Polyacrodus

Polyacrodus was a genus of prehistoric shark, composed of several species, with fossils found in North America, South America, Europe, Asia, and Africa. The name Polyacrodus means “tooth with many bumps”, and it was officially named by the German paleontologist Otto Jaekel in 1889 (1).

Jaekel, Otto (1889). “Die Selachier aus dem oberen Muschelkalk lothringens”. Abhandlungen zur Geologischen Specialkarte von Elsass-Lothringen, volume 3, issue 4 (1889). Pages 273-332.

Polyacrodus belonged to a group of sharks called the “hybodonts”, named in reference to the shark Hybodus. In fact, the teeth of this animal were originally classified as belonging to Hybodus. However, Otto Jaekel recognized that while the crowns were similar in shape, the root structures were different, and therefore they ought to be classified as a different genus (2).

Polyacrodus teeth have been found within sediments deposited in freshwater, so it is certain that this animal was able to tolerate being in freshwater, like the modern-day Bull Shark (3).

While fragmentary remains of fin spines and even coprolites have been attributed to Polyacrodus or to some similar animal (4), the only fossils which can currently be definitely attributed to the genus Polyacrodus are teeth (5).

Teeth of Polyacrodus. Bratvold, Janne; Delsett, Lene Liebe; Hurum, Jorn Harald (2018). “Chondrichthyans from the Grippia bonebed (Early Triassic) of Marmierfjellet, Spitsbergen”. Norwegian Journal of Geology, volume 98, issue 2 (2018). Page 198.
https://www.researchgate.net/publication/328183978_Chondrichthyans_from_the_Grippia_bonebed_Early_Triassic_of_Marmierfjellet_Spitsbergen

In general shape, Polyacrodus teeth bear a similarity to the teeth of dogfish, bramble sharks, nurse sharks, and other sharks that spend much of their lives cruising close to the sea’s bottom. This makes a certain amount of sense, since these sharks are what are called “benthic”, meaning that they spend much of their time near the sea floor. Furthermore, body fossils of hybodont sharks bear a resemblance to many species of benthic sharks today. Therefore, it’s likely that Polyacrodus spent much of its time cruising along the bottom of estuaries or rivers. Benthic sharks like nurse sharks and dogfish are opportunistic and have a wide-ranging diet, including worms, freshwater shrimp, fish, and scavenging whatever tasty morsels it came across. Did Polyacrodus possess fleshy barbels near its nose, the way that many bottom-dwelling sharks do today? Maybe. What about spiracles? Did it possess breathing holes on the top of its head, behind the eyes, the way that rays and many species of benthic sharks do? Perhaps, perhaps not.

Below is an illustration which I made of the prehistoric shark Polyacrodus. I decided to give it a brown coloration as a way to camouflage itself against river bottoms and the murky waters of bays and estuaries. The drawing measures 10 inches long from tip to tip, and was drawn on printer paper with No.2 pencil and Crayola and Prismacolor colored pencils.

Polyacrodus. © Jason R. Abdale (May 4, 2022)

There are currently fourteen species of Polyacrodus known to science. Nearly all of these are found in rocks dated to the Permian and Triassic Periods, making it one of the few animals that was able to survive the Permian Mass Extinction of 251 MYA, which is estimated to have killed off 95% of all living things (6). However there is one species, Polyacrodus parvidens, whose fossils are found in European rocks dated from the middle Jurassic Period to the middle Cretaceous Period. It was originally named Hybodus parvidens by Arthur S. Woodward, but was later re-classified as a species of Polyacrodus (7). Teeth which have been attributed to Polyacrodus parvidens have also been found within the Cedar Mountain Formation of Utah (8), the Cloverly Formation of Montana (9), and the Twin Mountains Formation of Texas (10). In all cases, these date to the middle of the Cretaceous Period. It’s likely that the fossil teeth which have been found within these layers have been mis-identified by paleontologists, because it is extremely unlikely that this single species of Polyacrodus could continue to exist after all of its related species had gone extinct so many millions of years earlier. It’s possible that the Cretaceous fossils which have been attributed to Polyacrodus actually belong to another hybodont genus, such as Lissodus or Lonchidion. However, until the entire genus Polyacrodus undergoes a serious re-examination, and until paleontologists decide to change its identification to something new, its current identification will remain.

Source citations

  1. Mindat. “Polyacrodus”.
  2. Mindat. “Polyacrodus”; Erik Anderson Stensio, Triassic Fishes from Spitzbergen, Part I. Vienna: Adolf Hausen, 1921. Page 27.
  3. Archibald, J. D.; Sues, Hans-Dieter; Averianov, A. O.; King, C.; Ward, D. J.; Tsaruk, O. A.; Danilov, I. G.; Rezvyi, A. S.; Veretennikov, B. G.; Khodjaev, A. (1998). “Précis of the Cretaceous Paleontology, Biostratigraphy and Sedimentology at Dzharakuduk (Turonian?-Santonian), Kyzylkum Desert, Uzbekistan”. In Lucas, Spencer G.; Kirkland, James I.; Estep, John W., eds. Lower and Middle Cretaceous Terrestrial Ecosystems. New Mexico Museum of Natural History and Science, bulletin no. 14 (1998). Page 24.
  4. Kirkland, James I.; Suarez, Marina; Suarez, Celina; Hunt-Foster, ReBecca (2016). “The Lower Cretaceous in East-Central Utah—The Cedar Mountain Formation and its Bounding Strata”. Geology of the Intermountain West, volume 3 (October 2016). Pages 133, 137, 151, 184.
  5. Maisey, John G. (1982). “The Anatomy and Interrelationships of Mesozoic Hybodont Sharks”. American Museum Novitiates, no. 2724 (April 14, 1982). Pages 28, 38.
  6. Mindat. “Polyacrodus”.
  7. Mindat. “Polyacrodus”; Woodward, Arthur Smith (1916). “The Fossil Fishes of the English Wealden and Purbeck Formations”. Palaeontographical Society (1916). Page 12.
  8. Gillette, David., ed. “Vertebrate Paleontology in Utah”. Miscellaneous Publications 99-1, Utah Geological Survey (1999). Page 225; Kirkland, James I.; Suarez, Marina; Suarez, Celina; Hunt-Foster, ReBecca (2016). “The Lower Cretaceous in East-Central Utah—The Cedar Mountain Formation and its Bounding Strata”. Geology of the Intermountain West, volume 3 (October 2016). Pages 133, 137, 151, 184.
  9. Oreska, Matthew P. J.; Carrano, Matthew T.; Dzikiewicz, Katherine M. (2013). “Vertebrate paleontology of the Cloverly Formation (Lower Cretaceous), I: faunal composition, biogeographic relationships, and sampling”. Journal of Vertebrate Paleontology, volume 33, issue 2. Pages 268-269.
  10. Fischer, Jan (2008). “Brief synopsis of the hybodont form taxon Lissodus BROUGH, 1935, with remarks on the environment and associated fauna”. Paläontologie, Stratigraphie, Fazies (16), Freiberger Forschungshefte, C 528 (January 2008). Page 14.

Bibliography

Books
Stensio, Erik Anderson. Triassic Fishes from Spitzbergen, Part I. Vienna: Adolf Hausen, 1921.
https://ia903004.us.archive.org/5/items/triassicfishesfr11921sten/triassicfishesfr11921sten.pdf.

Articles
Archibald, J. D.; Sues, Hans-Dieter; Averianov, A. O.; King, C.; Ward, D. J.; Tsaruk, O. A.; Danilov, I. G.; Rezvyi, A. S.; Veretennikov, B. G.; Khodjaev, A. (1998). “Précis of the Cretaceous Paleontology, Biostratigraphy and Sedimentology at Dzharakuduk (Turonian?-Santonian), Kyzylkum Desert, Uzbekistan”. In Lucas, Spencer G.; Kirkland, James I.; Estep, John W., eds. Lower and Middle Cretaceous Terrestrial Ecosystems. New Mexico Museum of Natural History and Science, bulletin no. 14 (1998). Pages 21-27.

Fischer, Jan (2008). “Brief synopsis of the hybodont form taxon Lissodus BROUGH, 1935, with remarks on the environment and associated fauna”. Paläontologie, Stratigraphie, Fazies (16), Freiberger Forschungshefte, C 528 (January 2008). Pages 1-23.
https://www.researchgate.net/publication/248399734_Brief_synopsis_of_the_hybodont_form_taxon_Lissodus_BROUGH_1935_with_remarks_on_the_environment_and_associated_fauna/link/00b7d51dfdb2a21b90000000/download.

Gillette, David., ed. “Vertebrate Paleontology in Utah”. Miscellaneous Publications 99-1, Utah Geological Survey (1999).

Kirkland, James I.; Suarez, Marina; Suarez, Celina; Hunt-Foster, ReBecca (2016). “The Lower Cretaceous in East-Central Utah—The Cedar Mountain Formation and its Bounding Strata”. Geology of the Intermountain West, volume 3 (October 2016). Pages 101-228. https://giw.utahgeology.org/giw/index.php/GIW/article/view/11.

Maisey, John G. (1982). “The Anatomy and Interrelationships of Mesozoic Hybodont Sharks”. American Museum Novitiates, no. 2724 (April 14, 1982). Pages 1-48.
https://digitallibrary.amnh.org/bitstream/handle/2246/5337//v2/dspace/ingest/pdfSource/nov/N2724.pdf?sequence=1&isAllowed=y.

Oreska, Matthew P. J.; Carrano, Matthew T.; Dzikiewicz, Katherine M. (2013). “Vertebrate paleontology of the Cloverly Formation (Lower Cretaceous), I: faunal composition, biogeographic relationships, and sampling”. Journal of Vertebrate Paleontology, volume 33, issue 2. Pages 264-292.
https://www.researchgate.net/publication/236886285_Vertebrate_Paleontology_of_the_Cloverly_Formation_Lower_Cretaceous_I_Faunal_Composition_Biogeographic_Relationships_and_Sampling.

Woodward, Arthur Smith (1916). “The Fossil Fishes of the English Wealden and Purbeck Formations”. Palaeontographical Society (1916). Pages 1-48.

Websites
Mindat. “Polyacrodus”. https://www.mindat.org/taxon-P34508.html. Accessed on March 26, 2022.



Categories: Paleontology, Uncategorized

Tags: , , , , , , , , , , , , , , , , , , , , , , , , ,

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: