with a 2023 study, [6] but this study was finally criticized and Paleospondylus returned to being a tetrapodomorph larvae. [7]

See also

Related Research Articles

<span class="mw-page-title-main">Chordate</span> Phylum of animals having a dorsal nerve cord

A chordate is a deuterostomic animal belonging to the phylum Chordata. All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (synapomorphies) that distinguish them from other taxa. These five synapomorphies are a notochord, a hollow dorsal nerve cord, an endostyle or thyroid, pharyngeal slits, and a post-anal tail. The name "chordate" comes from the first of these synapomorphies, the notochord, which plays a significant role in chordate body plan structuring and movements. Chordates are also bilaterally symmetric, have a coelom, possess a closed circulatory system, and exhibit metameric segmentation.

<span class="mw-page-title-main">Chondrichthyes</span> Class of jawed cartilaginous fishes

Chondrichthyes is a class of jawed fish that contains the cartilaginous fish or chondrichthyans, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or bony fish, which have skeletons primarily composed of bone tissue. Chondrichthyes are aquatic vertebrates with paired fins, paired nares, placoid scales, conus arteriosus in the heart, and a lack of opercula and swim bladders. Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates.

<span class="mw-page-title-main">Vertebrate</span> Subphylum of chordates with backbones

Vertebrates are deuterostomal animals with bony or cartilaginous axial endoskeleton — known as the vertebral column, spine or backbone — around and along the spinal cord, including all fish, amphibians, reptiles, birds and mammals. The vertebrates consist of all the taxa within the subphylum Vertebrata and represent the overwhelming majority of the phylum Chordata, with currently about 69,963 species described.

<span class="mw-page-title-main">Hagfish</span> Family of eel-shaped, slime-producing animal

Hagfish, of the class Myxini and order Myxiniformes, are eel-shaped jawless fish. Hagfish are the only known living animals that have a skull but no vertebral column, although they do have rudimentary vertebrae. Hagfish are marine predators and scavengers who can defend themselves against other larger predators by releasing copious amounts of slime from mucous glands in their skin.

<span class="mw-page-title-main">Agnatha</span> Infraphylum of jawless fish

Agnatha is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both living (cyclostomes) and extinct species. Among recent animals, cyclostomes are sister to all vertebrates with jaws, known as gnathostomes.

<span class="mw-page-title-main">Gnathostomata</span> Infraphylum of vertebrates

Gnathostomata are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, living gnathostomes have true teeth, paired appendages, the elastomeric protein of elastin, and a horizontal semicircular canal of the inner ear, along with physiological and cellular anatomical characters such as the myelin sheaths of neurons, and an adaptive immune system that has the discrete lymphoid organs of spleen and thymus, and uses V(D)J recombination to create antigen recognition sites, rather than using genetic recombination in the variable lymphocyte receptor gene.

<span class="mw-page-title-main">Jaw</span> Opposable articulated structure at the entrance of the mouth

The jaws are a pair of opposable articulated structures at the entrance of the mouth, typically used for grasping and manipulating food. The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of humans and most animals.

<span class="mw-page-title-main">Theria</span> Subclass of mammals in the clade Theriiformes

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<span class="mw-page-title-main">Craniate</span> Clade of chordates, member of the Craniata

A craniate is a member of the Craniata, a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini (hagfishes), Hyperoartia, and the much more numerous Gnathostomata. Formerly distinct from vertebrates by excluding hagfish, molecular and anatomical research in the 21st century has led to the reinclusion of hagfish as vertebrates, making living craniates synonymous with living vertebrates.

<span class="mw-page-title-main">Acanthodii</span> Paraphyletic group of cartilaginous fishes

Acanthodii or acanthodians is an extinct class of gnathostomes. They are currently considered to represent a paraphyletic grade of various fish lineages basal to extant Chondrichthyes, which includes living sharks, rays, and chimaeras. Acanthodians possess a mosaic of features shared with both osteichthyans and chondrichthyans. In general body shape, they were similar to modern sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteians.

<span class="mw-page-title-main">Cyclostomi</span> Superclass of jawless fishes

Cyclostomi, often referred to as Cyclostomata, is a group of vertebrates that comprises the living jawless fishes: the lampreys and hagfishes. Both groups have jawless mouths with horny epidermal structures that function as teeth called ceratodontes, and branchial arches that are internally positioned instead of external as in the related jawed fishes. The name Cyclostomi means "round mouths". It was named by Joan Crockford-Beattie.

<i>Casineria</i> Species of tetrapodomorph (fossil)

Casineria is an extinct genus of tetrapodomorph which lived about 340–334 million years ago in the Mississippian epoch of the Carboniferous period. Its generic name, Casineria, is a latinization of Cheese Bay, the site near Edinburgh, Scotland, where the holotype fossil was found. When originally described in 1999, it was identified as a transitional fossil noted for its mix of basal (amphibian-like) and advanced (reptile-like) characteristics, putting it at or very near the origin of the amniotes, the group containing all mammals, birds, modern reptiles, and other descendants of their reptile-like common ancestor. However, the sole known fossil is lacking key elements such as a skull, making exact analysis difficult. As a result, the classification of Casineria has been more controversial in analyses conducted since 1999. Other proposed affinities include a placement among the lepospondyls, seymouriamorphs, "gephyrostegids", or as a synonym of Caerorhachis, another controversial tetrapod which may have been an early temnospondyl.

<span class="mw-page-title-main">Turtle shell</span> Shield for the ventral and dorsal parts of turtles

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

Elpistostegalia is a clade containing Panderichthys and all more derived tetrapodomorph taxa. The earliest elpistostegalians, combining fishlike and tetrapod-like characters, such as Tiktaalik, are sometimes called fishapods. Although historically Elpistostegalia was considered an order of prehistoric lobe-finned fishes, it was cladistically redefined to include tetrapods.

<span class="mw-page-title-main">Evolution of fish</span> Origin and diversification of fish through geologic time

The evolution of fish began about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fish lineages belong to the Agnatha, or jawless fish. Early examples include Haikouichthys. During the late Cambrian, eel-like jawless fish called the conodonts, and small mostly armoured fish known as ostracoderms, first appeared. Most jawless fish are now extinct; but the extant lampreys may approximate ancient pre-jawed fish. Lampreys belong to the Cyclostomata, which includes the extant hagfish, and this group may have split early on from other agnathans.

<i>Entelognathus</i> Placoderm fish from the late Ludlow epoch of the Silurian period

Entelognathus primordialis is an early placoderm from the late Silurian of Qujing, Yunnan, 419 million years ago.

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References

  1. Thomson, K.S. (2004). "A Palaeontological Puzzle Solved?". American Scientist. 92 (3): 209–211. doi:10.1511/2004.47.3425. JSTOR   27858385.
  2. Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 33. ISBN   1-84028-152-9.
  3. Hirasawa, T; Oisi, Y; Kuratani, S (2016). "Palaeospondylus as a primitive hagfish". Zoological Letters. 2 (1): 20. doi: 10.1186/s40851-016-0057-0 . PMC   5015246 . PMID   27610240.
  4. Johanson, Zerina; Smith, Moya; Sanchez, Sophie; Senden, Tim; Trinajstic, Kate; Pfaff, Cathrin (2017). "Questioning hagfish affinities of the enigmatic Devonian vertebrate Palaeospondylus". Royal Society Open Science. 4 (7): 170214. Bibcode:2017RSOS....470214J. doi:10.1098/rsos.170214. PMC   5541543 . PMID   28791148.
  5. Hirasawa, Tatsuya; et al. (25 May 2022). "Morphology of Palaeospondylus shows affinity to tetrapod ancestors". Nature . 50 (7912): 109–112. doi:10.1038/s41586-022-04781-3 . Retrieved 25 May 2022.
  6. Brownstein, Chase Doran (August 2023). "Palaeospondylus and the early evolution of gnathostomes". Nature. 620 (7975): E20–E22. doi:10.1038/s41586-023-06434-5. ISSN   1476-4687.
  7. Hirasawa, Tatsuya; Kuratani, Shigeru (24 August 2023). "Reply to: Palaeospondylus and the early evolution of gnathostomes". Nature. 620 (7975): E23–E24. doi:10.1038/s41586-023-06435-4. ISSN   0028-0836.