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Early mammalian social behaviour revealed by multituberculates from a dinosaur nesting site


When sociality evolved and in which groups remain open questions in mammalian evolution, largely due to the fragmentary Mesozoic mammal fossil record. Nevertheless, exceptionally preserved fossils collected in well-constrained geologic and spatial frameworks can provide glimpses into these more fleeting aspects of early mammalian behaviour. Here we report on exceptional specimens of a multituberculate, Filikomys primaevus gen. nov., from the Late Cretaceous of Montana, primarily occurring as multi-individual, monospecific aggregates of semi-articulated skulls and skeletons within a narrow stratigraphic (~9 cm thick) and geographic (<32 m2) interval. Taphonomic and geologic evidence indicates that F. primaevus engaged in multigenerational, group-nesting and burrowing behaviour, representing the first example of social behaviour in a Mesozoic mammal. That F. primaevus was a digger is further supported by functional morphological and morphometric analyses of its postcranium. The social behaviour of F. primaevus suggests that the capacity for mammals to form social groups extends back to the Mesozoic and is not restricted to therians. Sociality is probably an evolutionarily labile trait that has arisen numerous times during mammalian evolution.

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Fig. 1: Skeletal elements of the five F. primaevus individuals preserved in MOR 10908 at Egg Mountain.
Fig. 2: Spatial distribution and composition of the monospecific F. primaevus fossil aggregates at Egg Mountain.
Fig. 3: Evidence for burrowing capabilities in F. primaevus.

Data availability

All µCT data are available as .tiff stacks through MorphoSource: All measurements and other data used in these analyses are provided in the Supplementary Tables.


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We thank the Museum of the Rockies, Beatrice R. Taylor Paleontological Resource Area and J. Horner for permission and support to excavate Egg Mountain, and the 2010–2016 Egg Mountain field crews for collecting the specimens described herein; R. Masek for his masterful preparation of the specimens; M. Ouchida for preparing the in situ line drawings, skeletal reconstruction and artistic reconstruction of the Egg Mountain multituberculates; A. L. Brannick and M. Rivin for µCT scanning specimens; J. Scannella and A. Atwater of the Museum of the Rockies for curatorial assistance; J. R. Moore for his geological insights at Egg Mountain; D. W. Krause for allowing the study of the holotype of Ptilodus kummae, among other multituberculate postcranial specimens; B. Minjin for allowing the study of multiple Mongolian multituberculate skeletons, and M. Novacek, J. Meng, and J. Galkin for access to the holotype of Cimexomys judithae, all housed at the American Museum of Natural History; C. S. Scott for allowing the study of unpublished multituberculate specimens at the Royal Tyrrell Museum; W. A. Clemens and P. A. Holroyd for access to University of California Museum of Paleontology specimens; D. L. Brinkman for facilitating access to the Yale Peabody Museum collections; D. M. Boyer for access to and advice about MorphoSource µCT data; D. M. Grossnickle, A. L. Brannick, M. R. Whitney, B. T. Hovatter, J. R. Claytor, P. K. Wilson and C. A. E. Strömberg for helpful feedback on the manuscript and figures; R. R. Rogers, A. K. Behrensmeyer and T. M. Bown for helpful discussions about Egg Mountain taphonomy; and D. W. Krause, A. Weil, B. Minjin and C. S. Scott for discussions about multituberculates. Financial support was from the National Science Foundation grant nos 0847777 (D.J.V.) and 1325674 (D.J.V. and G.P.W.M.), a National Science Foundation Graduate Research Fellowship (L.N.W.), the Doris O. and Samuel P. Welles Research Fund (L.N.W.), the University of Washington Department of Biology (L.N.W. and G.P.W.M.) and the Burke Museum (L.N.W. and G.P.W.M.).

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L.N.W., G.P.W.M., D.J.V. and E.J.S. designed the study. L.N.W. wrote the paper with substantial input from all authors. W.J.F. led the ichnological analysis. M.C. led the multivariate analysis. All authors contributed to the analyses and revising the paper. G.P.W.M., D.J.V. and E.J.S. supervised all research activities. L.N.W. and G.P.W.M. are both corresponding authors.

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Correspondence to Lucas N. Weaver or Gregory P. Wilson Mantilla.

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Extended data

Extended Data Fig. 1 Hypothesized phylogenetic relationships and comparative dental morphology of Filikomys primaevus and other North American multituberculates.

a, Simplified cladogram showing changes in cheek tooth morphology among North American cimolodontans. Filikomys is morphologically intermediate between basal cimolodontans like Cimexomys and ptilodontoids like Mesodma. We place Filikomys as a basal ptilodontoid based on a number of shared, derived craniodental character states. See Supplementary Discussion for further discussion. b, MOR 10908D, right lateral view of cranium and occlusal view of right lower dentition. c, MOR 10908A, right ventrolateral view of skull and near complete upper dentition (only I2 missing). d, MOR 10908D, ventral view of cranium and lateral views of right and left dentaries. e, MOR 11750, lateral view of left dentary. f, MOR 10908C, left ventrolateral view of skull and complete post-incisor upper dentition. Scale bar = 2 mm.

Extended Data Fig. 2 Results from phylogenetic parsimony analysis on multituberculate character-taxon matrix containing 51 taxa and 130 characters.

Illustrated is the 50% majority-rule consensus tree generated from the 356 shortest trees retained. Tree length = 478; Consistency Index = 0.43; Retention Index = 0.76. The numbers adjacent to certain nodes are bootstrap (above) and Bremer support (below) values; these values were only illustrated for nodes that had a bootstrap value of 40% or more and/or a Bremer support value of 2 or more. Multituberculata (node 1) is highlighted in blue, Cimolodonta (node 2) is highlighted in red, and Ptilodontoidea (node 3) is highlighted in yellow.

Extended Data Fig. 3 The geography, geology, taphonomy, and fossil collecting at the Egg Mountain locality (Museum of the Rockies site TM-006).

a, Locality map of the field area at the Willow Creek Anticline, Two Medicine Formation near Choteau, Montana, USA. The Egg Mountain locality (EM) is indicated with an arrow. b, Stratigraphic section measured at Egg Mountain. See Supplementary Discussion for detailed discussion of Egg Mountain geology. c, Close up of Units 1–3, which were quarried from 2010 to 2016. These sections are subdivided into 12 separate jackhammer passes (JHP). JHPs are stratigraphically parallel cuts that proceeded at ~10 cm-deep intervals through the quarry. The JHPs that produced F. primaevus fossils are indicated by the skeleton illustrations. The ichnofauna (see legend below) indicate that Egg Mountain sediments were subaerially exposed. d, Photograph of Egg Mountain quarry taken from the north. Note the thick rusty limestones in the foreground, which poke through the siltstone in the main quarry towards the southern quarry wall. A smaller discontinuous limestone lens is visible in the east wall of the main quarry. eg, Representative dinosaur specimens from Egg Mountain. e, Tyrannosaurid tooth showing the longitudinal fracturing common at the site. f, Shed hadrosaur tooth. One of the most common fossils are shed hadrosaur teeth of small dimensions. g, Unidentified hadrosaur bone, one of the few large bones recovered from the site. This specimen is from a partial, associated hadrosaur skeleton. In contrast to the bones of small mammals and squamates from the site, these large bones are in poor condition showing surface corrosion and numerous breaks. The close position of the fragments suggests that the breakdown of these bones occurred post depositionally, either subaerially or within the soils. h, Grid system of the main quarry (MQ) set up 11 m north by 9 m west (N1–11/W5–13); each quadrant is one square-meter. Yellow square indicates the portion of the quarry where F. primaevus fossils were found. Pins labeled 1–11 indicate where specific F. primaevus specimens were found (corresponding to Fig. 2 in the main text), pins labeled 81 and 82 represent the location of the two collections that make up MOR 10908. i, MOR 10908 as found in situ in the field. Skull (MOR 10908 A) visible in right lateral view at top left of image with its pelvic girdle visible in lower left. Portions of its vertebral column were lost in missing block to left. Postcrania of other individuals are partially visible to the right of this specimen, including the impression of a femur. Orientation as in Fig. 1j in the main text. Parts ad and h were modified from ref. 29. Abbreviations: F, femur; Ls, micritic limestone; MQ, main quarry; Pg, pelvic girdle; Sk, skull; Sltst, siltstone; UQ, upper quarry. Scale bars = 1 cm (e), 0.5 cm (f), and 2 cm (g, i).

Extended Data Fig. 4 Evidence for both subadult and adult individuals of Filikomys primaevus at Egg Mountain.

MOR 10908E and 10908B exhibit features characteristic of skeletally immature, subadult mammals31: (i) unfused cranial sutures, (ii) MOR 10908E is the smallest (based on dentary length) of all the individuals preserved in MOR 10908, (iii) both have open lower incisor roots (orange arrows), indicating that the tooth was still erupting, (iv) MOR 10908E has unworn cheek teeth and MOR 10908B has only very minor cheek-tooth wear, and (v) the long bones are missing their epiphyses, indicating that they were unfused at the time of death. MOR 10908D, 10908C, and 10908A exhibit features characteristic of skeletally mature, adult mammals31: (i) fused cranial sutures, (ii) MOR 10908A is the largest (based on dentary length) of all the individuals preserved in MOR 10908, (iii) all have closed lower incisor roots (blue arrows), indicating that the tooth was fully erupted, (iv) the cheek teeth of MOR 10908C and 10908A are heavily worn, and (v) the long bones have fused epiphyses. See Supplementary Discussion for more information. Scale bars = 5 mm.

Extended Data Fig. 5 Results from canonical variate analyses (CVAs) of linear measurements taken from the postcranial skeleton of Filikomys primaevus.

a, c, e, Plots of canonical function 1 (CF1) and CF2 from CVA of the right (a) and left (c) sides of MOR 10908A, and the left side of MOR 10908C (e). b, d, f, Plots of structure correlations among the osteological indices and CF1 and CF2 of the right (b) and left (d) sides of MOR 10908A, and the left side of MOR 10908C (f). In all three analyses, F. primaevus is classified as fossorial. For explanation of osteological index abbreviations see Supplementary Discussion.

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Weaver, L.N., Varricchio, D.J., Sargis, E.J. et al. Early mammalian social behaviour revealed by multituberculates from a dinosaur nesting site. Nat Ecol Evol 5, 32–37 (2021).

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