Ontogenetic disturbances are disturbances affecting the course of development of an individual organism. In the context of teeth and jaw, ontogenetic disturbances can be recognized as disturbances of tooth shape (form), size and number, tooth position and malocclusion, tooth structure, disturbances of tooth eruption, and congenital malformations and disturbances of bone formation and growth of the skeleton affecting jaws and teeth. “Double teeth” is a term for a developmental anomaly caused by congenital, inherited, acquired and/or idiopathic factors resulting in the union of two and, on more rare occasions, three adjacent teeth. The union itself is caused by the physical pressure between two adjacent teeth during their growth and development. Clinical literature indicates that “double teeth” usually occur in three different manners: concrescence, gemination, and fusion (Alt and Türp, 1998, McKinney and Olmo, 2022).

Tooth concrescence is a rare abnormality in which two or more teeth are joined with cementum (Levitas, 1965). Acquired concrescence is a condition where two or more teeth fuse at their cementum surfaces after they have fully erupted or completed their root formation. On the other hand, if this fusion occurs during tooth development, it is called true concrescence, and it is commonly observed between second and third maxillary molars or between the third molar and a supernumerary molar, in either deciduous or permanent dentition (Alt and Türp, 1998, Wang et al., 2022). It has been reported to occur in around 0.8 % of permanent dental extractions (Palermo & Davies-House, 2016).

Gemination is a dental condition in which a single tooth bud or follicle tries to split, leading to the formation of two crowns on a single tooth root. This condition is most seen in incisors and canines, but it can also affect premolars. A distinguishing feature of gemination is the presence of an incisal groove or depression on the affected tooth. It's worth noting that the overall number of teeth in the mouth is not affected by gemination (Alt and Türp, 1998, Knežević et al., 2002, Mohan et al., 2013). The prevalence rate of unilateral gemination is 0.5 % in deciduous teeth and 0.1 % in permanent dentition while the prevalence of bilateral cases is 0.01–0.04 % in primary dentition and only 0.02–0.05 % in permanent dentition (Rao et al., 2013).

Teeth fusion occurs when two normally tooth germs touch each other and subsequentially join (Neves et al., 2002). This can happen when the tooth germs are located very close to each other or when a supernumerary (extra) tooth bud develops too close to a normally developing tooth bud (McKinney & Olmo, 2022). When the fusion process takes place, the entire length of the teeth may be involved, in which case cementum, dentine and enamel are shared. The tooth count reveals a missing tooth with the anomalous tooth counted as one unless the fusion occurred with a supernumerary tooth (More and Tailor, 2012, Regezi et al., 2012). The most frequently fused teeth are mandibular incisors, followed by maxillary incisors, while in molars this disorder is very rare (Khurana and Khurana, 2014, Le Gall et al., 2011, Woelfel, 1990). The prevalence is between 0.5 % and 2.5 % in primary teeth and 0.1 % in permanent dentition (Karumaran et al., 2020, Pindborg, 1970, Sunny et al., 2013). No sex differences have been recorded so far (Cetinbas et al., 2007) with higher incidence reported in individuals of Asian and Native American origin (White & Pharoah, 1994).

There are many reports of “double teeth” in recent populations. For example, Açıkel et al. (2018) recorded 50 cases of primary fused teeth in 40 patients among a total of 13,450 pediatric patients who attended the Pediatric Dental Clinic in northern Turkey between 2015 and 2017. Similar frequencies of various forms of “double teeth” have been reported by Mader (1979), Duncan & Helpin (1987), and Martinez, Segura (2018) in their systematic literature reviews of these anomalies in modern settings. However, such examples in archaeological contexts are still rare. Irish (2024) suggests that the prevalence of this anomaly in certain archaeological populations from Africa is between 0.048 % and 0.098 % depending on the region and time period. In most cases the fusion of primary (deciduous) teeth is reported (Benazzi et al., 2010, Borowska et al., 2024, Drusini and Swindler, 1994, Padgett, 2018, Silva and Silva, 2018, Smith and Wojcinski, 2011, Šarkić et al., 2022, Tritsaroli, 2018, Yamamoto, 1989) with only a few examples of the involvement of the permanent teeth (Collina et al., 2021, Forshaw, 2019, Irish, 2024, Phillips et al., 2021), and so far, only one from Europe (Collina et al., 2021). Making a correct diagnosis of “double teeth” in archaeological specimens can also be challenging. Therefore, the purpose of this paper is to present and discuss a case of fusion of permanent maxillary anterior teeth by using multiple lines of evidence (archaeological, macroscopic as well as micro-CT scanning) from the late medieval period (14th-15th century CE) burial phase from Sisak in continental Croatia, and to provide a comprehensive overview of similar known cases from archaeological contexts globally. To our knowledge, this is the first known archaeological example of the fusion of permanent maxillary incisors from Europe and one of the very few known cases of the fusion of permanent teeth in ancient populations. A detailed study of this particular case, but also the additional research of skeletal collections from different temporal and geographic contexts will also help us to better understand etiology of this anomaly in contemporary populations.