User:Macrophyseter/sandbox6

Macrophyseter/sandbox6; Temporal range: Early Eocene - Pleistocene, 53.3–1.1 Ma PreꞒ Ꞓ O S D C P T J K Pg N
	Fossil teeth of Parotodus benedenii
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Subclass:	Elasmobranchii
Order:	Lamniformes
Family:	incertae sedis
Genus:	†Parotodus; Cappetta, 1980
Type species
	†Parotodus benedenii; Le Hon, 1871
Species
	†P. benedenii Le Hon, 1871 (type); †P. pavlovi Menner, 1928; †P. mangyshlakensis Kozlov, 1999;

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Parotodus, commonly known as the false mako shark, is an extinct genus of mackerel shark that lived approximately 53 to one million years ago during the Eocene and Pleistocene epochs. Its teeth, which are found worldwide, are often prized by fossil collectors due to their rarity. The scarcity of fossils is because Parotodus likely primarily inhabited open oceans far away from the continents.

Description

Initially appearing as a small shark, Parotodus gradually increased in size over geologic time and by the Neogene period became one of the largest sharks of its time.^[4] A 1999 study estimated the genus to have measured up to 7.6 meters (25 ft) in length.^[5] The teeth of Parotodus are distinctively curved and rarely show feeding damage, which suggests that it mainly preyed on soft-bodied animals. Paleontologists speculate that this included other sharks, including the contemporaneous Megalodon.^[6]

Classification and evolution

Taxonomic ambiguity

The taxonomic placement of Parotodus remains highly uncertain. The consensus holds that the genus is a mackerel shark of the order Lamniformes, but scientists remain unsure as to which family it belongs to, which in addition brings to debate whether the type species is even congeneric with the other taxa traditionally included within Parotodus. This is because P. benedenii is only known from teeth, and they hold ambiguous characteristics that converge with multiple lamniform families. Scientists have proposed the placement of Parotodus within one of five possible families: the mega-toothed sharks (Otodontidae), white sharks (Lamnidae), cardabiodontids (Cardabiodontidae), thresher sharks (Alopiidae),^[4]^[7] and archaeolamnids (Archaeolamnidae).^[4] However, none of these proposals have strong evidence for support.^[7]

The 2008 MS thesis of paleontologist Heike Mewis attempted a phylogenetic analysis of various extinct sharks based on dental characteristics, which recovered Parotodus as a sister group to a clade containing the Otodontidae and Lamnidae (in other words, Parotodus was of neither family). However, there were caveats to the analysis that makes it difficult to determine its reliability. Mewis (2008) did not include thresher sharks in the analysis, and the data matrix for Parotodus contained many undefined characters, both of which increase the chance that the results are inaccurate.^[7]

As an otodontid

As a cardabiodontid

The hypothesis of a cardabiodontid identity was developed by Siverson (1999) upon the discovery of the Late Cretaceous shark Cardabiodon,^[8] which lived between 95 to 91 mya.^[9] Siverson noted a number of key dental similarities between the new genus and P. benedenii such as: curved crowns in most tooth types, a combination of curved lateral cusplets in upper teeth and straight cusplets in lower teeth (in Oligocene P. benedenii), the enlargement of the front lower lateroposterior teeth, consistently broad lingual necks in all tooth types, and the absence of median root grooves.^[8]^[9] Siverson further elaborated on the connections in a 2008 correspondence with Bourdon. He observed that the teeth of very young Cardabiodon venator, the geologically youngest representative of the genus, are almost identical with P. benedenii in shape, noting that the two species would be indistinguishable if the teeth were of the same size. This also introduced a hypothesis that Parotodus may have evolved as a paedomorph of Cardabiodon. Siverson also pointed out that while there are many dental similarities between P. benedenii and Otodus, there are far more between the former and Cardabiodon.^[10]

Siverson & Lindgren (2005) rejected the inclusion of P. mangyshlakensis within Parotodus and believe that the species has no evolutionary relations with P. benedenii. The study justified this based on two key differences. First, the lingual necks in P. mangyshlakensis are large in anterior teeth but small in lateroposterior teeth, whereas the lingual necks in Cardabiodon and P. benedenii are consistently large in all tooth types. Second, the teeth of P. mangyshlakensis exhibit massive lingual protuberance, which is absent in Cardabiodon and P. benedenii.^[9]

The main weakness of this hypothesis is the 60-million-year gap between Cardabiodon and P. benedenii, which makes it difficult to prove that the similarities between the two sharks are indeed an evolutionary relationship. But if the hypothesis is indeed correct, this would make Parotodus a Lazarus taxon that disappeared from the fossil record to reappear after an extended time.^[9]^[4] Cardabiodon is an antitropical shark that only appears in cooler waters at high latitudes, and Siverson & Lindgren (2005) noted that shark fossils between the Cretaceous and Oligocene are generally rare in such locations, which may explain the current lack of fossil evidence.^[9] Canevet (2019) also suggested Parotodus' habitation of open oceans and general rarity in continental fossil deposits as another explanation for the gap.^[4]

As an archaeolamnid

Canevet (2019) tendered a new hypothesis that postulates Parotodus as a descendant of the Late Cretaceous subspecies Archaeolamna kopingensis judithensis, thus making it an archaeolamnid. This is justified by the strong dental similarities between Archaeolamna and P. pavlovi such as: a large lingual neck, strongly compressed roots in the anterior teeth, a U-shaped inner edge of the root lobes in all but the most rear tooth positions, and the presence of lateral cusplets. There are nevertheless some notable differences between Archaeolamna kopingensis judithensis and P. pavlovi, such as the exclusive presence of marked folds on the external face of the teeth in the former and differences in the development of lateral cusplets in lateral teeth. Canevet also considered the assignment of P. pavlovi within Parotodus as tentative because it is possible that the species may better align with Archaeolamna instead.^[4]

This model follows the evolutionary hypothesis connecting the Eocene species to P. benedenii, adding that Archaeolamna kopingensis judithensis evolved into P. pavlovi. However, it also suffers a near 20-million-year gap, with currently no known Maastrichtian or Paleocene fossils connecting Archaeolamna and Parotodus.^[4]

As a lamnid

As a thresher shark

This was first suggested by Herman (1979), which noted that the dentition of P. benedenii resemble those in modern thresher sharks both in superficial shape and a shared absence of marked size differences between the anterior and lateral teeth. Extant thresher shark teeth are much smaller than Parotodus, but during the Miocene very large species of threshers such as the giant thresher (Alopias grandis) and serrated giant thresher (Alopias palatasi) that reached similar sizes to P. benedenii were present, which would mitigate size differences as a factor against this hypothesis. However, the way the roots are vascularized in P. benedenii teeth are very different than in known thresher sharks, which is a major weakness to a thresher shark identity.^[4]^[7]

Herman (1979) neglected to address P. pavlovi, which would potentially provide another weakness. Bourdon (2008) noted that this can be neutralized if scientists can successfully sever the evolutionary link between P. benedenii and the Eocene species, which he believed to be likely based on the evidence presented by Siverson & Lindgren (2005). However, Bourdon opinionated that P. benedenii was more likely a thresher shark than a cardabiodontid.^[10]

Classification

Due to the general scarcity and ambiguity of fossils, the familiar placement remains uncertain. Several proposals suggest classifying the genus as a mega-toothed shark, cardabiodont, archaeolamnid, thresher shark, or white shark.^[4]^[7]

Currently, three valid species are generally recognized within Parotodus. These include P. benedenii, P. pavlovi, and P. mangyshlakensis. However, some scientists, especially those who identify Parotodus as a cardabiodont, do not recognize the latter two as members of the genus.^[4]

A fourth species was reported by Ward, Nakatani, and Bernard in a 2017 poster from the Oligocene of Japan. The poster stated that the new species is to be named after Yasuhiro Fudouji, the paleontologist who discovered the type specimens, and will be formally described in an upcoming paper. However, the scientific name was not explicitly stated to avoid an accidental invalidation of the taxon.^[6]

Fossil records

The genus is often regarded as a rare species despite its presence in ocean deposits worldwide. As a result, it is often prized by fossil collectors. Paleontologists believe that Parotodus likely inhabited primarily open oceans like the modern oceanic whitetip shark and blue shark. This would explain why fossils of a cosmopolitan animal are so rare, given that open oceans are seldom represented in terrestrial fossil deposits. This hypothesis is additionally supported by how Parotodus teeth are unusually common in nodule deposits under the Pacific and Indian Oceans and on islands located far away from continental lands.^[4]

Parotodus fossils have been recovered from fossil deposits in the Azores, Mallorca, Malta, Europe, Madagascar, Kazakhstan, Japan, South Africa, New Zealand, Australia, Peru, California, the East Coast of the United States, and dredged from the Pacific and Indian Oceans.^[4]

References

^ Iserbyt, A.; De Schutter, P.J. (2012). "Quantitative analysis of Elasmobranch assemblages from two successive Ypresian (early Eocene) facies at Marke, western Belgium". Geologica Belgica. 15 (3): 147–156.
^ Steurbaut, E. (2006). "Ypresian". In Dejonghe, L. (ed.). Current status of chronostratigraphic units named from Belgium and adjacent areas. Vol. 9. pp. 73–93. {{cite book}}: |journal= ignored (help)
^ Boessenecker, S.J.; Boessenecker, R.W.; Geisler, J.H. (2018). "Youngest record of the extinct walrus Ontocetus emmonsi from the Early Pleistocene of South Carolina and a review of North Atlantic walrus biochronology" (PDF). Acta Palaeontologica Polonica. 63 (2): 279–286.
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l Canevet, J. (2019), "Le genre Parotodus: le faux requin-mako", Fossiles (in French), no. 37, pp. 39–50
^ Kent, B.W. (1999). "Speculations on the Size and Morphology of the Extinct Lamnoid Shark, Parotodus benedeni (le Hon)" (PDF). The Mosasaur. 6: 11–15.
^ ^a ^b Ward, D.J.; Nakatani, D.; Bernard, E.L. (2017), A new species of Parotodus (Lamniformes: Otodontidae) from the Oligocene of Japan, The Palaeontological Association, doi:10.13140/RG.2.2.33171.58404
^ ^a ^b ^c ^d ^e Kent, B.W. (2018). "The Cartilaginous Fishes (Chimaeras, Sharks, and Rays) of Calvert Cliffs, Maryland, USA". In Godfrey, S. J. (ed.). The Geology and Vertebrate Paleontology of Calvert Cliffs, Maryland, USA. pp. 45–157. ISSN 1943-6688. {{cite book}}: |journal= ignored (help)
^ ^a ^b Siverson, M. (1999). "A new large lamniform shark from the uppermost Gearle Siltstone (Cenomanian, Late Cretaceous) of Western Australia". Transactions of the Royal Society of Edinburgh: Earth Sciences. 90 (1): 49–66. doi:10.1017/S0263593300002509.
^ ^a ^b ^c ^d ^e Siverson, M.; Lindgren, J. (2005). "Late Cretaceous sharks Cretoxyrhina and Cardabiodon from Montana, USA" (PDF). Acta Palaeontologica Polonica. 50 (2): 301–314.
^ ^a ^b Bourdon, J. (2008). "Parotodus". elasmo.com.