Plastomenus

Plastomenus Temporal range: Early Paleocene to Lutetian PreꞒ Ꞓ O S D C P T J K Pg N
Fossil in the Field Museum of Natural History.
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	Testudines
Suborder:	Cryptodira
Family:	Trionychidae
Subfamily:	† Plastomeninae
Genus:	† Plastomenus Cope, 1873
Type species
†Trionyx thomasii Cope, 1872
Species [1]
†P. joycei Lyson, Petermann & Miller, 2021 †P. thomasii (Cope, 1872) †P. vegetus (Gilmore, 1919)

Plastomenus is an extinct genus of turtle that inhabited western North America during the early Paleogene period. ^[2]

Evolution

Plastomenus belongs to the clade Pantrionychidae, represented by softshell turtles in modern times. It is the type genus of the subfamily Plastomeninae (also treated as the family Plastomenidae), a group of extinct turtles that lived from the Late Cretaceous to the Eocene. ^[2] The type species, P. thomasii, was first described by Edward Drinker Cope in 1872. ^{[3] [4]}

The following species are known: ^{[3] [5]}

• †P. joyceiLyson, Petermann & Miller, 2021 - Early Paleocene of Colorado, USA (Denver Formation) ^[2]
• †P. thomasii(Cope, 1872) - Early to Middle Eocene (Ypresian to Lutetian) of Wyoming, USA (Bridger Formation)
• †P. vegetus(Gilmore, 1919) - Early Paleocene of New Mexico, USA (Nacimiento Formation) ^[2]

Many other species formerly placed in this genus, from both North America and Asia, have been reclassified into other genera, synonymized with one of the extinct species, or are now considered nomina dubia . ^{[5] [6]}

Phylogeny

Based on Joyce & Lyson (2017): ^[7]

Plastomeninae	Aspideretoides Atoposemys Gilmoremys Plastomenini Helopanoplia Hutchemys Plastomenus

Alternatively, Plastomenus may be most closely related to Gilmoremys. ^[2]

Description

A cast of †Plastomenus thomasii with bite marks. The shell is approximately 14.75 cm long.

Plastomenus turtles are distinguished by their low, rounded shells and elongated skulls, which set them apart from other contemporary softshell turtles. ^[8]

References

1. "Plastomenus ✝". www.mindat.org. Hudson Institute of Mineralogy. Retrieved 23 March 2025.
2. Lyson, Tyler R.; Petermann, Holger; Miller, Ian M. (2 January 2021). "A new plastomenid trionychid turtle, Plastomenus joycei , sp. nov., from the earliest Paleocene (Danian) Denver Formation of south-central Colorado, U.S.A." . Journal of Vertebrate Paleontology. 41 (1). Bibcode:2021JVPal..41E3600L. doi:10.1080/02724634.2021.1913600.
3. "PBDB Taxon". paleobiodb.org. Retrieved 2025-04-11.
4. "Biographical memoir of Edward Drinker Cope 1840-1897" (PDF). Archived from the original (PDF) on 2025-04-14.
5. Vitek, Natasha S.; Joyce, Walter G. (2015). "A Review of the Fossil Record of New World Turtles of the Clade Pan-Trionychidae" . Bulletin of the Peabody Museum of Natural History. 56 (2): 185–244. Bibcode:2015BPMNH..56..185V. doi:10.3374/014.056.0204. ISSN   0079-032X.
6. Georgalis, Georgios L.; Joyce, Walter G. (2017). "A Review of the Fossil Record of Old World Turtles of the Clade Pan-Trionychidae" . Bulletin of the Peabody Museum of Natural History. 58 (1): 115–208. Bibcode:2017BPMNH..58..115G. doi:10.3374/014.058.0106. ISSN   0079-032X.
7. Joyce, Walter G.; Lyson, Tyler R. (2017). "The shell morphology of the latest Cretaceous (Maastrichtian) trionychid turtle Helopanoplia distincta". PeerJ. 5: e4169. doi: 10.7717/peerj.4169 . ISSN   2167-8359. PMC   5733369 . PMID   29259848.
8. Evers, Serjoscha W.; Chapelle, Kimberley E. J.; Joyce, Walter G. (December 2023). "Cranial and mandibular anatomy of Plastomenus thomasii and a new time-tree of trionychid evolution". Swiss Journal of Palaeontology. 142 (1): 1. Bibcode:2023SwJP..142....1E. doi: 10.1186/s13358-023-00267-5 . PMC   10020266 . PMID   36941994.