Acanthodii

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"Spiny shark" redirects here. For the spiny true sharks, see Echinorhinus.

Acanthodii
Temporal range: Early Silurian–Permian
Nerepisacanthus, a Silurian acanthodian
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Eugnathostomata
Class: †Acanthodii
Owen, 1846
Orders

Acanthodii or acanthodians is an extinct class of

epidermis was covered with tiny rhomboid platelets like the scales of holosteians (gars, bowfins).[1]

The popular name "spiny sharks" is because they were superficially shark-shaped, with a streamlined body, paired

stem-sharks". Acanthodians had a cartilaginous skeleton, but their fins had a wide, bony base and were reinforced on their anterior margin with a dentine spine. As a result, fossilized spines and scales are often all that remains of these fishes in ancient sedimentary rocks. The earliest acanthodians were marine, but during the Devonian, freshwater species became predominant.[citation needed
]

Acanthodians have been divided into four orders:

diplacanthids; they had robust bony shoulder girdles and many small sharp spines ("intermediate" or "prepelvic" spines) between the pectoral and pelvic fins. The climatiiform subgroup Diplacanthida has subsequently been elevated to its own order, Diplacanthiformes. Ischnacanthiforms were predators with tooth plates fused to their jaws. Acanthodiforms were filter feeders with a single dorsal fin, toothless jaws, and long gill rakers. They were the last and most specialized off the traditional acanthodians, as they survived up until the Permian period.[citation needed
]

Characteristics

Three acanthodians from the Early Devonian of Great Britain: Mesacanthus (an acanthodiform), Parexus (a "climatiiform"), and Ischnacanthus (an ischnacanthiform)
Museum für Naturkunde, Berlin
Various acanthodians, from top left clockwise: Cheiracanthus, Acanthodes, Climatius, Ischnacanthus, Parexus, Gyracanthus. center: Diplacanthus.

The scales of Acanthodii have distinctive ornamentation peculiar to each order. Because of this, the scales are often used in determining relative age of sedimentary rock. The scales are tiny, with a bulbous base, a neck, and a flat or slightly curved diamond-shaped crown.

Despite being called "spiny sharks", acanthodians predate sharks. Scales that have been tentatively identified as belonging to acanthodians, or "shark-like fishes" have been found in various Ordovician strata, though, they are ambiguous, and may actually belong to jawless fishes such as

bony fishes were already showing their potential to dominate the waters of the world, and their competition proved too much for the spiny sharks, which died out in Permian
times (approximately 250 million years ago).

Many palaeontologists originally considered the acanthodians close to the ancestors of the bony fishes.[

gill arch of some ancestral jawless fishes that had a gill skeleton made of pieces of jointed cartilage.[citation needed
]

Taxonomy and phylogeny

See also: List of acanthodian genera

In a study of early jawed vertebrate relationships, Davis et al. (2012) found acanthodians to be split among the two major clades

bony fish and tetrapods, has led to revisions of this phylogeny: acanthodians were then considered to be a paraphyletic assemblage leading to cartilaginous fish, while bony fish evolved from placoderm ancestors.[4]

Burrow et al. 2016 provides vindication by finding chondrichthyans to be nested among Acanthodii, most closely related to Doliodus and Tamiobatis.[2] A 2017 study of Doliodus morphology points out that it appears to display a mosaic of shark and acanthodian features, making it a transitional fossil and further reinforcing this idea.[5]

Phylogeny after[6]

Evolutionary history

The oldest remains attributed acanthodian-grade chondrichthyans are

Fanjingshania and Qianodus from the Early Silurian of China, dating to around 439 million years ago.[7][8] Compared to other contemporary groups of fish, acanthodians were relatively morphologically and ecologically conservative. Acanthodians rose in diversity during the Late Silurian, reaching their apex of diversity during the Lochkovian stage of the Early Devonian, declining during the Pragian but rising again during the following Emsian, which was followed by a decline in diversity during middle-Late Devonian. The diversity of the group was consistently low but stable during the Carboniferous, slightly decreasing going into the Permian.[9] The youngest records of the group are isolated scales and fin spines from Middle-Late Permian strata in the Paraná Basin of Brazil.[10]

References

  1. S2CID 4462506
    .
  2. ^
    doi:10.26879/601
    .
  3. S2CID 4304310
    .
  4. S2CID 87470088
    .
  5. hdl:2246/6701
    .
  6. PMID 28403168
    .
  7. S2CID 252570103
    .
  8. S2CID 252569771
    .
  9. S2CID 250629392
    .
  10. S2CID 129538432
    .

Further reading

External links
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