Prodiplocynodon

Prodiplocynodon is an extinct genus of basal crocodyloid crocodylian. It is one of the only crocodyloids known from the Cretaceous and existed during the Maastrichtian stage.[1] The only species of Prodiplocynodon is the type species P. langi from the Lance Formation of Wyoming, known only from a single holotype skull lacking the lower jaw.[2]

Prodiplocynodon
Temporal range: Late Cretaceous,
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Crocodilia
Superfamily: Crocodyloidea
Genus: †Prodiplocynodon
Mook, 1941
Type species
†Prodiplocynodon langi
Mook, 1941

The skull was collected by the American Museum Expedition of 1892 from exposures near the Cheyenne River in Niobrara County. It was described by Charles C. Mook of the American Museum of Natural History in 1941.[2] The generic name means "before Diplocynodon" because Mook saw close similarities between the holotype skull and that of the alligatoroid Diplocynodon from the Eocene of Europe.

Description

Most of the cranial sutures that outline individual bones of the skull are not visible in the holotype, and are often obscured by cracks. However, the overall shape of Prodiplocynodon is similar to that of basal alligatoroids.[1] Many of the features seen in Prodiplocynodon are common among eusuchians. The skull is short and triangular, being around 50 centimetres (20 in) in length. The orbits, or eye sockets, are quite large and subtriangular. The teeth are short and somewhat sharp, and in comparison to modern crocodiles show little variation. The orbits face directly upwards, but this may have been the result of slight compression in the holotype skull. The external nasal aperture, the opening for the nostrils, is very large. There is a constriction at the point of contact between the premaxilla and maxilla which would have been an area of reception for a large mandibular tooth. In Prodiplocynodon, the constriction is not deep, being intermediate between that of Alligator and Crocodylus.[2]

Classification

Mook suggested that Prodiplocynodon may be ancestral to alligatorids and crocodylids because it possessed features of both families. However, Mook also noted that some of the features observed in Prodiplocynodon that are also found in modern crocodylians may be the result of evolutionary convergence,[2] and thus Prodiplocynodon was also proposed to be an alligatorine.[3][4]

Phylogeny

Prodiplocynodon was not included in a phylogenetic study until 1996. In that study, Prodiplocynodon was excluded from the Alligatorinae because it lacked all seven of the unequivocal synapomorphies that were proposed for the clade. According to the 1996 study, characters that exclude Prodiplocynodon from Alligatorinae include the presence of a distinct lateral constriction between the premaxilla and maxilla, a contact between the nasal and lacrimal, and the lack of posterior massive crushing teeth. The 1996 analysis considered Prodiplocynodon to be the sister taxon to the Alligatorinae rather than the Crocodylinae because in Prodiplocynodon, the jugal-lacrimal suture is much shorter than the ventral border of the orbit. However, the authors of the study mentioned that this character is also seen in some derived crocodylines, and is lost in some ingroups of Alligatorinae.[5]

Successive phylogenetic studies have placed Prodiplocynodon as a basal member of Crocodyloidea along with Asiatosuchus,[6][7][8] as shown in the cladogram below:

Crocodyloidea 

Prodiplocynodon langi

Asiatosuchus germanicus

Crocodylus affinis

Dormaal crocodyloid

Crocodylus acer

Brachyuranochampsa eversolei

 Crocodylidae 

Tomistominae*

Crocodylinae

*Note: Tomistominae, which includes the extant False Gharial, is sometimes considered to be a gavialoid lineage on the basis of genetic evidence.[9][10]


A 2018 tip dating study by Lee & Yates simultaneously using morphological, molecular (DNA sequencing), and stratigraphic (fossil age) data established the inter-relationships within Crocodilia,[11] which was expanded upon in 2021 by Hekkala et al. using paleogenomics by extracting DNA from the extinct Voay.[12]

The below cladogram shows the results of the latest studies, which placed Prodiplocynodon outside of Crocodyloidea, as more basal than Longirostres (the combined group of crocodiles and gavialids).[11]

Crocodylia

Alligatoroidea

Prodiplocynodon†

Asiatosuchus†

"Crocodylus" affinis†

"Crocodylus" depressifrons†

"Crocodylus" acer†

Brachyuranochampsa†

Mekosuchinae

Longirostres
Crocodyloidea

"Crocodylus" megarhinus†

Crocodylidae

Gavialoidea

extinct basal Gavialoids†

Gavialidae

Gavialis

Tomistoma

References
  1. Brochu, C. A. (2003). "Phylogenetic approaches toward crocodylian history" (PDF). Annual Review of Earth and Planetary Sciences. 31 (31): 357–97. Bibcode:2003AREPS..31..357B. doi:10.1146/annurev.earth.31.100901.141308.
  2. Mook, C. C. (1941). "A new crocodilian from the Lance Formation" (PDF). American Museum Novitates (1128): 1–5.
  3. Steel, R. (1973). "Crocodilia". Handbuch der Paläoherpetologie. Stuttgart and Portland: Gustav Fischer Verlag. pp. 116pp.
  4. Carroll, R. L. (1988). Vertebrate Paleontology and Evolution. New York: W H. Freeman and Company. pp. 698pp. ISBN 9780716718222.
  5. Wu, X.-C.; Brinkman, D. B.; Russell, A. P. (1996). "A new alligator from the Upper Cretaceous of Canada and the relationships of early eusuchians" (PDF). Palaeontology. 39 (2): 351–375. Archived from the original (PDF) on 2011-09-28.
  6. Brochu, C. A. (2000). "Phylogenetic relationships and divergence timing of Crocodylus based on morphology and the fossil record". Copeia. 2000 (3): 657–673. doi:10.1643/0045-8511(2000)000[0657:pradto]2.0.co;2. S2CID 85824292.
  7. Brochu, C. A.; Gingerich, P. D. (2000). "New tomistomine crocodylian from the middle Eocene (Bartonian) of Wadi Hitan, Fayum Province, Egypt" (PDF). Contributions from the Museum of Paleontology: The University of Michigan. 30: 251–268.
  8. Hua, S.; Jouve, S. (2004). "A primitive marine gavialoid from the Paleocene of Morocco" (PDF). Journal of Vertebrate Paleontology. 24 (2): 341–350. doi:10.1671/1104. S2CID 86497272.
  9. Densmore, L. D.; Owen, R. D. (1989). "Molecular systematics of the order Crocodilia". American Zoologist. 29 (3): 831–841. doi:10.1093/icb/29.3.831.
  10. Brochu, C. A. (2004). "A new Late Cretaceous gavialoid crocodylian from Eastern North America and the phylogenetic relationships of Thoracosaurs" (PDF). Journal of Vertebrate Paleontology. 24 (3): 610–633. doi:10.1671/0272-4634(2004)024[0610:anlcgc]2.0.co;2. S2CID 131176447.
  11. Michael S. Y. Lee; Adam M. Yates (27 June 2018). "Tip-dating and homoplasy: reconciling the shallow molecular divergences of modern gharials with their long fossil". Proceedings of the Royal Society B. 285 (1881). doi:10.1098/rspb.2018.1071. PMC 6030529. PMID 30051855.
  12. Hekkala, E.; Gatesy, J.; Narechania, A.; Meredith, R.; Russello, M.; Aardema, M. L.; Jensen, E.; Montanari, S.; Brochu, C.; Norell, M.; Amato, G. (2021-04-27). "Paleogenomics illuminates the evolutionary history of the extinct Holocene "horned" crocodile of Madagascar, Voay robustus". Communications Biology. 4 (1): 505. doi:10.1038/s42003-021-02017-0. ISSN 2399-3642. PMC 8079395. PMID 33907305.
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