Journal of Mammalian Evolution

, Volume 25, Issue 3, pp 301–313 | Cite as

Origin of the Lateral Wall of the Mammalian Skull: Fossils, Monotremes and Therians Revisited

  • A.W. CromptonEmail author
  • C. Musinsky
  • G.W. Rougier
  • B.-A.S. Bhullar
  • J. A. Miyamae
Original Paper


To interpret the fundamental differences in the structure and origin of the braincase sidewalls of monotremes, multituberculates, and therians, we examined MicroCT scans of a mammaliaform, Morganucodon; two non-mammalian cynodonts, Massetognathus and Probainognathus; a stem therian, Vincelestes; a juvenile and adult monotreme, Ornithorhynchus; and two marsupials, Monodelphis and Didelphis. The skull of Morganucodon resembles the pattern predicted for an early mammal: the descending flanges of the frontal and parietal cover the lateral surface of the orbitosphenoid and the palatine forms most of the medial wall of the orbit. In monotremes, the lateral region of the chondrocranium ossifies to form a long presphenoid/orbitosphenoid complex. During the transition from early mammals to extant mammals the height of the alisphenoid decreased drastically, the anterior lamina extended anteriorly to form part of the sidewall while the lateral surface of the orbitosphenoid was exposed by the dorsal withdrawal of the frontal and parietal. By contrast, in multituberculates and therians the lateral edges of the frontals extended further ventrally and the orbitosphenoid was reduced to a smaller orbital exposure below the frontals. In multituberculates the alisphenoid decreased in height, replaced by an anterior extension of the anterior lamina. The palatine withdrew from the orbital wall, replaced by a dorsally directed expansion of the maxilla. Extant therians have lost the anterior lamina. The inferior edges of the frontal followed the further ventral migration of the orbitosphenoid. The alisphenoid and parietal form most of the braincase sidewall.


Braincase sidewall Orbitosphenoid Monotremes Ictidosaurs Morganucodon Vincelestes 



Thanks to the Museum of Comparative Zoology and the Department of Organismic and Evolutionary Biology at Harvard for their support.

Supplementary material

10914_2017_9388_MOESM1_ESM.pdf (3.6 mb)
ESM 1 Morganucodon watsoni (IVPP 8682) a, stereo of the anterior view of the braincase, with frontals, parietals and right petrosal omitted; b, stereo of the internal view of the right side of the braincase (with bones on the left side omitted) and interpretation (PDF 3726 kb)
10914_2017_9388_MOESM2_ESM.pdf (2.2 mb)
ESM 2 Ornithorhynchus anatinus (AMNH 2013). Stereo view of the internal view of the right side of the braincase (PDF 2267 kb)
10914_2017_9388_MOESM3_ESM.mp4 (7.9 mb)
ESM 3 Ornithorhynchus anatinus (AMNH 2013). Video showing half rotation of the orbitosphenoids (shades of pink), presphenoid (purple), palatine (mauve), mesethmoid (green), and pterygoids (yellow orange), basisphenoid (red); followed by a completion of the rotation with additional bones added: anterior lamina, squamosal and parietal (shades of blue), maxilla (yellow), dentary, frontal, supraoccipital, basioccipital, exoccipital and petrosal, (shades of green) (MP4 8052 kb)
10914_2017_9388_MOESM4_ESM.pdf (5.3 mb)
ESM 4 Stereo photographs of Vincelestes neuquenianus (MACN-N 04). a, partial view of the left side of the braincase; b. internal view of the left side of the braincase; c, ventral view of braincase; d, dorsal view of the braincase; e, horizontal section through the braincase to illustrate the relation of the orbital flanges of the palatine to the presphenoid/orbitosphenoid (PDF 5391 kb)
10914_2017_9388_MOESM5_ESM.mpg (39.5 mb)
ESM 5 Probainognathus jenseni (MCZ 4280). Video showing rotation of the orbitosphenoid and fragment of the frontal (MPG 40415 kb)
10914_2017_9388_MOESM6_ESM.mpg (103.1 mb)
ESM 6 Morganucodon watsoni (IVPP 8682). Video showing rotation of segmented bones of the skull (MPG 105554 kb)
10914_2017_9388_MOESM7_ESM.mp4 (83.6 mb)
ESM 7 Vincelestes neuquenianus (MACN-N05). Video showing rotation of the orbitosphenoids/presphenoid (pink), palatines (red), mesethmoid (light blue), and pterygoids (dark red), followed by rotation of the whole skull with the left side roof and zygomatic arch removed, also partially segmented (MP4 85,647 kb)


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Copyright information

© Springer Science+Business Media New York 2017

Authors and Affiliations

  1. 1.Museum of Comparative ZoologyHarvard UniversityCambridgeUSA
  2. 2.Department of Anatomical Sciences & NeurobiologyUniversity of LouisvilleLouisvilleUSA
  3. 3.Department of Geology & GeophysicsYale UniversityNew HavenUSA

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