MORPHOMETRICAL FEATURES OF THE CAVE BEAR AND BROWN BEAR HEAD SKELETON: A COMPARATIVE STUDY

Abstract

Abstract: The extinct cave bear (CB) is often depicted as a large heavy animal with a prominent, massive skull and severely shortened pelvic limbs. The size and robustness of the CB are comparable to those of the largest living ursids. Great similarities between CB and the brown bear (BB) species prompted our morphometric comparison. The goal of this study was to elucidate the potential differences in the morphometric skull characteristics of CB and BB species. Craniometric measurements were performed on the skulls of both bear species and were compared to identify craniometric features indicative of possible adaptations in both bear species. The results revealed a marked difference in skull size; however, the shape of the CB cranium is generally known to be similar to that of the BB. In CB, the total length of the skull was approximately 1.5 times longer, and the external bony nasal aperture was larger due to the relatively shorter nasal bones. The nose length and median palate length were relatively longer in the CB, and the infraorbital foramen was located more caudally and closer to the zygomatic process. The infraorbital channel of the CB was located over to the second superior molar roots whereas in BB the latter extends beyond the roots of the first to the second superior molar roots. A marked difference was the non-existence of the three most anterior premolars in the maxilla and mandible of the CB, Higher body of the mandible, which, together with the extensive biting surfaces of the cheeks teeth, indicates a predominantly plant-based diet of the CB. The braincase length was considerably shorter in the CB, resulting in a relatively small neurocranium volume. The pronounced frontal fossa in the CB skull continues caudally into a strongly developed frontal area, which gives the CB skull a prominent steep profile. In summary, comparative craniometry showed that CB had a smaller neurocranial volume and had herbivore-adapted jaws and teeth. These metric features of the head skeleton may be related to a lower adaptability to extreme climatic conditions to which they were exposed to during the last Pleistocene glacial period, which may have contributed to their extinction. Key words: head skeleton; skull; mandible; craniometrical features; bear MORFOMETRIČNE ZNAČILNOSTI SKELETA GLAVE JAMSKEGA IN RJAVEGA MEDVEDA: PRIMERJALNA ŠTUDIJA Izvleček: Izumrli jamski medved je pogosto upodobljen kot robustna, težka žival z masivno lobanjo in izrazito skrajšanima medeničnima okončinama. Po velikosti ga lahko primerjamo z največjimi vrstami današnjih medvedov. Domnevne podobnosti med jamskim in rjavim medvedom so spodbudile našo primerjalno študijo, s katero želimo pojasniti morebitne razlike v morfometričnih značilnostih lobanj, iz česar je mogoče sklepati na možne prilagoditve pri obeh vrstah. Oblika lobanje je pri jamskem medvedu precej podobna obliki pri rjavem medvedu, ugotovili pa smo izrazito razliko v velikosti. Pri jamskem medvedu je celotna dolžina lobanje približno 1,5-krat daljša, zunanja koščena odprtina vhoda v nosno votlino pa je zaradi razmeroma krajših nosnih kosti obsežnejša. Dolžina nosu in mediana nebna dolžina sta pri jamskem medvedu daljši. Podočnična odprtina leži nekoliko kavdalneje in bližje ličnemu loku. Posledično se podočnični kanal pri jamskem medvedu nahaja le nad koreninami drugega molarja, medtem ko slednji pri rjavem medvedu sega še nad korenine prvega molarja. Razlika je bila opazna tudi v neizraslih prvih treh premolarjih tako v zgornji kot spodnji čeljusti jamskega medveda ter izrazitejšem telesu spodnje čeljustnice. Vse to, vključno z obsežnimi griznimi ploskvami molarjev, nakazuje pretežno rastlinsko prehrano. Dolžina možganske vot- line je pri jamskem medvedu opazno krajša, kar se odraža v sorazmerno majhni prostornini. Globoka čelna jama se kavdalno strmo nadaljuje v močno razvito čelno področje. Manjša prostornina možganske votline, rastlinski hrani prilagojene čeljusti in površine zob ter nekatere druge metrične značilnosti okostja glave bi lahko bile vzročno povezane z nižjo prilagodljivostjo ekstremnim podnebnim razmeram, katerim je v zadnjem pleistocenskem ledeniškem obdobju jamski medved neuspešno kljuboval in kar bi lahko znatno prispevalo k njegovemu izumrtju. Ključne besede: okostje glave; lobanja; spodnja čeljustnica; kraniometrične lastnosti; medved

Keywords

• head skeleton
• skull
• mandible
• craniometrical features
• bear