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Digitale Medien

Evolutionary analysis of the middle and inner ear of Late Jurassic multituberculates (1993)

Lillegraven, Jason A. ; Hahn, Gerhard

Springer

Journal of mammalian evolution 1 (1993), S. 47-74

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ISSN: 1573-7055

Schlagwort(e): ear region ; Jurassic ; Multituberculata ; petrosal

Quelle: Springer Online Journal Archives 1860-2000

Thema: Biologie

Notizen: Abstract We describe three previously unreported specimens of petrosal bones of paulchoffatiid multituberculate mammals, collected from strata of Late Jurassic age in the Guimarota lignite mine of Leiria, west-central Portugal. The new fossils allow correction, supplementation, and confirmation of anatomical details, thus refining knowledge of general adaptation in the ear region among Jurassic multituberculates. Virtually all observed characters in the paulchoffatiid otic region are primitive relative to homologous features seen among Late Cretaceous and younger representatives of the Multituberculata; we recognize few unique otic specializations in paulchoffatiids that would preclude ancestry to later multituberculates. The plesiomorphic nature of paulchoffatiid ear regions provides no evidence in support of the hypothesis of a special, sister-group relationship between multituberculates and Late Cretaceous/Cenozoic marsupials plus placentals. Used in isolation, objective evidence derived from paulchoffatiid ear regions is consistent with interpretation of multituberculate divergence from other mammals predating the stem to living monotremes and postdating the stem to extinct morganucodontids. More broadly based comparative studies among Mesozoic mammals, however, suggest that independent acquisition of similarly advanced “mammalian” features was a pervasive theme among evolutionary histories of early mammals, probably including multituberculates. Although the phylogenetic position of multituberculates relative to other mammalian groups has yet to be unequivocally resolved, we suggest that a very early divergence of the group remains a distinct possibility.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1007/BF01027599

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Digitale Medien

Use of the term “Trophoblast” for tissues in therian mammals (1985)

Lillegraven, Jason A.

New York, NY : Wiley-Blackwell

Journal of Morphology 183 (1985), S. 293-299

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ISSN: 0362-2525

Schlagwort(e): Life and Medical Sciences ; Cell & Developmental Biology

Quelle: Wiley InterScience Backfile Collection 1832-2000

Thema: Biologie , Medizin

Notizen: Use of the term “trophoblast” in descriptions of therian (marsupial and eutherian) mammals has caused confusion because of misinterpretations of blastular homologies and because of imprecise application in functional versus ontogenetic-phylogenetic senses. Marsupials follow the plan of early development characteristic of noneutheian amniotes. Eutherians, in contrast, are unique in the early determination of presumptive embryonic versus extraembryonic cells through formation of inner cell mass versus trophoblastic (or trophectodermal) tissues, respectively. No cellular unit of the eutherian blastula is recognizable unequivocally as the homologue of a specific part of the protodermal marsupial blastula; progressive deletion of innovative but phylogenetically older ontogenetic steps probably figured importantly in the evolution of eutherian early embryogenesis. Because of marked differences in mode of formation and in cellular fates, homology of the blastocoel between marsupials and eutherians is questioned. It is suggested that use of the term “trophoblast” (1) be restricted to eutherians in discussions of ontogenesis or phylogenesis, and (2) be deemphasized in the functional sense (i.e., fetal-maternal exchanges) for marsupials, in favor of the more appropriate tissue terms of “choriovitelline” and “chorioallantoic” membranes. Integral to the origin of the eutherian style of embryogenesis was the evolution during Cretaceous time of neomorphic, extraembryonic tissues (i.e., trophoblast) having physiological properties that allowed the unique combination of (1) intimate apposition of fetal and maternal tissues and circulatory systems, along with (2) sustained, active morphogenesis. Marsupials have not achieved such a combination.