Lower Jaw Elements Research Articles

Unusual findings of cephalopod jaws in Mesozoic shallow water sandy facies

Jaws of fossil cephalopods, originally composed mainly of organic matter, are rare findings. They are sometimes can be found in nodules, in body chambers of their hosts (ammonoids and nautiloids), and in the fine-grained deposits, including Konservat-Lagerstätten. Findings of fossil coleoid jaws, which were completely organic and devoid of mineralized elements, are especially rare. Here, for the first time, numerous specimens of three-dimensional phosphatized cephalopod jaws, preserved in the extremely shallow-water coastal sandy facies of the Upper Jurassic (Volgian), Lower (Albian) and Upper Cretaceous (Cenomanian, Turonian, and Campanian), all of which have so far been widely considered only as a source of vertebrate fossils, are described. Both upper and lower cephalopod jaws were found, whereas lower jaw elements are more common. Most of the jaws were referred to as belonging to coleoids, judging by their shape and the ratio of the sizes of their outer and inner lamellae. The overall shape and rounded front tips of the lower jaws, as well as the relatively straight, not curved shape of the upper jaws suggest that they belonged to octobrachian coleoids. One of the jaws most likely belongs to a nautilid specimen. Some coleoid jaws from the Upper Cretaceous are relatively large, suggesting the large body size of their owners. It should be noted that elements of the jaw apparatus are the only remnants of cephalopods at most localities with coastal sandy facies. The study of these jaws sheds light on the diversity of cephalopods in coastal ecosystems of the Cretaceous time.

The first record of jaws of Boreal Valanginian ammonites (Cephalopoda, Polyptychitidae)

Jaws of ammonites which inhabited the Panboreal Superrealm during the Jurassic and Cretaceous are poorly known in comparison to those of Tethyan ammonoid faunas. This paucity may be explained by limited thickness, or even absence of an outer calcitic layer, in lower jaw elements (aptychi) of Boreal ammonites. Here we describe, for the first time, the jaws (both lower and upper) of ammonites of the Boreal family Polyptychitidae, of Early Cretaceous (Valanginian) age. Polyptychitid lower jaws are of the aptychus type, but have an unusual pointed and convex shape. However, lower jaws of Late Jurassic ancestors of polyptychitids (Craspeditidae) as well as Middle Jurassic cardioceratids (Pseudocadoceras) have a near-identical shape, as do previously described aptychi of the Late Cretaceous genera Neogastroplites and Placenticeras (Hoplitoidea). The close resemblance of lower jaws of evolutionarily distant ammonites may be linked to a similar lifestyle, but more data are needed to substantiate this. Upper jaws of polyptychitid are closely similar to previously described upper jaws of Jurassic ammonites, which indicates the conservatism of this part of the jaw apparatus. Together with shells and jaws of the Valanginian ammonites described herein, jaws of coleoids (likely belemnites) as well as arm hooks (onychites) have been found.

A revised Eurogondwana model: Late Cretaceous notosuchian crocodyliforms and other vertebrate taxa suggest the retention of episodic faunal links between Europe and Gondwana during most of the Cretaceous

We describe new remains of the enigmatic Late Cretaceous crocodyliform Doratodon carcharidens coming from the Santonian Csehbánya Formation of Hungary. The material includes isolated upper and lower jaw elements and teeth that represent the earliest occurrence of this genus. Previous reports of Doratodon restricted the range of this taxon to the Campanian–Maastrichtian of the areas that are now Romania, Austria and Spain. Using data from both the new remains and the type series from the lower Campanian of Austria, our phylogenetic analysis finds D. carcharidens deeply nested within Gondwanan notosuchians as part of the predominantly South American clade Sebecosuchia. The suggested affinity of D. carcharidens is generally in agreement with previous cladistic assessments and confirms the presence of Gondwanan faunal elements in the Late Cretaceous western Tethyean archipelago (the region that is now southern Europe). Together with published reports on Gondwanan tetrapod immigrants from the Barremian, Albian, Cenomanian and Santonian of Europe this record is not compatible with a recently proposed paleobiogeographical model predicting a post-Hauterivian separation of Europe from Africa and subsequent biogeographical reconnection in the Campanian lasting till the Eocene. Instead, the European fossil record suggests that episodic faunal and geographical links were retained with Gondwana during most of the Cretaceous.

Comparison of cranial form and function in association with diet in natricine snakes

The skull of squamates has many functions, with food acquisition and ingestion being paramount. Snakes vary interspecifically in the frequency, size, and types of prey that are consumed. Natural selection should favor phenotypes that minimize the costs of energy acquisition; therefore, trophic morphology should reflect a snake's primary prey type to enhance some aspect of feeding performance. I measured 19 cranial variables for six natricine species that vary in the frequency with which they consume frogs and fish. Both conventional and phylogenetically corrected analyses indicated that fish-eating snakes have relatively longer upper and lower jaw elements than frog-eating snakes, which tended to have broader skull components. I also compared the ratio of the in-lever to the out-lever lengths of the jaw-closing mechanism [jaw mechanical advantage (MA)] among species. Fish-eating snakes had significantly lower MAs in the jaws than did the frog-eating snakes. This result suggests that piscivores have faster closing jaws and that the jaws of frog-eating snakes have higher closing forces. Cranial morphology and the functional demands of prey capture and ingestion appear to be associated with primary prey type in natricine snakes.

A jaw-dropping ear

A fossil from the Early Cretaceous provides insight into the evolution of the hearing apparatus in mammals. Anchoring the eardrum was, it seems, an essential step in freeing the middle ear from the jaw. See Article p.181 The lower jaw of reptiles is made up of several bones. In mammals, however, it consists of just one, the tooth-bearing dentary. Most of the rest have become the ossicles that transmit sound through the middle ear. This transformation is an iconic example of evolutionary change, but direct fossil evidence of the transition has been hard to find. That's why a fossil described by Jin et al. is so important. It is from a triconodont (a type of extinct mammal) from the Cretaceous period in China. In it, the lower-jaw elements have started to resemble middle-ear ossicles, but are still joined to the lower jaw by a sliver of ossified cartilage. This element, Meckel's cartilage, is an important part of the inner surface of the lower jaw; the new fossil shows that it was a vital piece in the evolutionary jigsaw that led to the formation of the mammalian middle ear.

A Preliminary Report on the Superficial Throat Musculature of the Microhylidae and Its Possible Role in Tongue Action

Micro-dissection of species of the Microhylidae reveals wide variation in the structure of the lower jaw elements and superficial throat musculature. The presence of posterolateral processes on the mentomeckelian bones makes it unlikely that tongue action is the same in these species as reported for Rana catesbeiana. The accessory slips of the intermandibularis posterior muscle are implicated, by their morphology, in feeding movements.