Holocephali

The subclass Holocephali ("complete heads") is a taxon of cartilaginous fish in the class Chondrichthyes. The earliest fossils are of teeth and come from the Devonian period. Little is known about these primitive forms, and the only surviving group in the subclass is the order Chimaeriformes. This group includes the rat fishes in the genus Chimaera, and the elephant fishes in the genus Callorhinchus. These fishes move by using sweeping movements of their large pectoral fins. They have long slender tails and live close to the seabed feeding on benthic invertebrates. They lack a stomach, food moving directly into the intestine.

Holocephali
Temporal range: 416–0 Ma Devonian–Recent
Hydrolagus colliei, a rat fish
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Holocephali
Orders

Characteristics

Members of this taxon preserve today some features of elasmobranch life in Paleozoic times, though in other respects they are aberrant. They live close to the bottom and feed on molluscs and other invertebrates. The tail is long and thin and they move by sweeping movements of the large pectoral fins. The erectile spine in front of the dorsal fin is sometimes venomous. There is no stomach (that is, the gut is simplified and the 'stomach' is merged with the intestine), and the mouth is a small aperture surrounded by lips, giving the head a parrot-like appearance. The only surviving members of the group are the rabbit fish (Chimaera), and the elephant fishes (Callorhinchus).[2][3]

Evolution

The fossil record of the Holocephali starts during the Devonian period.[4] The record is extensive, but most fossils are teeth, and the body forms of numerous species are not known, or at best poorly understood. Some experts further group the orders Petalodontiformes, Iniopterygiformes, and Eugeneodontida into the taxon "Paraselachimorpha", and treat it as a sister group to Chimaeriformes. However, as almost all members of Paraselachimorpha are poorly understood, most experts suspect this taxon to be either paraphyletic or a wastebasket taxon.

Based on genetic research, it is estimated the Holocephali split from the Elasmobranchii about 421 million years ago.[5] Analysis of the 280 million-year-old holocephalian Dwykaselachus demonstrates that early members of the group were more shark-like.[6]

Taxonomy according to Joseph Nelson, 2006[7]
Subclass Holocephali
  • †Superorder Paraselachimorpha
  • Superorder Holocephalimorpha
    • †Order Psammodontiformes*
    • †Order Copodontiformes
    • †Order Squalorajiformes
    • †Order Chondrenchelyiformes
    • †Order Menaspiformes
    • †Order Cochliodontiformes
    • Order Chimaeriformes (chimaeras)

† Extinct * position uncertain

gollark: They don't seem to have many WD disks in that.
gollark: Hmm. Perhaps seagate somewhat bad then.
gollark: ```ID# ATTRIBUTE_NAME FLAG VALUE WORST THRESH TYPE UPDATED WHEN_FAILED RAW_VALUE 1 Raw_Read_Error_Rate 0x002f 200 200 051 Pre-fail Always - 2 3 Spin_Up_Time 0x0027 179 171 021 Pre-fail Always - 2033 4 Start_Stop_Count 0x0032 001 001 000 Old_age Always - 168316 5 Reallocated_Sector_Ct 0x0033 200 200 140 Pre-fail Always - 0 7 Seek_Error_Rate 0x002f 200 200 051 Pre-fail Always - 0 9 Power_On_Hours 0x0032 079 079 000 Old_age Always - 15354 10 Spin_Retry_Count 0x0032 100 100 000 Old_age Always - 0 11 Calibration_Retry_Count 0x0032 100 100 000 Old_age Always - 0 12 Power_Cycle_Count 0x0032 098 098 000 Old_age Always - 2564192 Power-Off_Retract_Count 0x0032 200 200 000 Old_age Always - 320193 Load_Cycle_Count 0x0032 088 088 000 Old_age Always - 336830194 Temperature_Celsius 0x0022 126 080 000 Old_age Always - 21196 Reallocated_Event_Count 0x0032 200 200 000 Old_age Always - 0197 Current_Pending_Sector 0x0032 200 200 000 Old_age Always - 0198 Offline_Uncorrectable 0x0030 100 253 000 Old_age Offline - 0199 UDMA_CRC_Error_Count 0x0032 200 200 000 Old_age Always - 2200 Multi_Zone_Error_Rate 0x0008 200 200 000 Old_age Offline - 0240 Head_Flying_Hours 0x0032 088 088 000 Old_age Always - 8931```Wow, this disk is in a great state.
gollark: I don't see why I would want dead ones.
gollark: Certainly not to the extent that you'd expect piles of dead ones.

References
  1. Coates, M., Gess, R., Finarelli, J., Criswell, K., Tietjen, K. 2016. A symmoriiform chondrichthyan braincase and the origin of chimaeroid fishes. Nature. doi:10.1038/nature20806
  2. Pough, Janis & Heiser 2013, pp. 99,101, Table 5-1.
  3. Martin, lead section.
  4. Pough, Janis & Heiser 2013, pp. 103,105, Paleozoic Holocephalans.
  5. Renz, AJ; Meyer, A; Kuraku, S (2013). "Revealing less derived nature of cartilaginous fish genomes with their evolutionary time scale inferred with nuclear genes". PLoS One. 8: e66400. doi:10.1371/journal.pone.0066400. PMC 3692497. PMID 23825540.
  6. 280 million-year-old fossil reveals evolutionary origins of shark-like fishes
  7. Nelson 2006.

Bibliography
  • Martin, R. Aidan; et al. "Chimaeras — The Neglected Chondrichthyans". Biology of Sharks and Rays. ReefQuest Centre for Shark Research. Retrieved 2016-02-22.CS1 maint: ref=harv (link)
  • Nelson, Joseph S. (2006). Fishes of the World. ISBN 978-0-471-25031-9.CS1 maint: ref=harv (link)
  • Pough, F. Harvey; Janis, Christine M.; Heiser, John B. (2013). Vertebrate Life (9th,international ed.). ISBN 978-0-321-78235-9.CS1 maint: ref=harv (link)


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