Estimation of Divergence Dates for Monotremes From Comparisons of A-Lactalbumin Amino Acid Sequences.
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Abstract:
cx-Lactalbumins were isolated from milk of the platypus (Ornithorhynchus anatinus) and the echidna (Tachyglossus aculeatus). Their amino acid sequences were determined and compared with those of the cx- lactalbumins often eutherian and two marsupial species, using the computer programme ("Distances") to calculate the number of differences (substitutions) between a total of 36 pairs of cx-lactalbumins. As expected, the amino acid sequences of the monotreme cx-lactalbumins were more similar to each other than to those of other mammals, as were the sequences of the marsupial and the eutherian cx-lactalbumins. If one makes the common assumption that marsupials and eutherians diverged from each other 135 Myr ago then simple calculations from the data would suggest that the platypus and echidna lineages diverged 56 ± 8 (SD) Myr ago and that monotremes diverged from the other mammals 152 ± 29 Myr ago. These values are not inconsistent with the little that is known about the palaeontology of the monotremes and are very similar to those derived from previous studies on globin sequences. If, however, monotreme cx-lactalbumins evolved more slowly than the cx-lactalbumins of eutherians and marsupials, these dates could be underestimates.Keywords:
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Theria
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Living mammals are divided into three groups: (1) monotremes (“prototherians”, three genera of egg-layers of the Australian realm); (2) marsupials (“metatherians”, the diverse “pouched mammals” of the Australian realm and the Americas); and (3) eutherians (“placentals”, the rest of us, with the greatest taxonomic and adaptive complexity, and having nearly cosmopolitan distribution). The marsupials plus eutherians are referred to as “therians,” a subclass that has a rich history of diversity of extinct varieties in addition to the living orders (Cassiliano and Clemens, 1979; Kraus, 1979; Kielan-Jaworowska, Eaton and Bown, 1979). The phylogenetic relationship between monotremes and modern therians is contested. Traditional wisdom had it that monotremes have been distinct from therians since virtually the beginnings of mammalian history (e.g., Marshall, 1979) in the Late Triassic or Early Jurassic. Kemp (1983), however, favored a closer relationship between monotremes and therians, with their presumed divergence at some later, but paleontologically undefined, time.
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Abstract The reproductive strategies and the extent of development of neonates differ markedly between the three extant mammalian groups: the Monotremata, Marsupialia, and Eutheria. Monotremes and marsupials produce highly altricial offspring whereas the neonates of eutherian mammals range from altricial to precocial. The ability of the newborn mammal to leave the environment in which it developed depends highly on the degree of maturation of the cardio‐respiratory system at the time of birth. The lung structure is thus a reflection of the metabolic capacity of neonates. The lung development in monotremes ( Ornithorhynchus anatinus, Tachyglossus aculeatus ), in one marsupial ( Monodelphis domestica ), and one altricial eutherian ( Suncus murinus ) species was examined. The results and additional data from the literature were integrated into a morphotype reconstruction of the lung structure of the mammalian neonate. The lung parenchyma of monotremes and marsupials was at the early terminal air sac stage at birth, with large terminal air sacs. The lung developed slowly. In contrast, altricial eutherian neonates had more advanced lungs at the late terminal air sac stage and postnatally, lung maturation proceeded rapidly. The mammalian lung is highly conserved in many respects between monotreme, marsupial, and eutherian species and the structural differences in the neonatal lungs can be explained mainly by different developmental rates. The lung structure of newborn marsupials and monotremes thus resembles the ancestral condition of the mammalian lung at birth, whereas the eutherian newborns have a more mature lung structure. Anat Rec, 2009. © 2008 Wiley‐Liss, Inc.
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Eutheria
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Abstract The os paradoxum or dumb-bell-shaped bone is a paired bone occurring in the middle of the specialized bill of the platypus Ornithorhynchus anatinus . It has been variously considered as a neomorph of the platypus, as the homologue of the paired vomer of sauropsids, or as a part of the paired premaxillae. A review of the near 200-year history of this element strongly supports the os paradoxum as a remnant of the medial palatine processes of the premaxillae given its ontogenetic continuity with the premaxillae and association with the vomeronasal organ and cartilage, incisive foramen, and cartilaginous nasal septum. In conjunction with this hypothesis, homologies of the unpaired vomer of extant mammals and the paired vomer of extant sauropsids are also supported. These views are reinforced with observations from CT scans of O. anatinus , the Miocene ornithorhynchid Obdurodon dicksoni , and the extant didelphid marsupial Didelphis marsupialis . At the choanae, Obdurodon has what appears to be a separate parasphenoid bone unknown in extant monotremes.
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Capra hircus
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cx-Lactalbumins were isolated from milk of the platypus (Ornithorhynchus anatinus) and the echidna (Tachyglossus aculeatus). Their amino acid sequences were determined and compared with those of the cx- lactalbumins often eutherian and two marsupial species, using the computer programme ("Distances") to calculate the number of differences (substitutions) between a total of 36 pairs of cx-lactalbumins. As expected, the amino acid sequences of the monotreme cx-lactalbumins were more similar to each other than to those of other mammals, as were the sequences of the marsupial and the eutherian cx-lactalbumins. If one makes the common assumption that marsupials and eutherians diverged from each other 135 Myr ago then simple calculations from the data would suggest that the platypus and echidna lineages diverged 56 ± 8 (SD) Myr ago and that monotremes diverged from the other mammals 152 ± 29 Myr ago. These values are not inconsistent with the little that is known about the palaeontology of the monotremes and are very similar to those derived from previous studies on globin sequences. If, however, monotreme cx-lactalbumins evolved more slowly than the cx-lactalbumins of eutherians and marsupials, these dates could be underestimates.
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Platypus
Theria
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Monotreme
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Theria
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Monodelphis domestica
Evolution of mammals
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The claustrum, which comprises the claustrum proper and the endopiriform nucleus, is generally thought to be present in all mammals. Some previous reports of its possible absence in monotremes have appeared in the literature, but the question of its presence or absence in this clade has not been formally addressed. Whether monotremes have a claustrum is of some importance for formulating and evaluating hypotheses relating to the evolution of the structures in the lateral sector of the pallium across amniotes. Archival sets of sections through the brains of the platypus and the short-beaked echidna were examined and included material stained for seven different histochemical and immunohistochemical protocols. No cytoarchitectonically distinct claustrum could be identified in this material for either monotreme. We thus conclude that if monotremes have any cell population that is homolgous to the claustrum of therian mammals, it is entirely cryptic. A claustrum might have been present in ancestral mammals and lost in the monotreme clade, or it might have been gained at the origin of therian mammals. Nonetheless, its absence as a cytoarchitectonically discrete and identifiable structure in monotremes fails to support homology of the claustrum of therian mammals with any single part of the sauropsid pallium.
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This paper reviews present understanding of the evolution of the ornithorhynchids. An ancient family within the mammalian order Monotremata. Ornithorhynchidae today is represented only by the living platypus Ornithorhynchus analinus but has a history that probably predates the Tertiary and a past distribution that spanned at least three continents. Analysis of the palaeontological record has focused on the distinctive ornithorhynchid dentition, which in species of Monotrematum and Obdurodon was probably functional throughout life. The retention of functional dentition in concert with the great age of Ornithorhynchidae gives the platypus family a much larger role in analysis of the biogeographic and phylogenetic history of the monotremes than is given to the more specialised, edentate tachyglossids, or echidnas. A complete ornithorhynchid skull, recovered from Miocene deposits at Riversleigh in north-western Queensland, has allowed comparison between the cranium of a generally more plesiomorphic platypus and that of the living Or. anatinus, a study that answers some of the morphological questions posed by this enigmatic group while raising others. This review concludes with a discussion of the biogeography and palaeoecology of the family.
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The origin of placental mammals is the most important event in the mammalian evolutionary history because placentals make up more than 90% of all living mammals.Placental mammals are the world's most diverse mammal group characterized by a placenta that provides nourishment for unborn young.From bats to whales,from elephants to rodents,placental mammals are the most important mammal group of the world,playing crucial roles in modern ecosystems.Placental mammals are considered to be very important,because human and closely related primates are placentals.The problems as to when,where,and how eutherians (including placentals) originated in the Earth's history are some of the most important aspects in understanding the evolution.Chinese and American paleontologists have now reported their discovery of a remarkably well-preserved fossil from Jianchang County,Liaoning Province of China in a paper published in the prestigious journal Nature.Named Juramaia sinensis or the mother from China, this fossil from Middle Jurassic Tiaojishan Formation represents the earliest-known fossil of the eutherian-placental lineage.It shows that a new milestone in mammal evolution that was 35 million years earlier than the previous Cretaceous record.This new discovery fills an important gap in the fossil records and helps to calibrate modern DNA-based methods of dating the mammalian evolution.The age of Juramaia sinensis helps to establish the date when eutherian mammals diverged from other mammals:metatherians (whose descendants include marsupials such as kangaroos) and monotremes (such as the platypus).Understanding the beginning of placental mammals is a crucial issue in the evolutionary studies of all mammals.The date of an evolutionary divergence—when an ancestor species splits into two descendant lineages — is among the most important pieces of information that evolutionary biologists and paleontologists can have.Molecular studies can estimate the timing of evolution by a clock. But the molecular clock needs to be verified and tested by the fossil record.Prior to the discovery of Juramaia,DNA evidence suggested that eutherians should have shown up earlier in the fossil record—around 160 million years ago.Yet,the oldest known eutherian was 125 million years ago,previously represented by Eomaia from the Early Cretaceous Yixian Formation.This gap between molecular evidence and fossils was an important issue to be resolved for evolutionary biology and paleontology.The discovery of Juramaia gives a much earlier fossil evidence to corroborate the DNA findings,filling an important gap in the fossil record of early mammal evolution and helping to establish a new milestone,known as fossil calibration point,for mammalian evolutionary history.This Jurassic fossil provides new information about the earliest ancestors of today's placental mammals.By the scientists' analyses,Juramaia is either a great-grand-aunt or a 'great-grandmother' of all placental mammals that are thriving today.The fossil has an incomplete skull of about 22 mm long,part of the skeleton,and,remarkably,impressions of residual soft tissues such as hairs.It was an insectivorous mammal as indicated by its teeth.It was estimated to have a body mass of 13 grams.Most importantly,Juramaia's complete forelimb and hand bones enable paleontologists to interpret that it is a climbing mammal.This shows that the earliest eutherian evolution is correlated with the new adaptations.Eutherian mammals were a new lineage for the Jurassic Period.The adaptive features of the eutherians may have helped the new lineage to survive in a Jurassic ecosystem dominated by dinosaurs and other vertebrates.The ability to explore the trees and to escape to the canopy might have allowed eutherians to exploit a new niche,inaccessible and untapped by the majority of the Jurassic mammals that lived exclusively on the ground.
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