skip to main content


Title: The origin of placental mammal life histories
After the end-Cretaceous extinction, placental mammals quickly diversified, occupied key ecological niches and increased in size, but this last was not true of other therians. The uniquely extended gestation of placental young may have factored into their success and size increase, but reproduction style in early placentals remains unknown. Here we present the earliest record of a placental life history using palaeohistology and geochemistry, in a 62 million-year-old pantodont, the clade including the first mammals to achieve truly large body sizes. We extend the application of dental trace element mapping9,10 by 60 million years, identifying chemical markers of birth and weaning, and calibrate these to a daily record of growth in the dentition. A long gestation (approximately 7 months), rapid dental development and short suckling interval (approximately 30–75 days) show that Pantolambda bathmodon was highly precocial, unlike non-placental mammals and known Mesozoic precursors. These results demonstrate that P. bathmodon reproduced like a placental and lived at a fast pace for its body size. Assuming that P. bathmodon reflects close placental relatives, our findings suggest that the ability to produce well-developed, precocial young was established early in placental evolution, and that larger neonate sizes were a possible mechanism for rapid size increase in early placentals.  more » « less
Award ID(s):
1654949
NSF-PAR ID:
10454110
Author(s) / Creator(s):
; ; ; ; ; ; ; ; ;
Date Published:
Journal Name:
Nature
Volume:
610
ISSN:
0028-0836
Page Range / eLocation ID:
107-111
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. null (Ed.)
    The rise of mammals after the extinction of the dinosaurs remains one of the most enigmatic intervals in the evolution of mammals. A relatively sparse Paleocene fossil record and confusing interrelationships between taxa means that little is known of the evolution, ecology, and biology of these animals. As a result, the life history of these organisms is completely unstudied, despite likely playing a key role in the ability of these clades to rapidly proliferate and increase in body size in recovering ecosystems. However, intensive collection efforts in the San Juan Basin of New Mexico in the last decade have drastically improved the record of many Paleocene mammals, and offer the first opportunity to address questions about the life history of these animals. Here, we present preliminary results of an in-depth paleohistological analysis of Pantolambda bathmodon, an early, possibly gregarious pantodont, using an ontogenetic series of individuals. Pantodonts were bizarre, herbivorous eutherians of unknown phylogenetic affinity, and were among the first mammal lineages to reach large body sizes in the Paleocene. In examining both dental and skeletal records of growth from the same individuals, including a juvenile still bearing deciduous teeth, our study is among the most comprehensive paleohistological analyses of any fossil mammal. This intensive approach allows for unprecedented insights into the life history of this species. Neonatal lines in the teeth indicate that the deciduous premolars and the first upper molar were erupted prior to birth, similar to precocious, nidifugous mammals today. Daily incremental lines in the enamel and dentine suggest rapid crown formation times (~45–70 days) and a gestation period of at least 15 weeks. A stress line in the postcranial bones, recording an anomalous decrease in growth towards the end of this individual’s life, may represent the weaning event. In the absence of geochemical evidence, it is unclear which of two stress lines in the teeth corresponds to this event, but these lines occur roughly one and two months after birth, respectively. The weanling perished approximately 2.5 months after birth, weighing about 17 kg. An adult individual exhibiting severe wear on the dentition allows us to estimate maximum longevity in Pantolambda bathmodon at about 7 years. In comparison with life history data on living mammals from the PanTheria dataset, Pantolambda bathmodon had a gestation length and weaning duration below average for a placental of its adult body size (42 kg), but within the range of known variation. However, its lifespan was exceptionally short, falling outside the bounds of comparable living mammals. Together, these lines of evidence suggest a relatively rapid pace of life in Pantolambda bathmodon, despite its relatively large body size. Ongoing sampling of more individuals and geochemical analyses should allow for estimation of time to sexual maturity and help to confirm the identity of the weaning line, completing our picture of the life history of this pioneering species. 
    more » « less
  2. The rise of mammals after the extinction of the dinosaurs remains one of the most enigmatic intervals in the evolution of mammals. A relatively sparse Paleocene fossil record and confusing relationships between taxa means that little is known of the evolution, ecology, or biology of these animals. Accordingly, the life history of these organisms remains unstudied, despite likely playing a key role in the rapid proliferation and body size increase of these clades in recovering ecosystems. Here, we present results of an in-depth paleohistological analysis of Pantolambda bathmodon, an early, possibly gregarious pantodont, using a new ontogenetic series of specimens. Pantodonts were bizarre, herbivorous eutherians of unknown phylogenetic affinity, and were among the first mammal lineages to reach large body sizes in the Paleocene. In examining both dental and skeletal records of growth from the same individuals, including a juvenile still bearing deciduous teeth, our study is among the most comprehensive paleohistological analyses of any fossil mammal, allowing for unprecedented insights into the life history of this species. Neonatal lines in the teeth indicate that the deciduous premolars and the first upper molar erupted prior to birth, similar to precocious, nidifugous mammals today. Daily incremental lines in the enamel and dentine suggest rapid crown formation times (~70–180 days) and a gestation period of at least 20 weeks. A stress line in the teeth and postcranial bones, recording an anomalous decrease in growth towards the end of this individual’s life, may represent weaning. The weanling perished approximately 2.5 months after birth, weighing about 17 kg. Adult individuals exhibiting severe wear on the dentition allow us to estimate maximum longevity in Pantolambda bathmodon at about 7 years. In comparison with living mammals, Pantolambda bathmodon had gestation and weaning periods below average for a placental of its adult body size (42 kg), but within the range of known variation. However, its lifespan was exceptionally short, falling outside the bounds of comparable living mammals. Together, these lines of evidence suggest a rapid pace of life in Pantolambda bathmodon, despite its relatively large body size. Ongoing sampling of more individuals and geochemical analyses should allow for estimation of time to sexual maturity and help to confirm the identity of the weaning line, completing our picture of the life history of this pioneering species. 
    more » « less
  3. The end-Cretaceous mass extinction, 66 million years ago, profoundly reshaped the biodiversity of our planet. After likely originating in the Cretaceous, placental mammals (species giving live birth to well-developed young) survived the extinction and quickly diversified in the ensuing Paleocene. Compared to Mesozoic species, extant placentals have advanced neurosensory abilities, enabled by a proportionally large brain with an expanded neocortex. This brain construction was acquired by the Eocene, but its origins, and how its evolution relates to extinction survivorship and recovery, are unclear, because little is known about the neurosensory systems of Paleocene species. We used high-resolution computed tomography (CT) scanning to build digital brain models in 29 extinct placentals (including 23 from the Paleocene). We added these to data from the literature to construct a database of 98 taxa, from the Jurassic to the Eocene, which we assessed in a phylogenetic context. We find that the Phylogenetic Encephalization Quotient (PEQ), a measure of relative brain size, increased in the Cretaceous along branches leading to Placentalia, but then decreased in Paleocene clades (taeniodonts,phenacodontids, pantodonts, periptychids, and arctocyonids). Later, during the Eocene, the PEQ increased independently in all crown groups (e.g., euarchontoglirans and laurasiatherians). The Paleocene decline in PEQ was driven by body mass increasing much more rapidly after the extinction than brain volume. The neocortex remained small, relative to the rest of the brain, in Paleocene taxa and expanded independently in Eocene crown groups. The relative size of the olfactory bulbs, however, remained relatively stable over time, except for a major decrease in Euarchontoglires and some Eocene artiodactyls, while the petrosal lobules (associated with eye movement coordination) decreased in size in Laurasiatheria but increased in Euarchontoglires. Our results indicate that an enlarged, modern-style brain was not instrumental to the survival of placental mammal ancestors at the end-Cretaceous, nor to their radiation in the Paleocene. Instead, opening of new ecological niches post-extinction promoted the diversification of larger body sizes, while brain and neocortex sizes lagged behind. The independent increase in PEQ in Eocene crown groups is related to the expansion of the neocortex, possibly a response to ecological specialization as environments changed, long after the extinction. 
    more » « less
  4. Mammals survived the Chicxulub impact sixty-six million years ago and diversified into a wide variety of new ecological niches left by non-avian dinosaurs. Pantodonts, an enigmatic group, quickly achieved hefty postextinction body sizes to occupy large herbivore niches. We describe the first juvenile specimen of the Paleocene pantodont Pantolambda bathmodon (NMMNH P-27844) consisting of a partial skeleton including parts of the skull, a deciduous upper premolar series, nearly complete forelimbs, and elements of the carpus and hind limb. P-27844 is from the Torrejonian (~62.3 Ma) Tsosie Member of the Nacimiento Formation. P-27844 has the first deciduous teeth known for Pantolambda. dP2 and dP4 are submolariform with a triangular cross-section and a less developed protocone than adults. dP5 is molariform with a large paracone and metacone connected by wing-like cristae to form the w-shaped ectoloph typical of this genus’ molars. dP5 also has more pronounced conules than the molars. This molarization style of the ultimate premolar is seen across Pantodonta including in Alcidedorbignya inopinata, Barylambda faberi, and Coryphodon sp. The postcranial morphology of P-27844 is generally concordant with that of adults. This correspondence manifests particularly clearly in the forelimbs. The distal humerus exhibits the base of a posterolaterally directed epicondylar crest which likely anchored the anconeus and the extensor carpi radialis muscles, a deep radial fossa, and an open entepicondylar foramen. The ulna shows a welldeveloped anconeal process, a pronounced biceps and brachialis fossa, and a shallower groove to accommodate the abductor pollicis longus. The radius possesses a shallow pronator crest that originates near its distal end and extends about two-thirds of the way along the shaft. Interestingly, in contrast to adults, the radial shaft is straight rather than having moderate sigmoidal curvature and has not undergone epiphyseal fusion. Altogether, these osteological features illustrate that, even at its early ontogenetic stage, P-27844 possessed robust forelimb musculature. Using Developmental Mass Extrapolation from long bone measurements, P-27844’s body mass is estimated to be ~17 kg at time of death (~40% of adult body mass). Paleohistological analyses demonstrate the animal experienced a rapid pace of life for its body size and died ~2.5 months after birth. This specimen gives unprecedented insight into the early life history of Pantolambda. 
    more » « less
  5. ABSTRACT

    Mammals underwent a profound diversification after the end‐Cretaceous mass extinction, with placentals rapidly expanding in body size and diversity to fill new niches vacated by dinosaurs. Little is known, however, about the brains and senses of these earliest placentals, and how neurosensory features may have promoted their survival and diversification. We here use computed tomography (CT) to describe the brain, inner ear, sinuses, and endocranial nerves and vessels ofCarsioptychus coarctatus, a periptychid “condylarth” that was among the first placentals to blossom during the few million years after the extinction, in the Paleocene.Carsioptychushas a generally primitive brain and inner ear that is similar to the inferred ancestral eutherian/placental condition. Notable “primitive” features include the large, anteriorly expanded, and conjoined olfactory bulbs, proportionally small neocortex, lissencephalic cerebrum, and large hindbrain compared to the cerebrum. An encephalization quotient (EQ) cannot be confidently calculated because of specimen crushing but was likely very small, and comparisons with other extinct placentals reveal that many Paleocene “archaic” mammals had EQ values below the hallmark threshold of modern placentals but within the zone of nonmammalian cynodonts, indicative of small brains and low intelligence.Carsioptychusdid, however, have a “conventional” hearing range for a placental, but was not particularly agile, with semicircular canal dimensions similar to modern pigs. This information fleshes out the biology of a keystone Paleocene “archaic” placental, but more comparative work is needed to test hypotheses of how neurosensory evolution was related to the placental radiation. Anat Rec, 302:306–324, 2019. © 2018 Wiley Periodicals, Inc.

     
    more » « less