Mammal speciation before and after Cretaceous–Paleogene boundary + diprotodont incisors + animal body volume estimation.

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Ben Creisler

May 30, 2024, 3:34:54 PMMay 30
Ben Creisler

Recent papers:

Ignacio Quintero, Nicolas Lartillot and Hélène Morlon (2024)
Imbalanced speciation pulses sustain the radiation of mammals
Science 384(6699): 1007-1012
doi: 10.1126/science.adj2793

Editor’s summary

Mammals have drawn particular research attention because of their huge level of morphological variation. Until recently, most believed that the dinosaur decline at the end of the Cretaceous opened niches for mammals to fill, resulting in a burst of speciation. Quintero et al. developed a model that integrated phylogenetics and fossils to estimate diversification rates across all mammalian lineages and concluded that mammalian speciation rates were high well before the Cretaceous–Paleogene boundary. Rather than leading to a burst of diversification, the major environmental changes during the time filtered out more slowly speciating mammalian lineages, leading to domination of the group by those that speciated more rapidly. —Sacha Vignieri


The evolutionary histories of major clades, including mammals, often comprise changes in their diversification dynamics, but how these changes occur remains debated. We combined comprehensive phylogenetic and fossil information in a new “birth-death diffusion” model that provides a detailed characterization of variation in diversification rates in mammals. We found an early rising and sustained diversification scenario, wherein speciation rates increased before and during the Cretaceous-Paleogene (K-Pg) boundary. The K-Pg mass extinction event filtered out more slowly speciating lineages and was followed by a subsequent slowing in speciation rates rather than rebounds. These dynamics arose from an imbalanced speciation process, with separate lineages giving rise to many, less speciation-prone descendants. Diversity seems to have been brought about by these isolated, fast-speciating lineages, rather than by a few punctuated innovations.


Free pdf:

William M.G. Parker, Justin W. Adams, Eliza J. Campbell, Graeme Coulson, Gordon D. Sanson & Alistair R. Evans (2024)
Evergrowing incisors of diprotodont marsupials record age and life history
Archives of Oral Biology 106018


Analysis of incisor growth and wear in diprotodont marsupials.
Discovery that measurement of macropodid incisors informs age and sex.
Histology also finds secondary dentine and cementum inform age.
Generalisability is illustrated using dentally reduced Tarsipes rostratus.
Findings have applications in population ecology and palaeontology.



Tooth growth and wear are commonly used tools for determining the age of mammals. The most speciose order of marsupials, Diprotodontia, is characterised by a pair of procumbent incisors within the lower jaw. This study examines the growth and wear of these incisors to understand their relationship with age and sex.


Measurements of mandibular incisor crown and root length were made for two sister species of macropodid (kangaroos and wallabies); Macropus giganteus and Macropus fuliginosus. Histological analysis examined patterns of dentine and cementum deposition within these teeth. Broader generalisability within Diprotodontia was tested using dentally reduced Tarsipes rostratus – a species disparate in body size and incisor function to the studied macropodids.


In the macropodid sample it is demonstrated that the hypsodont nature of these incisors makes measurements of their growth (root length) and wear (crown length) accurate indicators of age and sex. Model fitting finds that root growth proceeds according to a logarithmic function across the lifespan, while crown wear follows a pattern of exponential reduction for both macropodid species. Histological results find that secondary dentine deposition and cementum layering are further indicators of age. Incisor measurements are shown to correlate with age in the sample of T. rostratus.


The diprotodontian incisor is a useful tool for examining chronological age and sex, both morphologically and microstructurally. This finding has implications for population ecology, palaeontology and marsupial evolution.


Free pdf:

Ruizhe Jackevan Zhao (2024)
Estimating body volumes and surface areas of animals from cross-sections.
PeerJ 12:e17479


Body mass and surface area are among the most important biological properties, but such information is lacking for some extant organisms and most extinct species. Numerous methods have been developed for body size estimation of animals for this reason. There are two main categories of mass-estimating approaches: extant-scaling approaches and volumetric-density approaches. Extant-scaling approaches determine the relationships between linear skeletal measurements and body mass using regression equations. Volumetric-density approaches, on the other hand, are all based on models. The models are of various types, including physical models, 2D images, and 3D virtual reconstructions. Once the models are constructed, their volumes are acquired using Archimedes’ Principle, math formulae, or 3D software. Then densities are assigned to convert volumes to masses. The acquisition of surface area is similar to volume estimation by changing math formulae or software commands. This article presents a new 2D volumetric-density approach called the cross-sectional method (CSM).


The CSM integrates biological cross-sections to estimate volume and surface area accurately. It requires a side view or dorsal/ventral view image, a series of cross-sectional silhouettes and some measurements to perform the calculation. To evaluate the performance of the CSM, two other 2D volumetric-density approaches (Graphic Double Integration (GDI) and Paleomass) are compared with it.


The CSM produces very accurate results, with average error rates around 0.20% in volume and 1.21% in area respectively. It has higher accuracy than GDI or Paleomass in estimating the volumes and areas of irregular-shaped biological structures.


Most previous 2D volumetric-density approaches assume an elliptical or superelliptical approximation of animal cross-sections. Such an approximation does not always have good performance. The CSM processes the true profiles directly rather than approximating and can deal with any shape. It can process objects that have gradually changing cross-sections. This study also suggests that more attention should be paid to the careful acquisition of cross-sections of animals in 2D volumetric-density approaches, otherwise serious errors may be introduced during the estimations. Combined with 2D modeling techniques, the CSM can be considered as an alternative to 3D modeling under certain conditions. It can reduce the complexity of making reconstructions while ensuring the reliability of the results.


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