The temporomandibular joint has a special relevance because, among other characteristics, it is classified as an appositional joint. This means that, unlike other synovial joints, it does not develop from a skeletal anlage that undergoes segmentation, but instead the components move toward one another by appositional growth (Liang, 2016; Radlanski, 2018).

It is also peculiar that the temporomandibular joint and the mammalian middle ear are interconnected by their evolutionary origin and their embryonic development (Antwal et al. 2013; Stocum & Roberts, 2018).

From an evolutionary point of view, the jaw joint of non-mammalian vertebrates, known as the primary jaw joint, forms between the quadrate and the articular. Biological and paleontological studies confirmed that in mammals these structures evolved into the middle ear ossicles, i.e., the incus and the malleus. This transformation occurred at the same time as the development of a new secondary jaw articulation, referred to as the temporomandibular joint, between the squamosal and the dentary bones (Reichert, 1837, Gaupp, 1912, Anthwal et al., 2013, Maier and Ruf, 2016).

Embryologically, the intimate link between both joints is explained because before the actual temporomandibular joint would have been formed, the proximal end of Meckel's cartilage, in continuity with the malleus, articulates with the incus. Therefore, the incudomalleolar joint remains attached to the mandible via Meckel’s cartilage and, after a slight time lag, the secondary temporomandibular joint develops (Anthwal and Thompson, 2016). Interestingly, there is a certain timeframe in which both joints exist simultaneously. However, until to date, it is not at all clear how these two joints can exist physiologically with or without interfering functionally.

The close jaw-ear relationship can still be found in adults since both structures are connected by ligaments, such as the discomalleolar and the spheonomandibular ligament (Pinto, 1962, Rodríguez Vázquez et al., 1992, Rodríguez-Vázquez et al., 1993, Rodríguez-Vázquez et al., 1998, Sencimen et al., 2008, Anthwal et al., 2020, Kokot, 2021).

Taken all together, first we want to address the question of how both joints develop spatiotemporally. For that purpose, we need a detailed description of their morphogenesis that can be achieved using 3D reconstructions to evaluate the auditory ossicles, Meckel’s cartilage, the mandible and the temporomandibular joint. More specifically, we want to discern at what time the primary and secondary jaw joints coexist simultaneously and discuss the possible mechanism for their separation.