Joint histology inAlligator mississippiensischallenges the identification of synovial joints in fossil archosaurs and inferences of cranial kinesis

Abstract

Archosaurs, like all vertebrates, have different types of joints that allow or restrict cranial kinesis, such as synovial joints and fibrous joints. In general, synovial joints are more kinetic than fibrous joints, because the former possess a fluid-filled cavity and articular cartilage that facilitate movement. Even though there is a considerable lack of data on the microstructure and the structure-function relationships in the joints of extant archosaurs, many functional inferences of cranial kinesis in fossil archosaurs have hinged on the assumption that elongated condylar joints are (i) synovial and/or (ii) kinetic. Cranial joint microstructure was investigated in an ontogenetic series of American alligators, Alligator mississippiensis. All the presumably synovial, condylar joints found within the head of the American alligator (the jaw joint, otic joint and laterosphenoid -postorbital (LS -PO) joint) were studied by means of paraffin histology and undecalcified histology paired with micro-computed tomography data to better visualize threedimensional morphology. Results show that among the three condylar joints of A. mississippiensis, the jaw joint was synovial as expected, but the otherwise immobile otic and LS-PO joints lacked a synovial cavity. Therefore, condylar morphology does not always imply the presence of a synovial articulation nor mobility. These findings reveal an undocumented diversity in the joint structure of alligators and show that crocodylians and birds build novel, kinetic cranial joints differently. This complicates accurate identification of synovial joints and functional inferences of cranial kinesis in fossil archosaurs and tetrapods in general.

The roles of joint tissues and jaw muscles in palatal biomechanics of the Savannah monitor (Varanus exanthematicus) and their significance for cranial kinesis

“…Sections of the otic joint revealed a unichondral joint in which the hylaline cartilage-capped epiphysis of the otic process of the quadrate abuts layers of fibrous connective tissue surrounding the paraoccipital process and squamosal. This morphology is also similar to that found in the otic joint of gecko (Payne et al, 2011) and alligator (Bailleul and Holliday, 2017).…”

“…Our results also support the findings of Mezzasalma et al (2014) that the histology of the joint may not necessarily be as important as the morphology for kinesis, as the soft tissue joint models (JL2-4) revealed similar patterns of strain and deformation. Furthermore, Bailleul et al (2016) and Bailleul and Holliday (2017) found gross bony morphology does not always reflect the types of connective tissues in archosaur skulls. Moreover, cranial joint tissues may behave considerably differently in dynamic, rather than static loading conditions.…”

The iliosacral joint in lizards: an osteological and histological analysis

“…As explained above, a fibrous‐based contact is indicative of a syndesmosis, which allows for limited mobility (Barnett et al ; Kardong, ). The presence of calcified cartilage, more distally along the margins of the sacral ribs, interposed between the subchondral bone and the fibres, suggests that this contact transitions to a synchondrosis towards the ISJ, similarly to what is described by Bailleul & Holliday () for some cranial joints of modern alligators.…”

Dinosaur paleohistology: review, trends and new avenues of investigation

“…The hypothesis that physical movement may be responsible for secondary cartilage formation has been previously proposed for some cranial joints in both birds and mammals (Murray, 1963; Murray & Drachman, 1969; Bareggi et al, 1994), but the degree and/or direction of such movement has not been quantified. Nevertheless, cranial tissues have great potential for more accurate inferences regarding skull structure and function (e.g., cranial kinesis or akinesis) in non-avian dinosaurs and other fossil archosaurs (e.g., see Bailleul & Holliday, 2017; Lessner et al, 2019).…”