Therian Mammals Started an Ecomorphical Evolutionary Radiation PRIOR to the Extinction of the Dinosaurs

Therian mammals experience an ecomorphological radiation during the Late Cretaceous and selective extinction at the K–Pg boundary

Authors:

Grossnickle et al

Abstract:

It is often postulated that mammalian diversity was suppressed during the Mesozoic Era and increased rapidly after the Cretaceous–Palaeogene (K–Pg) extinction event. We test this hypothesis by examining macroevolutionary patterns in early therian mammals, the group that gave rise to modern placentals and marsupials. We assess morphological disparity and dietary trends using morphometric analyses of lower molars, and we evaluate generic level taxonomic diversity patterns using techniques that account for sampling biases. In contrast with the suppression hypothesis, our results suggest that an ecomorphological diversification of therians began 10–20 Myr prior to the K–Pg extinction event, led by disparate metatherians and Eurasian faunas. This diversification is concurrent with ecomorphological radiations of multituberculate mammals and flowering plants, suggesting that mammals as a whole benefitted from the ecological rise of angiosperms. In further contrast with the suppression hypothesis, therian disparity decreased immediately after the K–Pg boundary, probably due to selective extinction against ecological specialists and metatherians. However, taxonomic diversity trends appear to have been decoupled from disparity patterns, remaining low in the Cretaceous and substantially increasing immediately after the K–Pg extinction event. The conflicting diversity and disparity patterns suggest that earliest Palaeocene extinction survivors, especially eutherian dietary generalists, underwent rapid taxonomic diversification without considerable morphological diversification.

Did Placentals and Marsupials Diverge 170 Million Years ago Starting at the Aalenian/Bajocian Jurassic?

The Interrelationships of Placental Mammals and the Limits of Phylogenetic Inference

Authors:

Tarver et al

Abstract:

Placental mammals comprise three principal clades: Afrotheria (e.g. elephants and tenrecs), Xenarthra (e.g. armadillos and sloths) and Boreoeutheria (all other placental mammals), the relationships among which are the subject of controversy and a touchstone for debate on the limits of phylogenetic inference. Previous analyses have found support for all three hypotheses, leading some to conclude that this phylogenetic problem might be impossible to resolve, due to the compounded effects of Incomplete Lineage Sorting (ILS) and a rapid radiation. Here we show, using a genome scale nucleotide dataset, microRNAs, and the reanalysis of the three largest previously published amino-acid datasets, that the root of Placentalia lies between Atlantogenata and Boreoeutheria. Although we found evidence for ILS in early placental evolution, we are able to reject previous conclusions that the placental root is a hard polytomy that cannot be resolved. Reanalyses of previous datasets recover Atlantogenata + Boreoeutheria and show that contradictory results are a consequence of poorly fitting evolutionary models; instead, when the evolutionary process is better-modelled, all datasets converge on Atlantogenata. Our Bayesian molecular clock analysis estimates that marsupials diverged from placentals 157-170 Ma, crown Placentalia diverged 86-100 Ma, and crown Atlantogenata diverged 84-97 Ma. Our results are compatible with placental diversification being driven by dispersal rather than vicariance mechanisms, postdating early phases in the protracted opening of the Atlantic Ocean.

Palaeoxonodon: a Stem Therian Mammal From Bathonian Jurassic Isle of Skye (or Lumper Attack!)

A lower jaw of Palaeoxonodon from the Middle Jurassic of the Isle of Skye, Scotland, sheds new light on the diversity of British stem therians

Authors:

Close et al

Abstract:

The Middle Jurassic was a key interval of mammalian evolutionary history that witnessed the diversification of the therian stem group. Great Britain has yielded a significant record of mammalian fossils from this interval, represented by numerous isolated jaws and teeth from the Bathonian of Oxfordshire and the Isle of Skye. This record captures a key period in early cladotherian evolution, with amphitheriids, peramurans and ‘stem zatherians’ displaying intermediate talonid morphologies that document the evolutionary assembly of tribosphenic molars. We present a mandible with near-complete dentition from the late Bathonian (c. 167.4–166.5 Ma) Kilmaluag Formation, near Elgol, Skye, representing the amphitheriid Palaeoxonodon ooliticus, previously known only from isolated teeth. The specimen sheds new light on the taxonomic diversity of British Middle Jurassic stem therians, as the morphological variation within the preserved tooth row encompasses that previously ascribed to three distinct species within two genera: Palaeoxonodon ooliticus, P. freemani and Kennetheridium leesi. Thus, both P. freemani and K. leesi are subjective junior synonyms of P. ooliticus. The dental formula of P. ooliticus (i4:c1:p5:m5) is intermediate between the primitively larger postcanine count (p5:m6–7) of Amphitherium and the reduced number in peramurans and tribosphenidans (p5:m3). Phylogenetic analyses of P. ooliticus generally confirm a close affinity with Amphitherium, but highlight the lack of strong empirical support for hypothesized patterns of divergences among early cladotherians.

 pop sci write up.

The Phylogenetics of 5 Mammals From Aptian/Albian Cretaceous Texas and Oklahoma

Taxonomic revision of tribosphenic mammals from the Lower Cretaceous Antlers Formation of Texas and Oklahoma, USA.

Author:

Averianov

Abstract:

There are five taxa of tribosphenic mammals in the Early Cretaceous Antlers Formation of Texas and Oklahoma, USA: a basal stem therian (Kermackia texana), stem therians near the eutherian-metatherian dichotomy (Holoclemensia texana and Pappotherium pattersoni), and stem marsupials (Atokatheridium boreni and Oklatheridium szalayi). K. texana has a primitive therian postcanine formula with three molars, replacement of p5, M3 with low protocone and no conules, lower molars with a large trigonid angle, oblique protocristid, paraconid smaller than metaconid (except m3), strong distal metacristid, narrow talonid, small talonid basin, and small entoconid (absent on m3). H. texana also has replacement in the fifth premolar locus and three molars. It is more derived in having a larger protoconal region with higher protocone and conules present, lack of distal metacristid, smaller trigonid angle, transverse protocristid, and wide talonid with larger talonid basin. It is similar to Eutheria by having M1 with reduced ectoflexus, semimolariform p5, and low trigonid angle with transverse protocristid. Holoclemensia cannot be referred to Eutheria because of the lack of the second rank postvallum/prevallid shear and unwinged conules. P. pattersoni (=Slaughteria eruptens) has replacement in the fifth premolar position, premolariform p5, and three molars (symplesiomorphies for Theria). It is more derived than Holoclemensia in having a wider and shorter talonid. Pappotherium is similar to Eutheria in having a low trigonid angle, transverse protocristid, and the cristid obliqua labial to the protocristid notch. It cannot be attributed to the Eutheria because of the narrow protoconal region, low protocone, small conules lacking internal cristae, postprotocrista not extending labially past the metacone base, and a small talonid basin. Atokatheridium boreni and Oklatheridium szalayi (=O. minax, syn. nov.) have four molars and emphasis on the postvallum/prevallid shear (large metacone on M2, strong postmetacrista, paraconid higher than metaconid). These taxa cannot be attributed to the Deltatheroida because of large protoconal region with winged conules. Oklatheridium is further different from the Deltatheroida in having a wider talonid and better developed entoconid.

Labes: a Maastrichtian Cretaceous Eutherian Mammal From France

A Late Cretaceous eutherian mammal from southwestern France

Authors:

Martin et al

Abstract:

The first jaw remain of the Late Cretaceous eutherian Labes is a mandibular fragment from the Late Cretaceous (Maastrichtian) locality Massecaps in Departement Herault in southwestern France. The mandible holds three double-rooted molars (m1–3) with broken trigonids and preserved talonids of which m1 apparently was the largest. The entoconid is slightly approximated to the hypoconulid (69–74 % of the distance between the hypoconulid and hypoconid). The talonid basin has a rectangular shape, and the molar roots are not fused. An upper molar (M1) from the same locality bears three roots; it is characterized by a strong ectoflexus and a well-developed parastylar lobe. The largest cusp is the conical pyramidal paracone, followed by the tetrahedral shaped, very low protocone, and the small, distally placed metacone. Previously, Labes was only known by a few isolated lower molars from the late Campanian of Champ-Garimond (France) and the Maastrichtian of El Molino near Quintanilla del Coco (northwestern Spain).

Stem Therian Amphibetulimus From Bathonian Jurassic Siberia

Stem therian mammal Amphibetulimus from the Middle Jurassic of Siberia

Authors:

Alexander Averianov, Thomas Martin, Alexey Lopatin and Sergei Krasnolutskii

Abstract:

Amphibetulimus krasnolutskii is known from the Middle Jurassic (Bathonian) Itat Formation of Krasnoyarsk Territory, West Siberia, Russia, by several edentulous and three dentigerous dental fragments, preserving p1, antepenultimate, and ultimate lower molars, and by an upper molar. It is unique among stem therians by widely open trigonids on the posterior lower molars, paraconids that are higher than the metaconids and have keeled mesiolingual vertical crests, pronounced unilateral hypsodonty of the lower molars and correlated unequal alveolar borders of the dentary ramus, and a linear Meckelian groove that is not connected to the mandibular foramen and extends along the pterygoid ridge. Amphibetulimus differs from more derived stem therians by a simple unicuspid talonid without an incipient talonid basin and a distinct labial cingulum on the upper molars. The lack of an ectotympanic facet and the long linear Meckelian groove extending onto the pterygoid ridge suggest that Amphibetulimus had a derived state of the transitional mammalian middle ear, where the ear ossicles were connected to the dentary not by a thick “ossified” Meckelian cartilage, but by a thin Meckelian cartilage, as in prenatal and early postnatal stages of some modern therians.

Biomechanically Comparing Therian Sabretooths

Comparative Biomechanical Modeling of Metatherian and Placental Saber-Tooths: A Different Kind of Bite for an Extreme Pouched Predator

Authors:

1. Stephen Wroe (a,b)
2. Uphar Chamoli (b,c)
3. William C. H. Parr (b)
4. Philip Clausen (a)
5. Ryan Ridgely (d)
6. Lawrence Witmer (d)

Affiliations:

a. School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW, Australia

b. School of Engineering, University of Newcastle, Callaghan, NSW, Australia

c. St. George Clinical School, University of New South Wales, Sydney, NSW, Australia

d. Department of Biomedical Sciences, Heritage College of Osteopathic Medicine, Ohio University, Athens, Ohio, United States of America

Abstract:

Questions surrounding the dramatic morphology of saber-tooths, and the presumably deadly purpose to which it was put, have long excited scholarly and popular attention. Among saber-toothed species, the iconic North American placental, Smilodon fatalis, and the bizarre South American sparassodont, Thylacosmilus atrox, represent extreme forms commonly forwarded as examples of convergent evolution. For S. fatalis, some consensus has been reached on the question of killing behaviour, with most researchers accepting the canine-shear bite hypothesis, wherein both head-depressing and jaw closing musculatures played a role in delivery of the fatal bite. However, whether, or to what degree, T. atrox may have applied a similar approach remains an open question. Here we apply a three-dimensional computational approach to examine convergence in mechanical performance between the two species. We find that, in many respects, the placental S. fatalis (a true felid) was more similar to the metatherian T. atrox than to a conical-toothed cat. In modeling of both saber-tooths we found that jaw-adductor-driven bite forces were low, but that simulations invoking neck musculature revealed less cranio-mandibular stress than in a conical-toothed cat. However, our study also revealed differences between the two saber-tooths likely reflected in the modus operandi of the kill. Jaw-adductor-driven bite forces were extremely weak in T. atrox, and its skull was even better-adapted to resist stress induced by head-depressors. Considered together with the fact that the center of the arc described by the canines was closer to the jaw-joint in Smilodon, our results are consistent with both jaw-closing and neck musculature playing a role in prey dispatch for the placental, as has been previously suggested. However, for T. atrox, we conclude that the jaw-adductors probably played no major part in the killing bite. We propose that the metatherian presents a more complete commitment to the already extreme saber-tooth ‘lifestyle’.

Herds Overhead: Nimbadon lavarackorum (Diprotodontidae), Heavyweight Marsupial Herbivores in the Miocene Forests of Australia

Herds Overhead: Nimbadon lavarackorum (Diprotodontidae), Heavyweight Marsupial Herbivores in the Miocene Forests of Australia

Authors:

1. Karen H. Black (a,*)

2. Aaron B. Camens (b,*)

3. Michael Archer (a)

4. Suzanne J. Hand (a)

Affiliations:

a. School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, New South Wales, Australia,

b. Department of Earth and Environmental Sciences, University of Adelaide, Adelaide, South Australia, Australia

Abstract:

The marsupial family Diprotodontidae (Diprotodontia, Vombatiformes) is a group of extinct large-bodied (60–2500 kg) wombat-like herbivores that were common and geographically widespread in Cenozoic fossil deposits of Australia and New Guinea. Typically they are regarded to be gregarious, terrestrial quadrupeds and have been likened in body form among placental groups to sheep, rhinoceros and hippopotami. Arguably, one of the best represented species is the zygomaturine diprotodontid Nimbadon lavarackorum which is known from exceptionally well-preserved cranial and postcranial material from the middle Miocene cave deposit AL90, in the Riversleigh World Heritage Area, northwestern Queensland. Here we describe and functionally analyse the appendicular skeleton of Nimbadon lavarackorum and reveal a far more unique lifestyle for this plesiomorphic and smallest of diprotodontids. Striking similarities are evident between the skeleton of Nimbadon and that of the extant arboreal koala Phascolarctos cinereus, including the powerfully built forelimbs, highly mobile shoulder and elbow joints, proportionately large manus and pes (both with a semi-opposable digit I) and exceedingly large, recurved and laterally compressed claws. Combined with the unique (among australidelphians) proportionately shortened hindlimbs of Nimbadon, these features suggest adept climbing ability, probable suspensory behaviour, and an arboreal lifestyle. At approximately 70 kg, Nimbadon is the largest herbivorous mammal to have occupied the forest canopies of Australia - an ecological niche that is no longer occupied in any Australian ecosystem and one that further expands the already significant niche diversity displayed by marsupials during the Cenozoic.