''Tremacyllus'' is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of hegetotheriids. It lived from the

Late Miocene The Late Miocene (also known as Upper Miocene) is a sub-epoch of the Miocene epoch (geology), Epoch made up of two faunal stage, stages. The Tortonian and Messinian stages comprise the Late Miocene sub-epoch, which lasted from 11.63 Ma (million ye ...

to the

Late Pleistocene The Late Pleistocene is an unofficial Age (geology), age in the international geologic timescale in chronostratigraphy, also known as Upper Pleistocene from a Stratigraphy, stratigraphic perspective. It is intended to be the fourth division of ...

(~7-0.012 Ma) and its fossilized remains were discovered in

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...

Description

This animal was approximately the size of a

hare Hares and jackrabbits are mammals belonging to the genus ''Lepus''. They are herbivores, and live solitarily or in pairs. They nest in slight depressions called forms, and their young are able to fend for themselves shortly after birth. The ge ...

, and both animals, while unrelated, must have been quite similar in appearance. Its skull had large orbits and strong lower

incisor Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, whe ...

s, similar to modern

lagomorph The lagomorphs are the members of the taxonomic order Lagomorpha, of which there are two living families: the Leporidae (hares and rabbits) and the Ochotonidae (pikas). The name of the order is derived from the Ancient Greek ''lagos'' (λαγ� ...

s. It was probably a fast animal, with long legs, although proportionally shorter than other similar animals such as ''

Pachyrukhos ''Pachyrukhos'' is an extinct genus of hegetotheriid notoungulate from the Early to Middle Miocene (Colhuehuapian-Friasian in the SALMA classification) of Argentina and Chile. Fossils of this genus have been found in the Collón Curá, Sarmien ...

'' or extant lagomorphs. Compared to its relative ''

Paedotherium ''Paedotherium'' is an extinct, potentially paraphyletic genus of Notoungulate, belonging to the family Hegetotheriidae, composed of small-sized, rodent or lagomorph-like Meridiungulata, South American ungulates. Four species are unambiguously re ...

'', ''Tremacyllus'' was slightly smaller and possessed several distinctive characteristics in its dentition: its

diastema A diastema (plural diastemata, from Greek διάστημα, space) is a space or gap between two teeth. Many species of mammals have diastemata as a normal feature, most commonly between the incisors and molars. More colloquially, the condition ...

was longer, the third upper molar was shorter or had the same size than the second molar, and the lower

premolar The premolars, also called premolar teeth, or bicuspids, are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant in the permanent set of teeth, making eight premolars total in the mouth ...

s were more overlapping and less molar-like. Furthermore, the symphysis of the mandible was shorter than in ''Paedotherium''.

Classification

The genus ''Tremacyllus'' was described in 1891 by

Florentino Ameghino Florentino Ameghino (born Giovanni Battista Fiorino Giuseppe Ameghino September 19, 1853 – August 6, 1911) was an Argentine naturalist, paleontologist, anthropologist and zoologist, whose fossil discoveries on the Argentine Pampas, especially ...

for the species ''Pachyrukhos impressus'', described by Ameghino himself a few years earlier. He named the type species ''Tremacyllus impressus'', from the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Early Pleistocene The Early Pleistocene is an unofficial sub-epoch in the international geologic timescale in chronostratigraphy, being the earliest division of the Pleistocene Epoch within the ongoing Quaternary Period. It is currently estimated to span the time ...

, and several other species were later described by Ameghino himself, such as ''T. chapalmalensis'', ''T. diminutus'' and ''T. novus'', and by

Gaetano Rovereto Gaetano Rovereto (15 November 1870 – 23 November 1952) was an Italian geologist and paleontologist who, though critical of the application of theories to geology, was in favour of cyclical processes to explain geological formations. He published o ...

, such as ''T. intermedius'' and ''T. incipiens'', the latter of which lived during the

. A 2017 study led by Renatta Sostillo, Esperanza Cerdeño and Claudia I. Montalvo however determined that the presumed differences between all the species of ''Tremacyllus'' could be explained by intraspecific variations, and therefore that the only valid species was the type, ''T. impressus''. However, a 2022 study by Armella, Ercolli, Bonini and Garcia-Lopez found sufficient proof to recognize ''T. incipiens'' as a valid species. ''Tremacyllus'' was a specialized member of the

Hegetotheriidae Hegetotheriidae is an extinct family of notoungulate mammals known from the Oligocene through the Pliocene of South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a r ...

, a group of small-sized lagomorph-like notoungulates. Particularly, ''Tremacyllus'' was close from the genera ''Pachyrukhos'' and ''Paedotherium'', and was one of the last hegetotheres known, as well as one of the last notoungulates.

Paleobiology

One of the exceptional characteristics of ''Tremacyllus'', as well as its relatives within Pachyrukhinae, such as ''Paedotherium'', is the presence of a real sciuromorph condition in its chewing apparatus, defined by an anterior portion of the

masseter muscle In human anatomy, the masseter is one of the muscles of mastication. Found only in mammals, it is particularly powerful in herbivores to facilitate chewing of plant matter. The most obvious muscle of mastication is the masseter muscle, since it ...

coming from a large

zygomatic plate In rodent anatomy, the zygomatic plate is a bony plate derived from the flattened front part of the zygomatic arch (cheekbone). At the back, it connects to the front (maxillary) root of the zygomatic arch, and at the top it is connected to the ...

reaching the rostrum ; a characteristic traceable within Hegetotheriidae since the Oligocene. Hence, those animals are the first case of a non-rodent mammal developing a sciuromorph condition. This morphology would have permitted them to explore ecological niches unavailable to the histrichomorph rodents that coexisted with them. This innovative acquisition seems to have appeared at the same time in sciuromorph rodents and pachyrukhins, and could be linked with the consumption of hard food. It is therefore supposed that the expansion of nut trees and cone trees caused by major environmental changes during the Eocene-Oligocene transition may have been the potential trigger for this convergent evolution.Marcos D. Ercoli; Alicia Álvarez; Adriana M. Candela (2019). "Sciuromorphy outside rodents reveals an ecomorphological convergence between squirrels and extinct South American ungulates". Communications Biology. 2: Article number 202. doi:10.1038/s42003-019-0423-5. PMC 6546766. PMID 31231692.

References

*F. Ameghino. 1891. Mamiferos y aves fosiles argentinas. --Especies nuevas, adiciones y correcciones. Revista Argentina Historia Natural 1(4):240-259 *E. Cerdeno and M. Bond. 1998. Taxonomic Revision and Phylogeny of Paedotherium and Tremacyllus (Pachyrukhinae, Hegetotheriidae, Notoungulata) from the Late Miocene to the Pleistocene of Argentina. Journal of Vertebrate Paleontology 18(4):799-811 *F. D. Seoane, S. R. Juñent, and E. Cerdeño. 2017. Phylogeny and paleobiogeography of Hegetotheriidae (Mammalia, Notoungulata). Journal of Vertebrate Paleontology 37(1):e1278547:1-13 * Marcos D. Ercoli; Alicia Álvarez; S. Rocío Moyano; Dionisios Youlatos; Adriana M. Candela (2020). "Tracing the Paleobiology of ''Paedotherium'' and ''Tremacyllus'' (Pachyrukhinae, Notoungulata), the Latest Sciuromorph South American Native Ungulates – Part I: Snout and Masticatory Apparatus". Journal of Mammalian Evolution. in press. . *Marcos D. Ercoli; Alicia Álvarez; Dionisios Youlatos; S. Rocío Moyano; Adriana M. Candela (2020). "Tracing the Paleobiology of ''Paedotherium'' and ''Tremacyllus'' (Pachyrukhinae, Notoungulata), the Latest Sciuromorph South American Native Ungulates – Part II: Orbital, Auditory, and Occipito-Cervical Regions". Journal of Mammalian Evolution. in press. {{Taxonbar, from=Q3888834 Typotheres Pleistocene mammals of South America Pliocene mammals of South America Miocene mammals of South America Chasicoan Huayquerian Montehermosan Chapadmalalan Uquian Neogene Argentina Fossils of Argentina Fossil taxa described in 1891 Taxa named by Florentino Ameghino Prehistoric placental genera