How Mesozoic Mammals Moved

A multivariate approach to infer locomotor modes in Mesozoic mammals

Authors:

Chen et al

Abstract:

Ecomorphological diversity of Mesozoic mammals was presumably constrained by selective pressures imposed by contemporary vertebrates. In accordance, Mesozoic mammals for a long time had been viewed as generalized, terrestrial, small-bodied forms with limited locomotor specializations. Recent discoveries of Mesozoic mammal skeletons with distinctive postcranial morphologies have challenged this hypothesis. However, ecomorphological analyses of these new postcrania have focused on a single taxon, a limited region of the skeleton, or have been largely qualitative.

For more comprehensive locomotor inference in Mesozoic mammals, we applied multivariate analyses to a morphometric data set of extant small-bodied mammals. We used 30 osteological indices derived from linear measurements of appendicular skeletons of 107 extant taxa that sample 15 orders and eight locomotor modes. Canonical variate analyses show that extant small-bodied mammals of different locomotor modes have detectable and predictable morphologies. The resulting morphospace occupation reveals a morphofunctional continuum that extends from terrestrial to scansorial, arboreal, and gliding modes, reflecting an increasingly slender postcranial skeleton with longer limb output levers adapted for speed and agility, and extends from terrestrial to semiaquatic/semifossorial and fossorial modes, reflecting an increasingly robust postcranial skeleton with shorter limb output levers adapted for powerful, propulsive strokes. We used this morphometric data set to predict locomotor mode in ten Mesozoic mammals within the Docodonta, Multituberculata, Eutriconodonta, “Symmetrodonta,” and Eutheria. Our results indicate that these fossil taxa represent five of eight locomotor modes used to classify extant taxa in this study, in some cases confirming and in other cases differing from prior ecomorphological assessments. Together with previous locomotor inferences of 19 additional taxa, these results show that by the Late Jurassic mammals had diversified into all but the saltatorial and active flight locomotor modes, and that this diversification was greatest in the Eutriconodonta and Multituberculata, although sampling of postcranial skeletons remains uneven across taxa and through time.

Agilodocodon: a Jurassic Arboreal Omnivorous Docodont From China

An arboreal docodont from the Jurassic and mammaliaform ecological diversification

Authors:

Meng et al

Abstract:

A new docodontan mammaliaform from the Middle Jurassic of China has skeletal features for climbing and dental characters indicative of an omnivorous diet that included plant sap. This fossil expands the range of known locomotor adaptations in docodontans to include climbing, in addition to digging and swimming. It further shows that some docodontans had a diet with a substantial herbivorous component, distinctive from the faunivorous diets previously reported in other members of this clade. This reveals a greater ecological diversity in an early mammaliaform clade at a more fundamental taxonomic level not only between major clades as previously thought.

Docofossor: A Mole-like Docodont Mammaliaform Hints at "Modern" Morphogenetic Mechanisms Predate Mammals

Evolutionary development in basal mammaliaforms as revealed by a docodontan

Authors:

Luo et al

Abstract:

A new Late Jurassic docodontan shows specializations for a subterranean lifestyle. It is similar to extant subterranean golden moles in having reduced digit segments as compared to the ancestral phalangeal pattern of mammaliaforms and extant mammals. The reduction of digit segments can occur in mammals by fusion of the proximal and intermediate phalangeal precursors, a developmental process for which a gene and signaling network have been characterized in mouse and human. Docodontans show a positional shift of thoracolumbar ribs, a developmental variation that is controlled by Hox9 and Myf5 genes in extant mammals. We argue that these morphogenetic mechanisms of modern mammals were operating before the rise of modern mammals, driving the morphological disparity in the earliest mammaliaform diversification.

An Unique Mammalian Assemblage From Early Cretaceous Siberia

Mammal remains from the Lower Cretaceous Bol'shoi Terekhtyul' locality in West Siberia, Russia

Authors:

Averianov et al

Abstract:

Four mammalian taxa have been identified based on upper molariform tooth and edentulous dentary fragments from the Bol'shoi Terekhtyul' locality of the Lower Cretaceous Ilek Formation (Krasnoyarsk Territory, Russia): Docodonta indet., Amphidontidae indet., Zhangheotheria indet. sp. A and sp. B. The docodontan from Bol'shoi Terekhtyul is smaller than Sibirotherium rossicum from the Ilek Formation at Shestakovo 1 locality (Kemerovo Province, Russia) and may belong to a distinct taxon. It is one of the younger docodontans in the fossil record. The mammal assemblage from the Bol'shoi Terekhtyul' locality is drastically different from the mammal assemblage of nearby localities of the Ilek Formation along Bol'shoi Kemchug River, which consists of Gobiconodontidae and Amphilestidae. The reason for this faunal discrepancy is unclear for the moment.

Docodon apoxys: A New Docodon Species From Oxfordian/Kimmeridgian/Tithonian Jurassic Colorado

A New Species of Docodon (Mammaliaformes: Docodonta) from the Upper Jurassic Morrison Formation and a Reassessment of Selected Craniodental Characters in Basal Mammaliaforms

Authors:

Rougier et al

Abstract:

We describe a new species of the basal mammaliaform Docodon, D. apoxys, sp. nov., represented by three nearly complete dentaries from the Upper Jurassic Morrison Formation of central Colorado. This species differs from other known species of Docodon in the relative heights of the principal molar cusps and in the size of the distal molars. In addition, we attribute a partial rostrum preserving much of the maxilla and most of the premaxilla, bearing a complete incisor and premolar count, to Docodon sp. These materials supply previously unknown morphology for this classic North American Jurassic taxon, and help establish a common morphology for Docodonta as a whole. While Docodon differs from the Portuguese docodont Haldanodon in the presence of only five upper incisors (none of which is entirely within the maxilla), an internarial bar is present in both taxa. The new specimens also unambiguously preserve a complete and undistorted angular process that differs conspicuously from the classical interpretation of the holotype of Docodon victor; this structure in docodonts closely resembles the angular process in australosphenidans and trechnotheres. We abandon the concept of a “pseudangular” process and we consider the angular process to be a homologous feature (where present) across Mammaliaformes.

Kimmeridgian Jurassic Postcranial Mammal Fossils From Portugal

Mammalian postcranial bones from the Late Jurassic of Portugal and their implications for forelimb evolution

Author:

Thomas Martin

Abstract:

Isolated bones of forelimb and pelvic girdle (two humeri, five ulnae, and an ilium) recovered from the Late Jurassic (Kimmeridgian) Guimarota coal mine in western central Portugal are attributed to the docodont Haldanodon exspectatus, dryolestoids, and a ?paulchoffatiid multituberculate. The larger of the two humeri is assigned to the dryolestid Dryolestes leiriensis based on size and shape. It clearly exhibits an incipient trochlea at the distal joint, suggesting that this derived character was well established among Late Jurassic dryolestidans, including Henkelotherium. Plesiomorphic characters are the prominent spherical radial condyle and the weakly developed ulnar condyle. An incipient medial keel is present in distal aspect of the humerus trochlea. The shallow olecranon fossa of the humerus corresponds to the small anconeal process of the radius. The smaller humerus with damaged distal joint is 50% smaller and cannot be assigned to a specific dryolestoid taxon. It is in the size range of the smaller Guimarota dryolestoids Krebsotherium, Drescheratherium, and Henkelotherium. One ulna differs from the dryolestoid ulnar shape by the nearly semicircular articular surface for the ulnar condyle of the humerus, the asymmetric olecranon with medial overhang, as well as the prominent anconeal process and is tentatively attributed to a paulchoffatiid multituberculate.