Palaeolagus spp.

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Palaeolagus spp.


Palaeolagus haydeni. © @ Nobu Tamura->.

Temporal range: Paleogene; Late Eocene to Early Miocene; Priabonian to end Aquitanian (Early Chadronian to end Arikareean); 37.2-20.43 Ma[1][2]

Scientific classification:

Life
Domain: Eukaryota
(unranked): Unikonta
(unranked): Opisthokonta
(unranked): Holozoa
(unranked): Filozoa
Kingdom: Animalia
Subkingdom: Eumetazoa
(unranked): Bilateria
Clade: Nephrozoa
Superphylum: Deuterostomia
Phylum: Chordata
Clade: Cephalochordata
Clade: Craniata
Subphylum: Vertebrata
Infraphylum: Gnathostomata
Clade: Eugnathostomata
Clade: Teleostomi
Superclass: Tetrapoda
Clade: Reptiliomorpha
Clade: Amniota
Clade: Synapsida
Clade: Eupelycosauria
Clade: Sphenacodontia
Clade: Sphenacodontoidea
Order: Therapsida
Clade: Eutheriodonta
Suborder: Cynodontia
Clade: Epicynodontia
Infraorder: Eucynodontia
Parvorder: Probainognathia
Superfamily: Chiniquodontoidea
Clade: Prozostrodontia
Clade: Mammaliaformes
Class: Mammalia
Clade: Holotheria
Superlegion: Trechnotheria
Legion: Cladotheria
Sublegion: Zatheria
Infralegion: Tribosphenida
Subclass: Theria
Clade: Eutheria
Infraclass: Placentalia
Subcohort: Exafroplacentalia
Magnorder: Boreoeutheria
Superorder: Euarchontoglires
(unranked): Glires
Order: Lagomorpha
Genus: †Palaeolagus
Species: †P. burkei
†P. haydeni
†P. hemirhizis
†P. hypsodus
†P. intermedius
†P. philoi
†P. primus
†P. temnodon

Palaeolagus ('ancient hare') was an extinct genus of stem-lagomorph that lived in North America during the Eocene, Oligocene, and early Miocene epochs.[1][2]

Description:
Superficially, Palaeolagus looked similar to modern rabbits. Upon closer inspection, however, Palaeolagus appears to have lacked the postcranial specializations modern rabbits have for saltatorial locomotion.[3] It also had an akinetic skull, unlike the kinetic skulls modern rabbits possess (thought to be an adaptation for shock absorption as they land on the ground).[4] As such, Palaeolagus was a non-cursorial, non-saltatorial genus that still scampered (although, it appears to show the early signs of evolving saltatorial locomotion).[3][4]

Diet:
Palaeolagus possessed hypsodont dentition.[1] This suggests it had a diet and/or feeding habits that placed a substantial amount of wear on its teeth.


A fossil of P. haydeni.

Phylogenetic placement:
Micro-CT scans of the skull of P. haydeni have revealed that its skull had several leporid-like and ochotonid-like characters in its skull. Out of 46 characters, it shared 21 with leporids and 13 with ochotonids. It possessed 7 characters shared in common with both families, but also possessed 5 characters with a morphology unlike any of the crown lagomorphs. This lends support to previous placements of Palaeolagus as a stem-lagomorph.[5]

References:

[1] Graham, A. (1999). Late Cretaceous and Cenozoic history of North American vegetation: north of Mexico. Oxford University Press on Demand.
[2] Paleobiology Database – Palaeolagus
[3] Scott, W. B., Jepsen, G. L., & Wood, A. E. (1940). The Mammalian Fauna of the White River Oligocene: Part III. Lagomorpha. Transactions of the American Philosophical Society, 28(3), 271-362.
[4] Bramble, D. M. (1989). Cranial specialization and locomotor habit in the Lagomorpha. American zoologist, 29(1), 303-317.
[5] Wolniewicz AS and Fostowicz-Frelik Ł (2021) CT-Informed Skull Osteology of Palaeolagus haydeni (Mammalia: Lagomorpha) and Its Bearing on the Reconstruction of the Early Lagomorph Body Plan. Front. Ecol. Evol. 9:634757. doi: 10.3389/fevo.2021.634757

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CT-Informed Skull Osteology of Palaeolagus haydeni (Mammalia: Lagomorpha) and Its Bearing on the Reconstruction of the Early Lagomorph Body Plan

Lagomorpha is a clade of herbivorous mammals nested within Euarchontoglires, one of the major placental groups represented today. It comprises two extant families with markedly different body plans: the long-eared and long-limbed Leporidae (hares and rabbits) and the short-eared and short-limbed Ochotonidae (pikas). These two lagomorph lineages diverged probably during the latest Eocene/early Oligocene, but it is unclear whether the last common ancestor of crown lagomorphs was more leporid- or more ochotonid-like in morphology. Palaeolagus, an early lagomorph dominant in western North America from the late Eocene to Oligocene is of particular importance for addressing this controversy. Here, we present new and comprehensive data on the cranial anatomy of Palaeolagus haydeni, the type species for the genus, based on micro-computed tomography (μCT). Our μCT data allow us to confirm, revise and score for the very first time the states of several leporid-like and ochotonid-like characters in the skull of Palaeolagus. This mixed cranial architecture differentiates Palaeolagus from the crown groups of Lagomorpha and supports its phylogenetic status as a stem taxon.

Ever since Dice (1929), Palaeolagus was recognized as belonging to Leporidae on the basis of the dentition, mandible, and muzzle morphology, the parts most frequently preserved in the fossil record (Dice, 1933; Wood, 1940; Dawson, 1958). However, recent phylogenetic studies of lagomorphs and Glires have, in general, supported the position of Palaeolagus as a stem lagomorph (Asher et al., 2005; Wible, 2007; Fostowicz-Frelik and Meng, 2013; Lopatin and Averianov, 2020).

Thus, the current study aimed at the reappraisal of the cranial morphology of P. haydeni in order to provide new morphological data as a base for deciding between the two phylogenetic hypotheses. Micro-CT scanning allowed us to study the internal cranial structures and scrutinize the whole skull, bone by bone, comparing it with the crown taxa in detail, using Wible’s (2007) character matrix as the basis for our comparisons. Our study of Palaeolagus revealed that its cranial morphology shares characters not only with extant leporids but also with Ochotona (Table 1). Out of 46 characters analyzed by us, in 21 Palaeolagus showed features characteristic of extant Leporidae, in 13 the features characteristic of Ochotonidae (represented by Ochotona and Prolagus) and in seven, shared with both clades. However, in five characters Palaeolagus displayed morphology unlike any of the crown groups of Lagomorpha. These were: the morphology of the orbital process of maxilla, which is elongated in Palaeolagus, the absence of the zygomatico-orbital canal, a broader posterior squamosal, the absence of piriform fenestra, and the presence of two mental foramina (Table 1). This last combination of features can be viewed as plesiomorphic for all Glires, because Palaeolagus shares them with Rhombomylus (see Meng et al., 2003; Wible, 2007), a basal gliroid mammal which is neither a lagomorph nor a rodent (Asher et al., 2005).

The described cranial characters of Palaeolagus are not characteristic of only one of the crown clades; thus, this lagomorph is better considered as a stem lagomorph. Its apparent mosaic morphology has at least two important implications. First, the fact that the early lagomorph Palaeolagus, which does not have an immediate relationship to either of the crown clades, exhibits a mixture of morphological characters typically associated either with ochotonids or leporids, indicates its independent position, already suggested by Fostowicz-Frelik and Meng (2013). Second, it allows us to assume that some of the cranial characters observed in extant taxa and regarded thus far as synapomorphies for the recent clades are most likely plesiomorphic.

The Palaeolagus lineage most probably originated in the late middle Eocene (see Fostowicz-Frelik and Tabrum, 2009), and it was already well represented in the early late Eocene (Dawson, 2008). Therefore, the skull of Palaeolagus haydeni is a very good reference point for advanced stem lagomorphs, which most probably gave rise to Leporidae in North America (see Fostowicz-Frelik, 2013). However, to reconstruct the ancestral cranial morphotype of Ochotonidae more data is needed from the Eocene Asian taxa, of which our knowledge is at present very fragmentary.

www.frontiersin.org/articles/10.3389/fevo.2021.634757/full

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Some paleoart of Palaeolagus being chased by Dinictis. Art by Anita Burbeck->.