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Xu, X., Z.X. Luo, and J.Y. Rong (2010) Recent advances in Chinese palaeontology. Proc Biol Sci 277:161-164 Discoveries are a driving force for progress in palaeontology. Palaeontology as a discipline of scientific inquiry has gained many fresh insights into the history of life, from the discoveries of many new fossils in China in the last 20 years, and from the new ideas derived from these fossils. This special issue of Proceedings of Royal Society B entitled Recent Advances in Chinese Palaeontology selects some of the very latest studies aimed at resolving the current problems of palaeontology and evolutionary biology based on new fossils from China. These fossils and their studies help to clarify some historical debates about a particular fossil group, or to raise new questions about history of life, or to pose a new challenge in our pursuit of science. These works on new Chinese fossils have covered the whole range of the diversity through the entire Phanerozoic fossil record. Gao, C.L., G.P. Wilson, Z.X. Luo, A.M. Maga, Q. Meng, and X. Wang (2010) A new mammal skull from the Lower Cretaceous of China with implications for the evolution of obtuse-angled molars and 'amphilestid' eutriconodonts. Proc Biol Sci 277:237-246 We report the discovery of Juchilestes liaoningensis, a new genus and species of eutriconodont mammal from the Lujiatun Site of the Lower Cretaceous Yixian Formation (123.2 +/- 1.0 Ma; Lower Aptian). The holotype preserves a partial skull and full dentition. Among eutriconodonts, its lower dentition is similar to taxa formerly assigned to the paraphyletic group of 'amphilestids'. Some have considered 'amphilestid' molars to represent the structural intermediate between the lower molars of the 'triconodont' pattern of cusps in alignment and the fully triangulate and more derived therian molars. However, 'amphilestid' taxa were previously represented only by the lower dentition. Our study reveals, for the first time, the upper dentition and skull structure of an 'amphilestid', and shows that at least some eutriconodonts have an obtuse-angled cusp pattern on molars in middle positions of the long molar series. Its petrosal is similar to those of other eutriconodonts and spalacotheroid 'symmetrodonts'. Our phylogenetic analyses suggest that (i) Juchilestes is most closely related to the Early Cretaceous Hakusanodon from Japan, in the same Eastern Asiatic geographic region; (ii) 'amphilestids' are not monophyletic; and (iii) eutriconodonts might not be a monophyletic group, although this hypothesis must be further tested. Ji, Q., Z.X. Luo, X. Zhang, C.X. Yuan, and L. Xu (2009) Evolutionary development of the middle ear in Mesozoic therian mammals. Science 326:278-281 Ruf, I., Z.X. Luo, J.R. Wible, and T. Martin (2009) Petrosal anatomy and inner ear structures of the Late Jurassic Henkelotherium (Mammalia, Cladotheria, Dryolestoidea): insight into the early evolution of the ear region in cladotherian mammals. J Anat 214:679-693 The petrosal anatomy and inner ear structure of Jurassic cladotherian mammals represent the ancestral morphological conditions (groundplan) from which modern therian mammals (marsupials and placentals) have evolved. We present the reconstruction of the petrosal and inner ear features of the Late Jurassic dryolestoid mammal Henkelotherium guimarotae from high-resolution computed tomography and three-dimensional imaging analysis. This study of Henkelotherium revealed a combination of derived and primitive features, including: cladotherian apomorphies, such as the promontorial sulcus for the internal carotid artery and reduced lateral trough; trechnotherian characters, such as an enclosed cochlear canaliculus for the perilymphatic duct, post-promontorial tympanic sinus and caudal tympanic process; in addition to plesiomorphic mammalian features, such as the cavum supracochleare and prootic canal. The inner ear of Henkelotherium shows a division between the utricle and saccule, a cochlear canal coiled through at least 270 degrees, a distinctive primary bony lamina for the basilar membrane, and a secondary bony lamina. The development of the primary and secondary bony laminae in the cochlear canal is suggested here to be correlated with the concurrent coiling of the bony canal and membranous duct of the inner ear cochlea, apomorphies of the more inclusive cladotherian clade that also represent the ancestral morphotype of modern therian mammals. Because these features are crucial for high-frequency hearing in extant therian mammals, their early appearance in Late Jurassic cladotherians suggests a more ancient origination for high-frequency hearing in mammalian history than previously thought. Luo, Z.X., P. Chen, G. Li, and M. Chen (2007) A new eutriconodont mammal and evolutionary development in early mammals. Nature 446:288-293 Detachment of the three tiny middle ear bones from the reptilian mandible is an important innovation of modern mammals. Here we describe a Mesozoic eutriconodont nested within crown mammals that clearly illustrates this transition: the middle ear bones are connected to the mandible via an ossified Meckel's cartilage. The connected ear and jaw structure is similar to the embryonic pattern in modern monotremes (egg-laying mammals) and placental mammals, but is a paedomorphic feature retained in the adult, unlike in monotreme and placental adults. This suggests that reversal to (or retention of) this premammalian ancestral condition is correlated with different developmental timing (heterochrony) in eutriconodonts. This new eutriconodont adds to the evidence of homoplasy of vertebral characters in the thoraco-lumbar transition and unfused lumbar ribs among early mammals. This is similar to the effect of homeobox gene patterning of vertebrae in modern mammals, making it plausible to extrapolate the effects of Hox gene patterning to account for homoplastic evolution of vertebral characters in early mammals. Luo, Z.X., Q. Ji, and C.X. Yuan (2007) Convergent dental adaptations in pseudo-tribosphenic and tribosphenic mammals. Nature 450:93-97 Tribosphenic molars of basal marsupials and placentals are a major adaptation, with the protocone (pestle) of the upper molar crushing and grinding in the talonid basin (mortar) on the lower molar. The extinct pseudo-tribosphenic mammals have a reversed tribosphenic molar in which a pseudo-talonid is anterior to the trigonid, to receive the pseudo-protocone of the upper molar. The pseudo-protocone is analogous to the protocone, but the anteriorly placed pseudo-talonid is opposite to the posterior talonid basin of true tribosphenic mammals. Here we describe a mammal of the Middle Jurassic period with highly derived pseudo-tribosphenic molars but predominantly primitive mandibular and skeletal features, and place it in a basal position in mammal phylogeny. Its shoulder girdle and limbs show fossorial features similar to those of mammaliaforms and monotremes, but different compared with those of the earliest-known Laurasian tribosphenic (boreosphenid) mammals. The find reveals a much greater range of dental evolution in Mesozoic mammals than in their extant descendants, and strengthens the hypothesis of homoplasy of 'tribosphenic-like' molars among mammals. Luo, Z.X. (2007) Transformation and diversification in early mammal evolution. Nature 450:1011-1019 Evolution of the earliest mammals shows successive episodes of diversification. Lineage-splitting in Mesozoic mammals is coupled with many independent evolutionary experiments and ecological specializations. Classic scenarios of mammalian morphological evolution tend to posit an orderly acquisition of key evolutionary innovations leading to adaptive diversification, but newly discovered fossils show that evolution of such key characters as the middle ear and the tribosphenic teeth is far more labile among Mesozoic mammals. Successive diversifications of Mesozoic mammal groups multiplied the opportunities for many dead-end lineages to iteratively evolve developmental homoplasies and convergent ecological specializations, parallel to those in modern mammal groups. Li, G., and Z.X. Luo (2006) A Cretaceous symmetrodont therian with some monotreme-like postcranial features. Nature 439:195-200 A new spalacotheriid mammal preserved with a complete postcranium and a partial skull has been discovered from the Yixian Formation of Liaoning, China. Spalacotheroid symmetrodonts are relatives to modern therians (combined group of marsupials and placentals) and are characterized by many skeletal apomorphies of therians. But unlike the closely related spalacotheroids and living therians, this new mammal revealed some surprisingly convergent features to monotremes in the lumbar vertebrae, pelvis and hindlimb. These peculiar features may have developed as functional convergence to locomotory features of monotremes, or the presence of lumbar ribs in this newly discovered mammal and their absence in its close relatives might be due to evolutionary developmental homoplasy. Analysis including this new taxon suggests that spalacotheroids evolved earlier in Eurasia and then dispersed to North America, in concordance with prevailing geodispersal patterns of several common mammalian groups during the Early Cretaceous period. Ji, Q., Z.X. Luo, C.X. Yuan, and A.R. Tabrum (2006) A swimming mammaliaform from the Middle Jurassic and ecomorphological diversification of early mammals. Science 311:1123-1127 A docodontan mammaliaform from the Middle Jurassic of China possesses swimming and burrowing skeletal adaptations and some dental features for aquatic feeding. It is the most primitive taxon in the mammalian lineage known to have fur and has a broad, flattened, partly scaly tail analogous to that of modern beavers. We infer that docodontans were semiaquatic, convergent to the modern platypus and many Cenozoic placentals. This fossil demonstrates that some mammaliaforms, or proximal relatives to modern mammals, developed diverse locomotory and feeding adaptations and were ecomorphologically different from the majority of generalized small terrestrial Mesozoic mammalian insectivores. Martin, T., and Z.X. Luo (2005) Paleontology. Homoplasy in the mammalian ear. Science 307:861-862 Luo, Z.X., and J.R. Wible (2005) A Late Jurassic digging mammal and early mammalian diversification. Science 308:103-107 A fossil mammal from the Late Jurassic Morrison Formation, Colorado, has highly specialized teeth similar to those of xenarthran and tubulidentate placental mammals and different from the generalized insectivorous or omnivorous dentitions of other Jurassic mammals. It has many forelimb features specialized for digging, and its lumbar vertebrae show xenarthrous articulations. Parsimony analysis suggests that this fossil represents a separate basal mammalian lineage with some dental and vertebral convergences to those of modern xenarthran placentals, and reveals a previously unknown ecomorph of early mammals. Dooley, A.C., N.C. Fraser, and Z.-X. Luo (2004) The earliest known member of the rorqual-gray whale clade (Mammalia, Xetacea). J. Vertebr. Paleontol. 24:453-463 Eobalaenoptera harrisoni, gen. et sp. nov., is described from a partial skeleton collected from the middle Miocene Calvert Formation of Virginia. Characteristics of this taxon, particularly of the petrosal, indicate that the new whale is a member of the clade that includes the Balaenopteridae (rorquals) and Eschrichtidae (gray whales) to the exclusion of "cetotheres" and the Balaenidae (right whales). Some of the probable synapomorphies of this clade include an elongate pars cochlearis, a tubular internal auditory meatus, the greater petrosal nerve foramen on the tympanic side of the petrosal, the stylomastoid fossa extending onto the posterior process of the petrosal, no medial groove on the pars cochlearis. four digits on each forelimb, depressed supraorbital processes, and ascending processes of maxillae extending onto the vertex. The approximate 14-million-year age of the specimen makes it the oldest known member of the clade by some 3 to 5 million years, and extends the fossil record of this clade closer to the divergence time estimated by some recent molecular studies. You, H.L., Z.X. Luo, N.H. Shubin, L.M. Witmer, Z.L. Tang, and F. Tang (2003) The earliest-known duck-billed dinosaur from deposits of late Early Cretaceous age in northwest China and hadrosaur evolution. Cretaceous Res. 24:347-355 A new dinosaur of Early Cretaceous age was recently discovered in the Gobi Desert of northwest China. It is more closely related to Late Cretaceous hadrosaurids than to Early Cretaceous iguanodontids. It occupies the most basal position in the phylogeny of all duck-billed dinosaurs, or the Hadrosauroidea. This early hadrosauroid sheds new light on the origin of the herbivorous feeding specializations of the Late Cretaceous duck-billed dinosaurs, and corroborates the view that the Iguanodontidae and the Hadrosauroidea are monophyletic clades, with the former characterized by an enlarged maxilla as the main mechanism for mastication, and the latter diagnosed by a smaller yet more mobile maxilla with an elaborate dental battery, separated by a diastema from the enlarged premaxilla. Our study also suggests that the Hadrosauroidea had most likely originated in Asia in the Early Cretaceous before this clade diversified and spread to other Laurasian continents during the Late Cretaceous. Shapiro, M.D., H.L. You, N.H. Shubin, Z.X. Luo, and J.P. Downs (2003) A large ornithomimid pes from the lower cretaceous of the Mazongshan Area, northern Gansu Province, People's Republic of China. J. Vertebr. Paleontol. 23:695-698 You, H.L., F. Tang, and Z.X. Luo (2003) A new basal Titanosaur (Dinosauria : Sauropoda) from the early cretaceous of China. Acta Geol. Sin-engl 77:424-429 A new dinosaur of Early Cretaceous age was recently discovered from the Mazongshan area of northwestern Gansu Province, China. The new dinosaur represents a new genus and species of Sauropoda, and is among the most basal members of Titanosauria. Its finding also suggests that titanosaurs might have originated in Asia no later than the Early Cretaceous. Luo, Z.-X., Q. Ji, J.R. Wible, and C.-X. Yuan (2003) An Early Cretaceous Tribosphenic mammal and metatherian evolution. Science 302:1934-1940 Derived features of a new boreosphenidan mammal from the Lower Cretaceous Yixian Formation (125 ma) of China suggest that it has a closer relationship to metatherians including extant marsupials, than to eutherians including extant placentals. This fossil extends the record of marsupial relatives with skeletal remains by 50 million years. It also has many foot structures known only from climbing and tree-living extant mammals, suggesting that early crown therians exploited diverse niches. New data from this fossil support the view that Asia was likely the center for the diversification of earliest metatherians and eutherians during the Early Cretaceous. Kielan-Jaworowska, Z., R.L. Cifelli, and Z.-X. Luo (2002) Dentition and relationships of the Jurassic mammal Shuotherium. Acta Palaeontol. Pol. 47:479-486 The Middle Jurassic mammal Shuotherium has lower molars that possess a trigonid and talonid, but are unique in having the talonid situated in front of the trigonid, rather than behind it, as in molars of usual tribosphenic pattern. Shuotherium dongi Chow and Rich, 1982 was based on a dentary bearing seven teeth, originally interpreted as three premolars and four molars. Based on comparison with other groups of early mammals, we reinterpret the premolar-molar boundary in the holotype of S. dongi, and propose a dental formula of four (or more) premolars and three molars. The ultimate lower premolar (previously identified as the first molar) has a completely developed trigonid and no talonid or pseudo-talonid. We hypothesize that the mesial cingulid on molars of Australosphenida is a highly plausible structural antecedent to the pseudo-talonid of Shuotherium. This and other shared, derived features support a relationship of Shuotherium and Australospherrida as sister-taxa. We hypothesize that the common ancestor of Shuotherium + Australosphenida had a global distribution no younger than early Middle Jurassic, and that the respective clades diverged prior to full separation of Gondwanan and Laurasian landmasses. Luo, Z.-X., Z. Kielan-Jaworowska, and R.L. Cifelli (2002) In quest for a phylogeny of Mesozoic mammals. Acta Palaeontol. Pol. 47:1-78 We propose a phylogeny of all major groups of Mesozoic mammals based on phylogenetic analyses of 46 taxa and 275 osteological and dental characters, using parsimony methods (Swofford 2000). Mammalia sense lato (Mammaliaformes of some authors) are monophyletic. Within mammals, Sinoconodon is the most primitive taxon. Sinoconodon, morganucodontids, docodonts, and Hadrocodium lie outside the mammalian crown group (crown therians + Monotremata) and are, successively, more closely related to the crown group. Within the mammalian crown group, we recognize a fundamental division into australosphenidan (Gondwana) and boreosphenidan (Laurasia) clades, possibly with vicariant geographic distributions during the Jurassic and Early Cretaceous. We provide additional derived characters supporting these two ancient clades, and we present two evolutionary hypotheses as to how the molars of early monotremes could have evolved. We consider two alternative placements of allotherians (haramiyids + multituberculates). The first, supported by strict consensus of most parsimonious trees, suggests that multituberculates (but not other alllotherians) are closely related to a clade including spalacotheriids + crown therians (Trechnotheria as redefined herein). Alternatively, allotherians can be placed outside the mammalian crown group by a constrained search that reflects the traditional emphasis on the uniqueness of the multituberculate dentition. Given our dataset, these alternative topologies differ in tree-length by only similar to0.6% of the total tree length; statistical tests show that these positions do not differ significantly from one another. Similarly, there exist two alternative positions of eutriconodonts among Mesozoic mammals, contingent on the placement of other major mammalian clades. Of these, we tentatively favor recognition of a monophyletic Eutriconodonta, nested within the mammalian crown group. We suggest that the "obtuse-angle symmetrodonts" are paraphyletic, and that they lack reliable and unambiguous synapomorphies. Ji, Q., Z.-X. Luo, C.-X. Yuan, J.R. Wible, J.-P. Zhang, and J.A. Georgi (2002) The ealiest known eutherian mammal. Nature 416:816-822 The skeleton of a eutherian (placental) mammal has been discovered from the Lower Cretaceous Yixian Formation of northeastern China. We estimate its age to be about 125 million years (Myr), extending the date of the oldest eutherian records with skull and skeleton by about 40 50 Myr. Our analyses place the new fossil at the root of the eutherian tree and among the four other known Early Cretaceous eutherians, and suggest an earlier and greater diversification of stem eutherians that occurred well before the molecular estimate for the diversification of extant placental superorders (104 64 Myr). The new eutherian has limb and foot features that are known only from scansorial (climbing) and arboreal (tree-living) extant mammals, in contrast to the terrestrial or cursorial (running) features of other Cretaceous eutherians. This suggests that the earliest eutherian lineages developed different locomotory adaptations, facilitating their spread to diverse niches in the Cretaceous. Luo, Z.-X., Z. Lielan-Jaworowska, and R.L. Cifelli (2001) In quest for a phylogeny of Mesozoic mammals. Acta Palaeontol. Pol. 47:1-78 We propose a phylogeny of all major groups of Mesozoic mammals based on phylogenetic analyses of 46 taxa and 275 osteological and dental characters, using parsimony methods (Swofford 2000). Mammalia sense lato (Mammaliaformes of some authors) are monophyletic. Within mammals, Sinoconodon is the most primitive taxon. Sinoconodon, morganucodontids, docodonts, and Hadrocodium lie outside the mammalian crown group (crown therians + Monotremata) and are, successively, more closely related to the crown group. Within the mammalian crown group, we recognize a fundamental division into australosphenidan (Gondwana) and boreosphenidan (Laurasia) clades, possibly with vicariant geographic distributions during the Jurassic and Early Cretaceous. We provide additional derived characters supporting these two ancient clades, and we present two evolutionary hypotheses as to how the molars of early monotremes could have evolved. We consider two alternative placements of allotherians (haramiyids + multituberculates). The first, supported by strict consensus of most parsimonious trees, suggests that multituberculates (but not other alllotherians) are closely related to a clade including spalacotheriids + crown therians (Trechnotheria as redefined herein). Alternatively, allotherians can be placed outside the mammalian crown group by a constrained search that reflects the traditional emphasis on the uniqueness of the multituberculate dentition. Given our dataset, these alternative topologies differ in tree-length by only similar to0.6% of the total tree length; statistical tests show that these positions do not differ significantly from one another. Similarly, there exist two alternative positions of eutriconodonts among Mesozoic mammals, contingent on the placement of other major mammalian clades. Of these, we tentatively favor recognition of a monophyletic Eutriconodonta, nested within the mammalian crown group. We suggest that the "obtuse-angle symmetrodonts" are paraphyletic, and that they lack reliable and unambiguous synapomorphies. Luo, Z-X., A.W. Crompton, and A.L. Sun (2001) A new mammaliaform from the Early Jurassic of China and evolution of mammalian characteristics. Science 292:1535-1540 A fossil from the Early Jurassic (Sinemurian, approximately 195 million years ago) represents a new lineage of mammaliaforms, the extinct groups more closely related to the living mammals than to nonmammaliaform cynodonts. It has an enlarged cranial cavity, but no postdentary trough on the mandible, indicating separation of the middle ear bones from the mandible. This extends the earliest record of these crucial mammalian features by some 45 million years and suggests that separation of the middle ear bones from the mandible and the expanded brain vault could be correlated. It shows that several key mammalian evolutionary innovations in the ear region, the temporomandibular joint, and the brain vault evolved incrementally through mammaliaform evolution and long before the differentiation of the living mammal groups. With an estimated body weight of only 2 grams, its coexistence with other larger mammaliaforms with similar "triconodont-like" teeth for insectivory within the same fauna suggests a great trophic diversity within the mammaliaform insectivore feeding guild, as inferred from the range of body sizes. Tang, F., Z.-X. Luo, Z.H. Zhou, H.L. You, J.A. Georgi, Z.L. Tang, and X.Z. Wang (2001) Biostratigraphy and palaeoenvironment of the dinosaur-bearing sediments in Lower Cretaceous of Mazongshan area, Gansu Province, China . Cretaceous Res. 22:115-129 This paper discusses the lithostratigraphy of the Xinminbao Group in the Mazongshan area of Gansu Province, northwestern China, and the correlation of its biota. The Xinminbao Group was deposited in a fluviolacustrine setting in Mesozoic graben basins under a semi-arid, subtropical climate. The fossil sites concentrated in the lower part of the group have yielded dinosaurs, mammals and other vertebrates that are comparable to the Aptian-Albian vertebrate assemblages that have been found in Mongolia and Siberia. Based on their invertebrate and vertebrate fossils, sediments of the lower part of the Xinminbao Group are considered to be Late Barremian-Aptian in age, although the middle and upper parts could be as young as Albian. The megafossil plants and conchostracans are comparable to those of the Yixian and Jiufuotang Formations in eastern China (Barremian-Aptian). The palynoflora is also comparable to that of the upper Lower Cretaceous elsewhere in northern China, although the presence of two angiosperm taxa, Asteropollis and Tricolpites, suggests that the main fossil sites are Albian. These biostratigraphic data for the Mazongshan area provide a new temporal calibration for the terrestrial biotas of the Lower Cretaceous of central Asia, where most of the known dinosaur-bearing sediments lack precise palynological, radiometric and magnetostratigraphic dating. Luo, Z.-X., R.C. Cifelli, and Z. Kielan-Jaworowska (2001) Dual evolution of tribosphenic mammals. Nature 409:53-57 Marsupials, placentals and their close therian relatives possess complex (tribosphenic) molars that are capable of versatile occlusal functions. This functional complex is widely thought to be a key to the early diversification and evolutionary success of extant therians and their close relatives (tribosphenidans). Long thought to have arisen on northern continents, tribosphenic mammals have recently been reported from southern landmasses. The great age and advanced morphology of these new mammals has led to the alternative suggestion of a Gondwanan origin for the group. Implicit in both biogeographic hypotheses is the assumption that tribosphenic molars evolved only once in mammalian evolutionary history. Phylogenetic and morphometric analyses including these newly discovered taxa suggest a different interpretation: that mammals with tribosphenic molars are not monophyletic. Tribosphenic molars evolved independently in two ancient (holotherian) mammalian groups with different geographic distributions during the Jurassic/Early Cretaceous: an australosphenidan clade endemic to Gondwanan landmasses, survived by extant monotremes; and a boreosphenidan clade of Laurasian continents, including extant marsupials, placentals and their relatives. Luo, Z.-X. (2000) Evolution - In search of the whales' sisters. Nature 404:235 Luo, Z.-X. (1999) Palaeobiology - A refugium for relicts. Nature 400:23 Luo, Z.-X., and P.D. Gingerich (1999) Transition from terrestrial ungulates to aquatic whales: transformation of the basicranium and evolution of hearing. Monogr. Ser. Mus. Paleontol., Papers Paleontol. 31:1-98 Ji, Q., Z.-X. Luo, and S. Ji (1999) A Chinese triconodont mammal and mosaic evolution of mammalian skeleton. Nature 398:326-330 Here we describe a new triconodont mammal from the Late Jurassic/Early Cretaceous period of Liaoning, China. This new mammal is represented by the best-preserved skeleton known so far for triconodonts which form one of the earliest Mesozoic mammalian groups with high diversity. The postcranial skeleton of this new triconodont shows a mosaic of characters, including a primitive pelvic girdle and hindlimb but a very derived pectoral girdle that is closely comparable to those of derived therians. Given the basal position of this taxon in mammalian phylogeny, its derived pectoral girdle indicates that homoplasies (similarities resulting from independent evolution among unrelated lineages) are as common in the postcranial skeleton as they are in the skull and dentition in the evolution of Mesozoic mammals. Limb structures of the new triconodont indicate that it was probably a ground-dwelling animal. Kielan-Jaworowska, Z., R.L. Cifelli, and Z.-X. Luo (1998) Alleged Cretaceous placental from down under. Lethaia 31:267-268 Luo, Z.-X. (1998) Homology and transformation of the cetacean ectotympanic bullae. Pp 269-301 in Evolutionary Emergence of Whales, Thewissen, J.G.M., Ed. Plenum Press, New York Hu, Y., Y. Wang, Z.-X. Luo, and C. Li (1997) A new symmetrodont mammal from China and its implications for mammalian evolution. Nature 390:137-142 A new symmetrodont mammal has been discovered in the Mesozoic era (Late Jurassic or Early Cretaceous period) of Liaoning Province, China. Archaic therian mammals, including symmetrodonts, are extinct relatives of the living marsupial and placental therians. However, these archaic therians have been mostly documented by fragmentary fossils. This newfossil taxon, represented by a nearly complete postcranial skeleton and a partial skull with dentition, is the best-preserved symmetrodont mammal yet discovered. It provides a new insight into the relationships of the major lineages of mammals and the evolution of the mammalian skeleton. Our analysis suggests that this new taxon represents a part of the early therian radiation before the divergence of living marsupials and placentals; that therians and multituberculates are more closely related to each other than either group is to other mammalian lineages; that archaic therians lacked the more parasagittal posture of the forelimb of most living therian mammals; and that archaic therians, such as symmetrodonts, retained the primitive feature of a finger-like promontorium (possibly with a straight cochlea) of the non-therian mammals. The fully coiled cochlea evolved later in more derived therian mammals, and is therefore convergent to the partially coiled cochlea of monotremes. Luo, Z.-X., and K. Marsh (1996) Petrosal (periotic) and inner ear of a Pliocene kogiine whale (Kogiinae, Odontoceti): Implications on relationships and hearing evolution of toothed whales. J. Vertebr. Paleontol. 16:328-348 This study describes the petrusal (periotic) bone and the inner ear of a fossil pygmy sperm whale (Kogiinae, Physeteridae, Cetacea) from the Yorktown Formation (Pliocene) of the Lee Creek Mines of North Carolina. A cladistic analysis of 36 petrosal characters among 18 cetacean taxa shows that extant and fossil kogiines are diagnosed by a plate-like and posteriorly oriented posterior process of the petrosal. The Physeteridae is diagnosed by two petrosal apomorphies: presence of an incudal process for the articulation of the incus, and an enlarged accessory ossicle. Petrosal apomorphies suggest that physeterids and ziphiids are sister groups among the odontocetes, and that tile Mysticeti and Odontoceti are both monophyletic. The cochlear spiral of the Pliocene kogiine petrosal has two full turns and a cochlear basal/height ratio of 0.6. Within the cochlear canal, the secondary bony lamina for the basilar membrane is present in one and one-quarter turns of the cochlea. The width of the basilar membrane, as inferred from the laminar gap, is much narrower in the fossil kogiine than in an Oligocene squalodontid-like whale. The primary bony lamina contains numerous foramina for the cochlear nerve. The spiral ganglion canal is well developed in one and three-quarter turns oi the cochlea. These cochlear structures are associated with high-frequency hearing in modern toothed whales. Comparison of the inner ear between kogiines and other odontocetes suggests that the fossil kogiine could hear about the same frequency ranges as extant delphinoids, and much higher frequencies than the Oligocene squalodontoid. The vestibule of the kogiine petrosal is a small and curved tube; thr semicircular canals are very reduced. These specialized features are the most unequivocal synapomorphies in the odontocete inner ears. Geisler, J., and Z.-X. Luo (1996) Petrosal and inner ear structures of Herpetocetus, their implications on relationships and hearing function of archaic mysticetes. J. Paleontol. 70:1045-1066 This paper describes the petrosal (periotic) and the inner ear of Herpetocetus sp., an archaic mysticete whale (Mysticeti, Mammalia) from the Yorktown Formation (Pliocene) of North Carolina, USA. Parsimony analysis of 28 petrosal characters of Herpetocetus sp. and 11 other cetacean taxa supports the monophyly of mysticetes and the division of odontocetes and mysticetes. The in-group taxa of this analysis are: Herpetocetus, Parietobalaena, Pelocetus, Balaenidae, Eschrichtius, and Balaenopteridae. Odontocetes and the archaeocete Zygorhiza were used as successive outgroups to root phylogenetic trees and to establish character polarities. Among the modem mysticetes, the Balaenopteridae (rorquals) and the Eschrichtiidae (gray whales) are more closely related to each other than either is to the Balaenidae (bowhead and right whales). Several Miocene ''cetotheriid'' mysticetes and balaenids share some resemblance in the petrosal, suggesting their affinities. Quantitative information of the inner ear of Herpetocetus sp. was obtained by serial sectioning and computer graphic reconstruction. Herpetocetus sp. is much less developed than odontocetes in the cochlear structures that are crucial for high frequency hearing. Some cochlear structures in this fossil mysticete resemble more closely the non-echolocating modem mysticetes than early fossil toothed whales, indicating a possible specialization in low frequency hearing. This suggests that the archaic mysticetes of the Miocene and Pliocene did not have high frequency hearing necessary for echolocation. Herpetocetus sp. is similar to modern mysticetes but different from odontocetes in the spherical shape of the vestibule. Luo, Z.-X., and E.R. eastman (1995) Petrosal and inner-ear of a squalodontoid whale - implications for evolution of hearing in Odontocetes. J. Vertebr. Paleontol. 15:431-442 This study focuses on the petrosal and bony structures of the inner ear of a fossil squalodontoid whale (Odontoceti, Mammalia) from the Chandler Bridge Formation (late Oligocene) of South Carolina. High frequency hearing in toothed whales (odontocetes) has been attributed to specialized bony structures and their associated membranes in the inner ear cochlea. Whales also have very peculiar vestibule and semicircular canals by comparison to terrestrial mammals. By using serial grinding and computerized reconstruction, we identified in this fossil squalodontoid whale several inner ear structures specialized for high frequency hearing: 1) a well developed secondary bony lamina for the basilar membrane in the basal 3/4 cochlear turn; 2) numerous foramina for the ganglion cells of the auditory nerve and a partially preserved spiral ganglion canal; 3) great size difference between the scala tympani and the scala vestibuli (including the scala media); and 4) wide separation between the cochlear turns. All these structures are correlated with high frequency hearing in modern odontocetes. The basilar membrane width, as inferred from the gap of the bony laminae preserved in the fossil, is greater in this squalodontoid than in modern delphinoids. The squalodontoid also has a higher cochlear spiral than modern delphinoids. These indicate that frequency of maximum sensitivity was lower in squalodontoids than in modern delphinoids. The vestibule of the squalodontoid is a very narrow and curved passage to the fenestra vestibuli. The semicircular canals are small and thin. These highly specialized structures suggest that specializations of the inner ear occurred prior to the diversification of all extant toothed whales, of which squalodontoids are the stem group. Luo, Z.-X., A.W. Crompton, and S.G. Lucas (1995) Evolutionary origins of the mammalian promontorium and cochlea. J. Vertebr. Paleontol. 15:113-121 Mammals have two apomorphies in the ear region: an elongated cochlear canal and an eminence on the tympanic side of the cochlear housing, known as the petrosal promontorium. In nonmammalian cynodonts, the cochlear recess is shorter, smaller, and oriented more medially than the mammalian cochlear canal; their cochlear housing is formed by the prootic, opisthotic, and basisphenoid. New fossil materials of two early mammals, Adelobasileus and Sinoconodon, reveal evidence on the evolutionary transformation of the mammalian cochlear canal and bony cochlear housing. Adelobasileus is more derived than any known non-mammalian cynodont in possessing an incipient promontorium, but more primitive than other mammals in retaining a vestigial basisphenoid wing. Sinoconodon has a fully developed petrosal promontorium but a short cochlea that extends only one third of the promontorium length. The cochlear canal of Morganucodon is twice as long as that of Sinoconodon, relative to both skull and promontorium. More derived mammals, such as Haldanodon and multituberculates, have an even longer cochlear canal. We propose that the promontorium transformed by expansion of the petrosal bone at the expense of the basisphenoid (probably also the basioccipital) through the transition from non-mammalian cynodonts to mammals. This change in ossification of the cochlear housing. resulted in a re-orientation of the cochlea and a better acoustic insulation of the inner ear. The mosaic of a fully developed promontorium and a short cochlear canal in Sinoconodon suggests that the formation of the promontorium preceded (and thus may be necessary for) the elongation of the mammalian cochlea. Luo, Z.-X. (1994) Sister taxon relationships of mammals and the transformations of the diagnostic mammalian characters. Pp 98-128 in The Shadow of Dinosaurs--Early Mesozoic Tetrapods, Fraser, N.C., and H.D. Sues, Ed. Cambridge University Press, Cambridge Luo, Z.-X., and A.W. Crompton (1994) Transformations of the quadrate (incus) through the transition from non-mammalian cynodonts to mammals. J. Vertebr. Paleontol. 14:341-374 The quadrate (incus) bone underwent important evolutionary transformations through the cynodont-mammal transition. The following character transformations played crucial roles in modifying the cynodont quadrate into the mammalian incus: 1) progressively greater rotation of the dorsal plate relative to the trochlea; 2) the contact facet of the dorsal plate becomes concave; 3) development of a constricted neck between the dorsal plate and the trochlea; 4) simplification of the quadrate-cranium joint, resulting in better mobility of the joint; and 5) introduction of a stapedial process (crus longum). The dorsal plate rotation, the concave contact facet, the constricted neck, the mobile joint of the quadrate and the cranium are also present in some advanced non-mammalian cynodonts. Broad phylogenetic distributions of these features suggest that the major features of the incus of early mammals, as represented by Morganucodon, originated much earlier in phylogenetic history among non-mammalian cynodonts. Apomorphies of the quadrate (incus) among the advanced non-mammalian cynodonts favor a sister-group relationship of tritheledontids and mammals. The hypothesis on the postdentary origin of the mammalian tympanic membrane is favored by transformation of the quadrate through the cynodont-mammal transition. Three most important modifications of the quadrate (incus) through the cynodont-mammal transition are: formation of the concave contact facet, progressively greater rotation of the dorsal plate, and decrease in the number of cranial bones articulating with the quadrate. These modifications would simplify the quadrate-cranial joint and increase the mobility of the quadrate (incus) relative to the cranium while a functioning tympanic membrane was maintained on the mandible, improving the sensitivity of the postdentary tympanum. Probainognathus is among the earliest known non-mammalian cynodonts with a concave contact facet and a rotated dorsal plate in the quadrate. Thus we hypothesize that it represents a critical step in the phylogenetic transformation that led to the origin of the modern mammalian middle ear and tympanic membrane. |
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