Diplacanthus crassisimus, copyright James St. John.

Belongs within: Gnathostomata.
Contains: Gyracanthidae, Climatiidae, Chondrichthyes, Ischnacanthidae, Acanthodidae.

Scleritomes: Not just an invert thing
Published 23 January 2008
Tchunacanthid scales, from (from Valiukevičius & Burrow, 2005).

Most of the problematic fossil taxa known only from their sclerites fall into the class of organisms dismissed by the sadly vertebrate-centric as “creepy-crawlies”. Nevertheless, the disarticulated scleritome issue is not unique to invertebrates.

The figure above shows scales of Silurian fish of the family Tchunacanthidae. This family was originally described by Karatajute-Talimaa & Smith (2003) as a new order, distinct from all others previously described (while Valiukevičius & Burrow seem a little sceptical of such a high ranking, they do still maintain the family’s distinctiveness). The interesting thing is that, so far, tchunacanthids are known only from scales.

Representative acanthodians, from here.

Tchunacanthidae are member of the Acanthodii, an extinct class of vertebrates found from the Silurian to the Permian. Acanthodians are sometimes referred as “spiny sharks”, a name that probably survives more because it sounds neat than because of its appropriateness for the actual animals. Though often regarded as more closely related to modern bony fishes and tetrapods than actual sharks, acanthodians resembled sharks in having a cartilaginous rather than a bony skeleton. As a result, acanthodian skeletons were rarely fossilised, and usually only the hard mineralised parts survived—teeth, scales and spines. Without the soft tissue holding them together, however, the fossils became disarticulated, just like the sclerites of a scleritome animal. Tchunacanthids are far from being the only family of non-bony fish known only from scattered pieces of armation—articulated specimens (except in those taxa that developed large bony plates) are the exception rather than the rule. Beyond the general features shared by all acanthodians, we are doomed to ignorance about what a living tchunacanthid looked like unless some day we are lucky enough to find one of those rare articulated fossils.

Systematics of Acanthodii
<==Acanthodii [Climatiida, Climatiiformes, Climatioidei]
    |--Diplacanthoidei [Diplacanthiformes]Z93
    |    |--GyracanthidaeZ93
    |    `--DiplacanthidaeV03
    |         |  i. s.: Ptychodictyon Gross 1973V03
    |         |           `--P. rimosum Gross 1973Z93
    |         |--TetanopsyrusDFC12
    |         `--+--Culmacanthus [Culmacanthidae]DFC12
    |            |    `--C. stewarti Long 1983BT12
    |            `--+--+--Gladiobranchus probaton Bernacsek & Dineley 1977DFC12, Z93
    |               |  `--Rhadinacanthus Traquair 1888DFC12, V03
    |               |       `--R. balticus Gross 1973Z93
    |               `--Diplacanthus Agassiz 1844DFC12, V03
    |                    |--D. carinatus Gross 1973V03
    |                    |--D. crassisimus Duff 1842V03
    |                    |--D. gravis Valiukevičius 1988V03
    |                    |--D. horridus Woodward 1892V03
    |                    |--D. kleesmentae Valiukevičius 1986V03
    |                    |--D. longispinus Agassiz 1844V03
    |                    |--D. poltnigi Valiukevičius 2003V03
    |                    `--D. solidus Valiukevičius 2003V03
    `--+--+--+--Ptomacanthus Miles 1973ZY13, V03
       |  |  |    `--P. anglicusBY99
       |  |  `--+--ClimatiidaeDFC12
       |  |     `--+--Brachyacanthus Egerton 1860DFC12, V03
       |  |        |    `--B. scutigerS93
       |  |        `--Parexus Agassiz 1845DFC12, V03
       |  |             `--P. anglicusB09
       |  `--+--Vernicomacanthus Miles 1973ZY13, V03
       |     `--+--Brochoadmones Bernacsek & Dineley 1977DFC12, V03
       |        `--+--Kathemacanthus Gagnier & Wilson 1996DFC12, V03
       |           |    `--K. rosulentusBY99
       |           `--+--ObtusacanthusDFC12
       |              |--Lupopsyrus Bernacsek & Dineley 1977DFC12, V03
       |              `--ChondrichthyesDFC12
       `--+--Euthacanthus [Euthacanthidae]ZY13
          |    `--E. macnicoli Powrie 1864V03
Acanthodii incertae sedis:
  Acanthospina Valiukevičius 2003V03
    `--*A. irregulare Valiukevičius 2003V03
  Markacanthus alius Valiukevičius 1988V03
  Pruemolepis wellsi Vieth-Schreiner 1983V03
  Tollichthys polaris Bystrov 1957V03
  Luetkeichthys borealis Bystrov 1957V03
    `--‘Gyracanthus’ sarlei (Hussakof & Bryant 1918)TBW05
    |--Lenacanthus Karatajute-Talimaa & Smith 2003 [Lenacanthidae]K-TS03
    |    `--*L. priscus Karatajute-Talimaa & Smith 2003K-TS03
    `--Tchunacanthus Karatajute-Talimaa & Smith 2003 [Tchunacanthidae]K-TS03
         `--*T. obruchevi Karatajute-Talimaa & Smith 2003K-TS03
  Acritolepis Valiukevičius 2003V03
    |--*A. ushakovi Valiukevičius 2003V03
    `--A. urvantsevi Valiukevičius 2003V03
  Erriwacanthus Ørvig 1967V03
  Ozarcus mapesaeGX17
  Striacanthus Hills 1931BT12
  Yealepis Burrow & Young 1999BY99
    `--*Y. douglasi Burrow & Young 1999BY99

*Type species of generic name indicated


[B09] Brazeau, M. D. 2009. The braincase and jaws of a Devonian ‘acanthodian’ and modern gnathostome origins. Nature 457: 305–308.

[BT12] Burrow, C. J., & S. Turner. 2012. Fossil fish taphonomy and the contribution of microfossils in documenting Devonian vertebrate history. In: Talent, J. A. (ed.) Earth and Life: Global biodiversity, extinction intervals and biogeographic perturbations through time pp. 189–223. Springer.

[BY99] Burrow, C. J., & G. C. Young. 1999. An articulated teleostome fish from the Late Silurian (Ludlow) of Victoria, Australia. Records of the Western Australian Museum Supplement 57: 1–14.

[DFC12] Davis, S. P., J. A. Finarelli & M. I. Coates. 2012. Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes. Nature 486: 247–250.

[GX17] Giles, S., G.-H. Xu, T. J. Near & M. Friedman. 2017. Early members of ‘living fossil’ lineage imply later origin of modern ray-finned fishes. Nature 549: 265–268.

[K-TS03] Karatajute-Talimaa, V., & M. M. Smith. 2003. Early acanthodians from the Lower Silurian of Asia. Transactions of the Royal Society of Edinburgh: Earth Sciences 93: 277–299.

[S93] Schultze, H.-P. 1993. Patterns of diversity in the skulls of jawed fishes. In: Hanken, J., & B. K. Hall (eds) 1993. The Skull vol. 2. Patterns of Structural and Systematic Diversity pp. 189–254. The University of Chicago Press.

[TBW05] Turner, S., C. J. Burrow & A. Warren. 2005. Gyracanthides hawkinsi sp. nov. (Acanthodii, Gyracanthidae) from the Lower Carboniferous of Queensland, Australia, with a review of gyracanthid taxa. Palaeontology 48: 963–1006.

[V03] Valiukevičius, J. 2003. Devonian acanthodians from Severnaya Zemlya Archipelago (Russia). Geodiversitas 25: 131–204.

Valiukevičius, J., & J. C. Burrow. 2005. Diversity of tissues in acanthodians with Nostolepis−type histological structure. Acta Palaeontologica Polonica 50 (3): 635–649.

[ZY13] Zhu, M., X. Yu, P. E. Ahlberg, B. Choo, J. Lu, T. Qiao, Q. Qu, W. Zhao, L. Jia, H. Blom & Y. Zhu. 2013. A Sliurian placoderm with osteichthyan-like marginal jaw bones. Nature 502: 188–193.

[Z93] Zidek, J. 1993. Acanthodii. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 589–592. Chapman & Hall: London.

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