Paroxyclaenidae is an extinctfamily of pantolestanmammals that ranges from the early Eocene to the early Oligocene. Most members of the family are found throughout Europe though the youngest and largest member, Welcommoides, lived in what is now Pakistan. Similar to a large amount of other more basal Cenozoic mammals, their taxonomic placement is not well understood with suggestions ranging from being within Carnivora to within the former orderCreodonta. Today, them and Pantolesta as a whole are considered Eutherian mammals that may be a part of another problematic order, Cimolesta.
Like a lot of early Cenozoic mammals, a lot of there diagnostic features are in relation to their dentition which is generally similar to other early 'condylarths' with the main differences being the large size and spacing of their premolars. Due to their dentition and postcranial anatomy, paroxyclaenids have been suggested to have been arborealfrugivores. Their extinction lines up generally with multiple climatic events during the late Eocene to the Eocene-Oligocene boundary which caused cooler and more arid conditions in Europe that affected a large amount of other animals in the region. The more stable conditions on the Indian subcontinent allowed them to last much longer outside of Europe than within. Currently there are two subfamilies recognized within Paroxyclaenidae: Merialinae and Paroxyclaeninae.
History and Classification
Similar to a number of early Cenozoic mammal families, the position of Paroxyclaenidae within mammals has been unstable with some authors placing them within Carnivora while other authors have placed them in a number of now non-monophyletic clades such as 'Insectivora', 'Condylarthra', and 'Creodonta'. The most common placement today is within another historically enigmatic group, Pantolesta.[1] The most current understanding of the family comes from a 1998 publication by Donald E. Russell and Marc Godinot that would not only name the oldest member of the group, Merialus martinae, but would also act as a general review of the previous fossils and describe the evolutionary history of the family. As a part of this, they would split the family into the two subfamilies ;Merialinae and Paroxyclaeninae.[2] It has been suggested by papers such as the 1993 publication by Clemens and Koenigswald that Paroxyclaenidae is the sister clade to Pantolestidae.[3]
Description
Skull of Paroxyclaenus sp.
Skull
The dentition of paroxyclaenids is generally similar to other earlier 'condylarths' though is also very specialized. Similar to other early 'condylarths', these animals had a total of four premolars and three molars. However, unlike these other groups, the posterior premolars were enlarged with them even being larger than the molars behind them. These molars would decrease in size posteriorly, making the first molar the largest in each part of the dentition. Along with this, the cheek teeth of paroxyclaenids were spaced. The enlarged canines and bunodonty of the molars were more similar to taxa such as Phenacodus. The main difference between the two subfamilies are the features of the fourth lower premolar. In Paroxyclaeninae, this tooth is larger and more molarized while the metaconid is absent in Merialinae. Another difference between the two subfamilies is the spacing between the paraconid and metaconid on the molars which are more widely spaced in Paroxyclaeninae than in Merialinae.[1]
Outside of the dentition, the skull of most paroxyclaenids isn't known with the two exceptions to this being Paroxyclaenus and Kopidodon. Both of these animals have decently robust skulls with wide snouts. The occipital regions of the skull were also well developed. The mandible of the genera possessed large infraorbital foramen which suggests that the snout most likely possessed a large amount of whiskers.[1]
Postcrania
Similar to a majority of the skull anatomy, the postcranial features of paroxyclaenids is poorly known from Paroxyclaenus and Kopidodon being the only two members that have preserved postcrania. From what is preserved, members of the family would have had generally robust bodies with short limbs and a long tail. The forelimbs were made up of a robust humerus in combination with a mobile elbow joint, both of which would have helped the animals climb. The hindlimbs would have been plantigrade with the ankle being flexible similarly to the elbow joint. Based on soft tissue preservation, Kopidodon would have had a bushy tail.[1]
Evolutionary history
The earliest records of Paroxyclaenidae are found in the early Ypresian in southern France with Merialus martinae having an estimated age of 55-56 mya. This genus, along with other members of Merialinae make up the earlier records. The other subfamily, Paroxyclaeninae, would later appear abruptly at 54-52 mya with this time lining up with a Mammal Dispersal Event that had been suggested to have taken place in Europe by more recent authors such as Hooker. A wide distribution of the family even in these earlier periods has been noted by authors and has lead to the suggestion that the family actually originated in the Paleocene rather than the Eocene. Even with this high diversity being present, it is rare to have more than one genus in a deposit. When more than one is present, the size of the genera differs by a large margin with two genera in the Paris Basin being 1.0 kilogram (2.2lb) and 1.5 kilograms (3.3lb).[1]
The older of the two subfamilies, Merialinae, would largely abruptly disappear in the late Ypressian, with previous authors suggesting that this was the youngest records of the subfamily.[1] However, in 2025, Floréal Solé and coauthors would describe the youngest member of not only the subfamily but the entire family as a whole with the genus, Welcommoides, dating to the lower Oligocene of Pakistan. Along with Welcommoides being the youngest member of the family, it would be the largest with an estimated mass of 3.990–4.476 kilograms (8.80–9.87lb).[4] In contrast to Merialinae, Paroxyclaeninae would largely diversify in the Paroxyclaeninae and would generally have more cases of contemporary genera than the other subfamily. The largest members of Paroxyclaeninae reached generally the same sizes as those within Merialinae. One of these larger members, Kopidodon, has been estimated to have a mass of from 2.8–3.0 kilograms (6.2–6.6lb) based on dentition and 3.0–5.0 kilograms (6.6–11.0lb) based on postcranial material.[1]
The youngest records of Paroxyclaenidae in Europe would be Paravulpavoides cooperi from the Bartonian of England with this being one of the smallest members of the family ( around 1 kilogram (2.2lb)). At the same time of the extinction of the family in Europe, a general overturn was taking place during the Bartonian–Priabonian boundary. This overturn would also affect other groups such as artiodactyls and palaeotheres that was most likely due to the increased aridity in Europe at the time.[1] This lack of aridification on the Indian subcontinent is suggested to be the reason that the group was able to last so much longer outside of Europe. Even with this being the case, another event has been suggested to be a cause for the extinction of the group, the Grande Coupure. This event took place at the Eocene-Oligocene boundary and is generally associated with the replacement of tropical forests with more temperate ones. This event had major effects on other arboreal groups in Europe, such as primates, with the only genus being present after the event being a mouse-sized omomyid. Due to the continued tropical conditions during this time, it has been suggested that Indian subcontinent acted as a refugium for these more arboreal groups.[4]
Paleoecology
Based on more complete fossils of genera like Kopidodon, members of Paroxyclaenidae would most likely have been arboreal herbivores with some members like Kopidodon being specifically frugivorous. This has been suggested due to not only the skeletal anatomy of the genus but also the presence of gut contents in Kopidodon. Though strange for more herbivorous animals, the large canines seen on taxa such as Paroxyclaenus could have been used to pull fruit off of branches.[1] David L Harrison would specifically make comparisons between the dentition of paroxyclaenids and those seen in fruit bats like Pteropus in his 2009 description of Paravulpavoides.[5] Based on a trend referred to as 'Kay's threshold', it is brought up that a majority of the protein in the animals' diet would have come from vegetation rather than insects. Though not a hard rule, primates under 0.5 kilograms (1.1lb) get most of their protein from insects while those over get it from vegetation; due to the gut contents of Kopidodon, it has been suggested that this threshold could have also been seen in Paroxyclaenidae.[1] The large premolars seen in members of the family has also been noted by multiple authors to potentially suggest a more durophagous diet than other similar groups of mammals.[4][5]
References
12345678910Solé, Floréal; Plateau, Olivia; Le Verger, Kévin; Phélizon, Alain (2019-02-08). "New paroxyclaenid mammals from the early Eocene of the Paris Basin (France) shed light on the origin and evolution of these endemic European cimolestans". Journal of Systematic Palaeontology. 17 (20): 1711–1743. doi:10.1080/14772019.2018.1551248. ISSN1477-2019.
↑Russell, Donald E.; Godinot, Marc (1988). "The paroxyclaenidae (Mammalia) and a new form from the early Eocene of Palette, France". Paläontologische Zeitschrift. 62 (3): 319–331. doi:10.1007/BF02989501.
↑Morlo, Michael; Gennell, Gregg F. (2003). "SMALL LIMNOCYONINES (HYAENODONTIDAE, MAMMALIA) FROM THE BRIDGERIAN MIDDLE EOCENE OF WYOMING: THINOCYON, PROLIMNOCYON, AND IRIDODON, NEW GENUS". CONTRIBUTIONS FROM THE MUSEUM OF PALEONTOLOGY. 31 (2).
12Harrison, David L. (2009). "A new genus of paroxyclaenid (Mammalia: Condylarthra: Paroxyclaenidae: Paravulpavoides) from the Upper Middle Eocene of Creechbarrow, Dorset, S. England". Cainozoic Research. 6 (12).
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