Gabriel A. Casal, Adriana M. Nillni, Mauro N. Valle, Ezequiel González Svoboda & Celina Tiedemann (2017)
Permineralization on dinosaur remains preserved in overflow deposits from Bajo Barreal Formation (Upper Cretaceous), central Patagonia, Argentina.
Revista Mexicana de Ciencias Geológicas (advance online publication)
The fossil deposit Cañadón Las Horquetas, placed in the centre-south of the Chubut Province, includes the Bajo Barreal Formation of early Cenomanian – late Turonian age. This site presents the greater abundance and diversity of fossil vertebrates in the San Jorge Gulf Basin. Thediagenetic processes reported herein represent a little addressed subject in taphonomic studies involving dinosaurs. We present for the first time information on the permineralization processes that affected the dinosaur remains preserved in overflow deposits from the Bajo Barreal Formation, as well as new sedimentological data of the studied unit. Samples from three specimens were studied by chemical analysis, polarization petrographic microscope, X-ray diffraction and scanning electron microscopy (SEM). These studies show that all the dinosaurs have had a similar diagenetic history, with the same pattern of mineral replacement. Besides, the similar chemical composition found in the fossils and in the bearing rocks suggest that the enrichment with new elements found in the studied dinosaur remains was due to the direct exchange with the sediments in which they were buried. The now reported diagenetic conditions can be preliminarily extended to the whole lower member of the Bajo Barreal Formation.
Neal Anthwal, Daniel J. Urban, Zhe-Xi Luo, Karen E. Sears & Abigail S. Tucker (2017)
Meckel’s cartilage breakdown offers clues to mammalian middle ear evolution.
Nature Ecology & Evolution 1, Article number: 0093 (2017)
A key transformation in mammalian ear evolution was incorporation of the primary jaw joint of premammalian synapsids into the definitive mammalian middle ear of living mammals. This evolutionary transition occurred in two steps, starting with a partial or ‘transitional’ mammalian middle ear in which the ectotympanic and malleus were still connected to the mandible by an ossified Meckel’s cartilage (MC), as observed in many Mesozoic mammals. This was followed by MC breakdown, freeing the ectotympanic and the malleus from the mandible and creating the definitive mammalian middle ear. Here we report new findings on the role of chondroclasts in MC breakdown, shedding light on how therian mammals lost the part of the MC connecting the ear to the jaw. Genetic or pharmacological loss of clast cells in mice and opossums leads to persistence of embryonic MC beyond juvenile stages, with MC ossification in mutant mice. The persistent MC causes a distinctive groove on the postnatal mouse dentary. This morphology phenocopies the ossified MC and Meckelian groove observed in Mesozoic mammals. Clast cell recruitment to MC is not observed in reptiles, where MC persists as a cartilaginous structure. We hypothesize that ossification of MC is an ancestral feature of mammaliaforms, and that a shift in the timing of clast cell recruitment to MC prior to its ossification is a key developmental mechanism for the evolution of the definitive mammalian middle ear in extant therians.
A key transformation in mammalian ear evolution was incorporation of the primary jaw joint of premammalian synapsids into the definitive mammalian middle ear (DMME) of living mammals. The quadrate and articular in the jaw joint of reptiles and the incus and malleus of the middle ear of mammals are homologues. These structures are formed by a type II collagen-expressing condensation continuous with the proximal part of Meckel’s cartilage (MC). The evolutionary transition from jaw joint to middle ear occurred in two steps. First, there was a partial or ‘transitional’ mammalian middle ear (PMME) in which the ectotympanic and malleus were still connected to the mandible by an ossified MC as observed in many Mesozoic mammals (Fig. 1a). This was followed by MC breakdown, freeing the ectotympanic and the malleus from the mandible and creating the DMME. The majority of MC is transient in mammalian development (Fig. 1b,d,e) but remains cartilaginous through to adulthood in other amniotes, such as extant squamates (Fig. 2f, Supplementary Fig. 1) and crocodiles17. The part of Meckel’s connected to the malleus and ectotympanic breaks down at postnatal day 2 (P2) in mice11, and around P20 in the marsupial opossum, Monodelphis domestica18. An understanding of the cellular processes involved in MC breakdown in extant placentals (mice) and marsupials (opossum) has important implications for the mechanisms underlying the transformation of the mammalian middle ear during this evolutionary transition.
During development, Meckel’s breakdown is associated with the onset of function of the squamosal–dentary jaw joint in the mouse and opossum, although timing differs18. The most proximal part of MC forms the malleus and incus, which undergo endochondrial ossification, whereas the slightly more distal part of MC is thought to transdifferentiate to become the anterior ligament of the malleus and the sphenomandibular ligament, breaking the hard tissue link between the ear and jaw. One general mechanism for remodelling of cartilage matrix is through the action of chondroclasts: large, multinucleated, haematopoietic-derived osteoclast-like cells that secrete enzymes that degrade cartilage. Haematopoietic-lineage clast cells are born as mononuclear precursors that travel along blood vessels to the site of activity where they are activated under the control of the transcription factor c-Fos to fuse and to form multinucleated mature clast cells.
Alida M. Bailleul, Lawrence M. Witmer & Casey M. Holliday (2017)
Cranial joint histology in the mallard duck (Anas platyrhynchos): new insights on avian cranial kinesis.
Anatomical Record 230(3): 444–460
DOI: 10.1111/joa.12562View/save citation
The evolution of avian cranial kinesis is a phenomenon in part responsible for the remarkable diversity of avian feeding adaptations observable today. Although osteological, developmental and behavioral features of the feeding system are frequently studied, comparatively little is known about cranial joint skeletal tissue composition and morphology from a microscopic perspective. These data are key to understanding the developmental, biomechanical and evolutionary underpinnings of kinesis. Therefore, here we investigated joint microstructure in juvenile and adult mallard ducks (Anas platyrhynchos; Anseriformes). Ducks belong to a diverse clade of galloanseriform birds, have derived adaptations for herbivory and kinesis, and are model organisms in developmental biology. Thus, new insights into their cranial functional morphology will refine our understanding of avian cranial evolution. A total of five specimens (two ducklings and three adults) were histologically sampled, and two additional specimens (a duckling and an adult) were subjected to micro-computed tomographic scanning. Five intracranial joints were sampled: the jaw joint (quadrate-articular); otic joint (quadrate-squamosal); palatobasal joint (parasphenoid-pterygoid); the mandibular symphysis (dentary-dentary); and the craniofacial hinge (a complex flexion zone involving four different pairs of skeletal elements). In both the ducklings and adults, the jaw, otic and palatobasal joints are all synovial, with a synovial cavity and articular cartilage on each surface (i.e. bichondral joints) ensheathed in a fibrous capsule. The craniofacial hinge begins as an ensemble of patent sutures in the duckling, but in the adult it becomes more complex: laterally it is synovial; whereas medially, it is synostosed by a bridge of chondroid bone. We hypothesize that it is chondroid bone that provides some of the flexible properties of this joint. The heavily innervated mandibular symphysis is already fused in the ducklings and remains as such in the adult. The results of this study will serve as reference for documenting avian cranial kinesis from a microanatomical perspective. The formation of: (i) secondary articular cartilage on the membrane bones of extant birds; and (ii) their unique ability to form movable synovial joints within two or more membrane bones (i.e. within their dermatocranium) might have played a role in the origin and evolution of modern avian cranial kinesis during dinosaur evolution.