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Facial sensation in archosaurs based on alligators

From: Ben Creisler

A recent paper (now formally published) about the facial nerves in
alligators and other archosaurs, with a press release and video link

Ian D. George & Casey M. Holliday (2013)
Trigeminal Nerve Morphology in Alligator mississippiensis and Its
Significance for Crocodyliform Facial Sensation and Evolution.
The Anatomical Record 296(4): 670–680
DOI: 10.1002/ar.22666

Modern crocodylians possess a derived sense of face touch, in which
numerous trigeminal nerve-innervated dome pressure receptors speckle
the face and mandible and sense mechanical stimuli. However, the
morphological features of this system are not well known, and it
remains unclear how the trigeminal system changes during ontogeny and
how it scales with other cranial structures. Finally, when this system
evolved within crocodyliforms remains a mystery. Thus, new
morphological insights into the trigeminal system of extant
crocodylians may offer new paleontological tools to investigate this
evolutionary transformation. A cross-sectional study integrating
histological, morphometric, and 3D imaging analyses was conducted to
identify patterns in cranial nervous and bony structures of Alligator
mississippiensis. Nine individuals from a broad size range were
CT-scanned followed by histomorphometric sampling of mandibular and
maxillary nerve divisions of the trigeminal nerve. Endocast volume,
trigeminal fossa volume, and maxillomandibular foramen size were
compared with axon counts from proximal and distal regions of the
trigeminal nerves to identify scaling properties of the structures.
The trigeminal fossa has a significant positive correlation with skull
length and endocast volume. We also found that axon density is greater
in smaller alligators and total axon count has a significant negative
correlation with skull size. Six additional extant and fossil
crocodyliforms were included in a supplementary scaling analysis,
which found that size was not an accurate predictor of trigeminal
anatomy. This suggests that phylogeny or somatosensory adaptations may
be responsible for the variation in trigeminal ganglion and nerve size
in crocodyliforms.


Press release and video: