The osteology of Rhombomylus (mammalia, glires): Implications for phylogeny and evolution of glires

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	The osteology of Rhombomylus (mammalia, glires): Implications for phylogeny and evolution of glires
	Meng, J; Hu, YM ; Li, CK; Meng, J (reprint author), Amer Museum Nat Hist, Div Paleontol, New York, NY 10024 USA.
	2003
发表期刊	BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY
ISSN	0003-0090
期号	275 页码:1-247
文章类型	Review
摘要	Rhombomylus is a gliroid mammal endemic to several early Eocene localities of central and eastern Asia. It is best represented by a collection of numerous jaws and teeth, dozens of juvenile and adult skulls. and associated postcranial specimens from the early Eocene Yu-huangding Formation at the Dajian village, Junxian County, Hubei Province, China. These specimens are the best preserved material of early nonrodent, nonlagomorph Glires known to date, and they provide a unique opportunity for us to understand anatomy, intraspecies variation, postnatal ontogeny, phylogeny, and evolution of basal gliroid mammals. This study opens with an introduction to the current knowledge of the phylogeny of gliroid mammals, a brief review of the research history of Rhombomylus, and a description of the locality where the specimens were collected. The core of the study consists of a taxonomic revision of the genus, a detailed description of the osteological morphology, extensive analyses on morphological characters, analyses on phylogeny, discussions on divergence time of the Glires, and analyses on functional morphology of mastication and locomotion. In comparing all known specimens assigned to the genus, we recognize only a single species. Rhombomylus turpanensis, for the genus and consider "R. laianensis" a junior synonym of R. turpanensis. This monotypic genus is rediagnosed. As a typical gliroid mammal, Rhombomylus possesses the following features: a single pair of enlarged, ever-growing incisors in both upper and lower jaws. the enamel band covering primarily the anterior and lateral surface of the incisor, a large diastema between the incisor and premolars, and a dental formula of dI2, CO, P3-P4, M1-M3/di2, c0, p3-p4. m1-m3. The single pair of upper incisors distinguishes Rhombomylus from duplicendentate Glires. Rhombomylus is more similar to Matutinia than to other simplicidentate Glires in having the following features: a broad hypocone shelf on the upper cheek tooth, a specialized zygomatic arch formed by the maxilla and jugal, an expanded mastoid of the petrosal that is exposed on the skull roof between the squamosal and parietal, an extensively interdigital suture between the premaxilla and frontal, a contribution of the ectotympanic to the medial wall of the glenoid fossa. and a distinct external meatus contacting the squamosal and blocking the postglenoid fossa posteriorly. Rhombomylus differs from Matutinia in being generally larger, in having higher tooth crowns, a stronger protocone and paracone, a complete vertical groove on the lingual surface of cheek teeth, and stronger crests on cheek teeth, and in lacking the carotid foramen in the auditory region. In general, Rhombomylus is more derived in morphology than is Matutinia. Although the general skull morphology of Rhombomylus is rodentlike, the dental morphology and detailed anatomy of the skull display unique Specialization of the genus. Upper tooth crowns are higher lingually than labially. Lophs are well developed. whereas cusps are not distinct. The incisor enamel is pauciserial and has two layers. with the outer layer being poorly differentiated. The enamel is thicker basally than apically in the same incisor and thicker in incisors of adult individuals than in juveniles. The basic enamel microstructure does not vary much at different sections of the same incisor. nor does it differ significantly in incisors from individuals of different ages. The thinnest enamel of the cheek teeth is on the occlusal surface, whereas the thickest is along the anterior surface of the upper tooth and the posterior wall of the lower tooth trigonid; these two regions are the main shearing surfaces of cheek teeth. Although the enamel microstructure varies from area to area on the same cheek tooth, there is always a region where the enamel microstructure is comparable to that of the incisor. The shearing facets on the cheek teeth bear numerous fine pits and striations. The orientation of the striations indicates a lateromedial movement of the mandible during mastication. Lack of enamel ridges on the occlusal surface and predominance of fine striations and pits on the wear facets of teeth suggest a dietary preference on soft vegetation. The skull in general is rodentlike, although the temporal region is primitively larger. Specialized cranial features include a strong zygomatic process of a complex shape, a complete bulla, expanded mastoid of the petrosal, exposure of the mastoid on the skull roof, loss of the internal carotid and stapedial arteries, the postglenoid foramen situated in the end of the glenoid fossa, and exclusion of the squamosal from the middle ear region. Cross sections show that the nasal cavity is simple and lacks any paranasal sinus except for possible existence of the maxillary sinus. The Jacobson's fossa is well developed. The ethmoid is similar to that of rodents in having four endoturbinals with the lamella of the second split to form two olfactory plates and a low number of ectoturbinals. Unlike other mammals in which the two olfactory plates of endoturbinal 11 inroll in the same direction, those of Rhombomylus inroll oppositely. In addition, entoturbinal III and IV fold dorsally instead of ventrally. The brain of Rhombomylus has relatively long olfactory lobes, small and unconvoluted cerebrum and cerebellum, and relatively narrow, long cerebrum. The postcranial skeleton of Rhombomylus is relatively conservative compared to the dentition and skull. The scapula is triangular in shape with distinct anterior and posterior angles. The clavicle is slim. The humerus has a low but prominent greater tubercle, a moderate deltopectoral crest, a moderate lateral epicondylar crest, and a deep olecranon fossa. Tile ulna has a large olecranon process and a deep trochlear notch. The hindlimb is Much longer than the forelimb. The femur has a high greater trochanter, a posteromedially directed lesser trochanter, a slender shaft, a deep patellar trochea, and an anteroposteriorly deep distal epiphysis. The tibia and fibula are also elongate and separate. Tile tibial crest is prominent. The upper ankle joint has the calcaneofibular contact. Based on the morphology of Rhombomylus the literature, and observation of specimens and casts of other mammals, we analyzed and tabulated 227 dental, cranial, and postcranial characters (608 states) among 50 terminal taxa. Characters were selected based on their use in previous studies, their amenability to description a,, discrete states, and a nonautapomorphic distribution. Terminal taxa include both fossil and extant genera and are representative of main Glires subgroups and of Other eutherians that have been previously considered to be relatives of gliroid mammals. Fossil Glires chosen for the studs are those represented by relatively complete material. Phylogenetic analyses were conducted using MacClade and PAUP*. Five strict consensus cladograms resulted from heuristic searches based oil three datasets, respectively: the dental, skeletal. and combined data matrix. Detailed information is provided for the preferred cladogram that was obtained from the combined data with all characters unordered. In all of tile cladograms, the clade of Glires is identified. In the preferred cladogram tile clade is supported by the highest Bremer index (5), branch length (45), and bootstrapping value (100). The sister group of the Glires is preferably the clade consisting of Anagale, Anagalopsis, and Pseudictops. In any case, zalambdalestids were only distantly related to the Glires. Within the Glires, clades of Duplicendentata. Simplicendentata, Eurymylidae, Lagomorpha, and Rodentia are identified. The clades of Duplicendentata and Lagomorpha are relatively robust, whereas that of the Simplicendentata is weakly Supported. Our analyses identified a clade consisting of Eurymylus. Heomys, Rhombomylus, and Matutinia, which are the core members of tile Eurymylidae. The phylogenetic position Of Sinomylus is unstable. In Rodentia, tile clade of tile Hystricognathi is robust. Tataromys forms the sister group of tile Hystricognathi-a clade named Hystricognathiformes. Sciurognath rodents are paraphyletic. Cavia is deeply nested in rodents its removal from rodents and attachment to tile primates or other clades significantly increases the steps of character transformations on tile cladogram. Phylogenetically constrained distributions of tile selected taxa indicate that the divergences of the Glires from other eutherian clades. between Lagomorpha and Rodentia. and among the main subgroups of rodents do not predate the Cretaceous-Tertiary boundary. Phylogenies and distributions of gliroid mammals at various levels support the traditional view that modern placental orders evolved after the Cretaceous-Tertiary boundary in an explosive pattern and disagree with hypotheses that attempt to reconcile the discrepancies between molecular clock dating and fossil records of modern placental divergences, These hypotheses include low preservation rates of Cretaceous mammals, lack of morphologically diagnostic features in Cretaceous taxa. unknown Cretaceous fossil records in southern continents, and deep superordinal branching of eutherian mammals in the Cretaceous. The masticatory apparatus and mastication of Rhombomylus are similar to those of lagomorphs in having upper check teeth wider than the lower ones. cheek teeth with transverse shearing ridges. main shearing facets On the anterior Surface of tile upper cheek tooth and tile posterior wall of the trigonid of the lower, the thickest enamel forming the main shearing edges, and power strokes of chewing unilaterally and buccilingually directed with both shearing and crushing. The masticatory apparatus and mastication of Rhombomylus are similar to those of rodents in absence of 13 and P2, an anteroposteriorly elongate glenoid fossa. a relatively long minimum working distance of the mandible, upper diastema longer than the lower one, and similarity in the general morphology of the mandible. The masticatory apparatus of Rhombomylus shows several primitive conditions compared with more advanced gliroid mammals. including posteriorly positioned masseter Muscles, larger coronoid process of the mandible and temporal region of the skull. slender incisors, enamel pauciserial, and primarily transverse movement of the mandible during mastication. The chewing cycle of Rhombomylus can be divided into six phases: rest position of the anisognathic dentition jaw opening and moving labially to the working side. vertical movement of the lower jaw to bring the upper and lower teeth into contacts transverse shearing between teeth; compression between the enlarged hypocone shelf and the trigonid and between the talonid basin and the protocone: and back to rest position. Conclusions on evolution of the masticatory apparatus of gliroid mammals include: (1) The propalinal movement of the mandible that permits gnawing and incisor sharpening is a gliroid synapomorphy. (2) Although all gliroid mammals can move lower jaws anteroposteriorly for incisal biting and sharpening, only some advanced forms are capable of propalinal mastication. The propalinal mastication is a derived condition and has been independently evolved several times in rodents. (3) The evolution of the gnawing function is prior to and independent of that of the propalinal chewing and is prior to the anterior shift of the jaw muscles. (4) Anterior shifting of the jaw muscles was originally and primarily to emphasize the strength of the incisal bite. (5) Modifications in jaw musculature occur in several directions independently among various gliroid groups. particularly rodents. (6) Acquisition of some cranial features such as the anteroposteriorly elongate glenoid fossa is originally related to gnawing and incisor sharpening. not to mastication. (7) Reduction in number of teeth is a general evolutionary trend in rodents; however, number of enamel bands functioning as grinding edges commonly increases. (8) The mastication in gliroid mammals evolved from primarily crushing and shearing to grindings the latter is more efficient for processing vegetable food with tough fabers. (9) The evolution of rodent mastication is a refining process that involves many aspects of the masticatory apparatus including the skull, mandible, jaw muscles. tooth morphologies. tooth size, enamel structures, and chewing rates. Refinement of various masticatory features evolved independently in different directions and in mosaic combinations. The heterochronical acquisition of masticatory features and the mosaic distributions of these features convey phylogenetic information at various levels for the Glires. Morphological analog with extant mammals indicate that the forelimbs of Rhombomylus do not show specialization toward either arboreal. terrestrial cursorial. or fossorial behavior. The forearms are probably capable of extensive movement in anteroposterior directions, but with limited abduction and adduction. They may also have the capacity for supination and pronation to some extent in food manipulation. The hindlimb of Rhombomylus shows many characteristics of terrestrial mammals. Some features even suggest capability of running or jumping. The hindlimb is much longer than the forelimb, indicating asymmetrical gait. Rhombomylus is basically a terrestrial mammal, capable of walking or jumping with an asymmetrical gait.; Rhombomylus is a gliroid mammal endemic to several early Eocene localities of central and eastern Asia. It is best represented by a collection of numerous jaws and teeth, dozens of juvenile and adult skulls. and associated postcranial specimens from the early Eocene Yu-huangding Formation at the Dajian village, Junxian County, Hubei Province, China. These specimens are the best preserved material of early nonrodent, nonlagomorph Glires known to date, and they provide a unique opportunity for us to understand anatomy, intraspecies variation, postnatal ontogeny, phylogeny, and evolution of basal gliroid mammals.
WOS标题词	Science & Technology ; Life Sciences & Biomedicine
关键词[WOS]	RABBIT ORYCTOLAGUS-CUNICULUS ; COMPARATIVE FUNCTIONAL-MORPHOLOGY ; RAT(RATTUS NORVEGICUS L) ; WILLEBRAND-FACTOR GENE ; RAT RATTUS NORVEGICUS ; REPUBLIC-OF-MONGOLIA ; OSSEOUS INNER-EAR ; 2 MURID RODENTS ; EUTHERIAN MAMMALS ; BRAIN TRAITS
收录类别	SCI
语种	英语
WOS研究方向	Biodiversity & Conservation ; Environmental Sciences & Ecology
WOS类目	Biodiversity Conservation ; Ecology
WOS记录号	WOS:000181325300001
引用统计
文献类型	期刊论文
条目标识符	http://119.78.100.205/handle/311034/3751
专题	中科院古脊椎所（2000年以后）古低等脊椎动物研究室古哺乳动物研究室
通讯作者	Meng, J (reprint author), Amer Museum Nat Hist, Div Paleontol, New York, NY 10024 USA.
作者单位	1.Amer Museum Nat Hist, Div Paleontol, New York, NY 10024 USA 2.Chinese Acad Sci, Inst Vertebrate Paleontol & Paleoanthropol, Beijing 100864, Peoples R China 3.CUNY City Coll, Grad Sch, Biol Program EEB, New York, NY 10031 USA
推荐引用方式 GB/T 7714	Meng, J,Hu, YM,Li, CK,et al. The osteology of Rhombomylus (mammalia, glires): Implications for phylogeny and evolution of glires[J]. BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY,2003(275):1-247.
APA	Meng, J,Hu, YM,Li, CK,&Meng, J .(2003).The osteology of Rhombomylus (mammalia, glires): Implications for phylogeny and evolution of glires.BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY(275),1-247.
MLA	Meng, J,et al."The osteology of Rhombomylus (mammalia, glires): Implications for phylogeny and evolution of glires".BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY .275(2003):1-247.

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