NOTE: This article was originally published on May 4, 2022. On December 30, 2025, it was updated and re-uploaded.
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 (Jaekel 1889, pages 273-332). It 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 (Mindat. “Polyacrodus”; Stensio 1921, page 27).
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 (Mindat. “Polyacrodus”). 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 it was later re-classified as a species of Polyacrodus (Mindat. “Polyacrodus”; Woodward 1916, page 12). In addition to fossils from Europe, teeth which have been attributed to Polyacrodus parvidens have also been found within the Cedar Mountain Formation of Utah (Cifelli et al. 1999, page 225; Kirkland et al. 2016, pages 133, 137, 151, 184), the Cloverly Formation of Montana (Oreska et al. 2013, pages 268-269), and the Twin Mountains Formation of Texas (Fischer 2008, page 14). 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.
While fragmentary remains of fin spines and even coprolites have been attributed to Polyacrodus (Kirkland et al. 2016, pages 133, 137, 151, 184), the only fossils which can currently be definitely attributed to the genus Polyacrodus are teeth (Maisey 1982, pages 28, 38). 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 (Archibald et al. 1998, page 24).
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
There is some extremely flimsy evidence that freshwater hybodont sharks lived within the Morrison Formation of western North America during the late Jurassic Period. In 1899, Othniel Charles Marsh reported that a single hybdodont tooth was found in the Atlantasaurus Beds (an old name for the Morrison Formation) near the village of Piedmont in western South Dakota. Marsh referred this tooth to the species Hybodus polyprion, which was previously known from England within rocks dated to the Bathonian Stage of the middle Jurassic Period (Marsh 1899, page 230). However, no description or images of the tooth were provided in Marsh’s report, and no collection ID code was given, so I have no idea where this tooth is kept. Furthermore, it’s also possible that Marsh made a mistake in identifying the rock layers as dating to the late Jurassic, because that area also has strata from the Chilson Member of the Lakota Formation which dates to the early Cretaceous Period. In fact, Marsh was adamant that the Chilson strata were Jurassic in age despite reports from several geologists and paleo-botanists stating that they dated to the early Cretaceous (Newton and Jenney 1880, pages 31, 43, 174-186; Ward 1894, pages 252-255; Marsh 1898, pages 114-115; Darton 1899, pages 385-386; Lindsay 2011, pages 216-239). In 2008, Hybodus polyprion was renamed to Secarodus polyprion, meaning “cutting/slicing tooth with multi saw”, with the authors noting numerous differences in tooth morphology compared to those of Hybodus (Rees and Underwood 2008, page 130). However, there’s another problem. Hybodus and Secarodus are marine genera. It’s more likely that the tooth from Piedmont-South Dakota – if it did indeed come from a hybodont shark – actually came from Lissodus or Polyacrodus, both of which are known from freshwater deposits. In fact, Rees and Underwood noted the similarity of Secarodus polyprion’s tooth structure to that of Polyacrodus (Rees and Underwood 2008, page 132). In 1989, it was reported that possible hybodont dorsal spines were found at Dinosaur National Monument near the Utah/Colorado border, as a locality dubbed Rainbow Park. However, these weren’t assigned to any particular genus (Chure and Engelmann 1989, page 12). Moreover, no mention was made of a hybodont dorsal spine in an updated list of Morrison species published in 2006 (Chure et al. 2006, pages 233-249), so it’s possible that this fossil was mis-identified earlier.
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.
Polyacrodus. © Jason R. Abdale (August 1, 2022).
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Websites
Mindat. “Polyacrodus”. https://www.mindat.org/taxon-P34508.html. Accessed on March 26, 2022.
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