Mammalian types are evolved from reptilian ancestors. All modern mammals have certain characteristics not present in their reptilian ancestors, including: a middle-ear with jaw-bones displaced and modified into ear-bones for enhanced hearing; hair (fur) to enhance maintenance of self-regulating body-temperature; milk production from modified sweat-glands [mammary glands along a milk-line]. Evolutionary theory would have primitive mammalian ancestors having initially one of those traits, which evolved into a more advanced form having two of those traits, and finally evolving into a form having all three. Descendants of that most-evolved form are the monotremes [duck-billed platypus, etc.], which evolved a lineage from which are descended the marsupials, from which evolved a lineage from which are descended the placentals. The earliest origins of mammals are murky, and I know of no known fossils for those primitive types that had only one or two mammalian traits, but not all three. What evidence do we have as to which arose first - - fur, better-hearing, or milk production?
Fur and mammary glands are not the kind of tissue that fossilizes readily. Ear bones are. This is going to be a challenge. I know there are a few bits of fossilized reptilian skin here and there. Is there any fossilized fur anywhere? Is there a piece of fossilized mammalian skin that shows evidence of hair follicles? Or fossilized skin with nipples? I thought it was you who told us a couple of days ago that the monotremes are in fact a separate evolutionary line from marsupials and placental mammals, evolved from different reptilian ancestors. Was that somebody else?
Yes they did. That's why Walter referred to the "reptilian ancestors" of mammals. Birds are descended from reptiles too. You can see the last vestiges of scales on their feet.
Which came first the chicken or the egg? Neither, it was the egg laying dinosaurs. Please Register or Log in to view the hidden image!
Fraggle: Here's the link I posted in both the "cloaca disadvantage" thread I posted a few days ago, and the "platypus" thread that Oleander posted a few weeks ago: "The order Monotremata (one-holed creatures) is comprised of two families, the Ornithorhynchidae, including the platypus, and the Tachyglossidae, including the long- and short-beaked spiny anteaters or echidnas. Monotremes are found only in Australia, Tasmania, and New Guinea. Monotremes are a derivative of an ancient mammal stock but there is no direct evidence of what it might have been. Monotremes are not closely related to marsupials or placental mammals, but rather they evolved from a distinct group of reptilian ancestors. Despite sharing some reptilian features, monotremes possess all the major mammalian characteristics: air breathing, endothermy (i.e., they are warm-blooded), mammary glands, a furred body, a single bone in the lower jaw, and three bones in the middle ear. Monotremes have a reptilian-like shoulder girdle with distinct coracoid bones and a T-shaped interclavicle. Other reptilian-like skeletal features are present, including certain ribs and vertebral processes, as well as epipubic or "marsupium" bones. These bones are rudimentary and analogous to those that support a pouch in present-day marsupials. However, it seems more likely that these bones are a vestige from reptilian ancestors, associated with the attachment of strong abdominal muscles to support large hindquarters. Unlike higher mammals with separate reproductive and excretory systems, monotremes have a cloaca, with only one external opening for excretion and reproduction, as in birds and reptiles. In male monotremes, the penis is used only for the passage of sperm and not for urination as in other mammals. The overall pattern of reproduction is mammalian with a brief, vestigial period of development of the young in an external, soft-shelled egg. Once fertilized in the oviduct, the egg is covered with albumen and a tough, leathery shell forms. The egg is rounded, large-yolked, and compressible, rather than brittle like the eggs of birds. Echidnas develop a temporary pouch to incubate the egg and care for the young. The platypus does not develop a pouch and typically lays a single egg in a leaf nest. The mammae lack nipples, so the young lick milk from two lobules in the echidna's pouch or from the abdominal fur of the platypus. A three to six month period of maternal care is typical for monotremes. Certain shrews and monotremes are the only venomous mammals. In echidnas, the poison gland is present, but non-functional. Only the male platypus is capable of producing the venom and conveying it to a horny spur on the back of the ankle. Delivered by a forceful jab of the hindlimbs, the venom is powerful enough to cause agonizing pain in humans and can kill a dog. Although the exact nature of the venom system is unknown, it may have originated as a defense against some long extinct predator. Today, dingoes occasionally prey on echidnas, but in historical terms, dingoes are relatively recent arrivals in Australia. Because echidnas are widely hunted as food and the platypus is quite sensitive to changes in its habitat, monotremes are considered vulnerable in status." quoted from: http://science.jrank.org/pages/4435/Monotremes.html You will note that it is not quite correct when it asserts that monotremes arose from a separate lineage of reptiles, compared to the marsupials and placentals. The marsupials and placentals both arose from the modern-monotreme mammalian ancestors, with the placentals later evolving from the modern-marsupial ancestors, retaining the embryo inside the mother for an even longer period of time compared to the marsupials, allowing for fuller development of the embryo [and in many species, the ability to walk at birth]. Thank goodness that the higher mammals evolved the ability to use the penis to pee! There are a host of other mammalian features in monotremes, but not in reptiles. I focused my query on the three most prominent that readily distinguish them from their reptilian ancestors. I'm not certain that dinosaurs arose before the most primitive mammals. Certainly it appears that birds arose long after the primitive mammals, though I do not have the estimated time sequences before me as I write. Anyone have good information as to the estimated time-line for the origins of the three main extant mammalian lineages? How does this compare with the time-line for the origin of feathers [feathered-dinosaurs being a precursor to flying-feathered-dinosaurs, aka birds]?
granpa: Udders are modified nipples, not modified pouches. Apparently, pouches were either lost when no longer necessary; or else they are a more advanced trait of modern marsupials, not present in the primitive marsupials from which the placentals evolved.
The only hypothesis I've ever seen about the origin of the mammary glands is that they are modified sweat glands. It's the only one in Wikipedia. Nonetheless, baby marsupials obtain milk from the wall of the pouch. Your question can be restated, "Did the milk-producing tissue in the wall of the pouch evolve into milk-producing teats with nipples attached?"
It's a shame that we have to rely on encyclopedias in discussing science. Some historical thoughts on the topic. Haldane proposed the sweat gland theory in 1964. He compared the suckling of sweat to a behaviour seen in some birds, which suck water from their parents' feathers in a hot and arid environment. This would indicate that hairs where there first, before lactation. It was a counter theory to one of Darwin. We all might have heard of this fellow. Darwin proposed that lactation evolved from a behaviour we are all familiar with in reptiles and birds: the incubation of eggs and young. he proposed several steps. 1. the evolution of a vascularized incubation area. 2. a preadaptation of this area resulting in cutaneous secretions either for adhesiveness to eggs or providing moisture to eggs. 3. the evolution of the marsupium (the pouch). 4. the evolution of lapping of the moisture by the young. 5. evolution of sucking behaviour. The evolution of hair is not a crucial factor here. Haldane's sweat theory wasn't really regarded as feasible by many. For it to work mammals had to evolve in hot arid environments. Also small mammals do not have that many sweat glands for physiological reasons. Therefore lactation should have developed in larger mammalian creatures, which might not have been that abundant. A lot of this old school discussion was based on old school concept such as ontogeny recapitulates phylogeny. Hence they looked at what came first during embryological development. mammary glands come before sweat and sebaceous glands. It's not really a line of evidence we would rely on heavily nowadays.
More recent speculations on lactation Here is the central thought: the modern reptiles have little to do with the reptilian ancestors of the mammalians. The lineages that gave rise to modern reptiles and synapsids giving rise to modern mammals have separated a long time ago. We know that modern reptiles do not produce 'milk' for their young. But is this also the case with synapsids? Synapsids have started evolving away from the other reptiles a long long time ago, and eventually one of the lineages resulted in the evolution of mammals. But mammary glands are a soft tissue structure and hence not readily fossilize. Let's just speculate instead then. If we cross check ontogeny with phylogeny we notice that in humans the mammary ridge appears in the ontogeny at a time that corresponds to the early evolution of synapsids in the phylogeny. Could they already have had 'primitive' mammary glands? Were mammalian ancestors capable of suckling? Suckling is accomplished by having a secondary palate. it makes it possible to breath through the nose and eat at the same time. Some reptiles (for instance crocodiles) have a secondary palate and therefore it is usually not seen as a good marker for suckling in fossils. Cynognathus did have a fused secondary plate, indicating it might suckled. Or it might not of course. reference - the very very speculative article: Lawrence W Swan. Bioscience, 40, p376 1990.
No it isn’t. :bugeye: Encyclopaedias have traditionally been an invaluable source of accurate knowledge and information for school children and lay people alike for over 200 years. I would say that's pretty much the audience here at SciForums. Granted, Wikipedia is a different concept to the traditional encyclopaedias, and any info obtained from it needs to be treated with care. But I’m sure you are well aware of the Nature paper that compared numerous scientific articles between Encyclopaedia Britannica and Wikipedia and found that the later was only marginally behind the former in terms of scientific accuracy. Encyclopaedia Britannica dispute the methodology, but of course they would.
So you are saying it is not a shame that a discussion stalls because they have to rely on encyclopedias? What a weird attitude.
no i am asking whether the pouch evolved to hold the milk after it was produced by the sweat glands thereby becoming an udder.
Narrowing down the answer. It appears that fur appeared before ear-bones, as some primitive mammals, with fur, show absence of ear-bones having evolved (unlike all modern extant mammals). https://www.nytimes.com/2017/08/09/...170813&nl=morning-briefing&nlid=78646289&te=1 "The fossils of the new species, Maiopatagium and Vilevolodon, are exquisitely preserved, revealing many details of their anatomy. ... Dr. Luo and his colleagues discovered that the two new species are even more distantly related to living gliders than Volaticotherium. They belong to an extinct lineage called haramiyidans, which diverged from the ancestors of all living mammals over 200 million years ago. As a result, they had only some of the traits that define mammals today. While they had fur and were warm-blooded like living mammals, they were more like reptiles in some respects. They had not yet evolved the tiny chain of bones that allow living mammals to hear, for example."
I like, just wanted to know why you think a new thread should be started when this is a continuation of an existing thread, with new information recently developed?
I'm very patient, and very determined. The original thread was a query to direct new investigation. New research now uncovers fossils that sheds light on the original query. Still more investigation work to be done. Did milk come before ear-bones, or after? We now know that furry animals arose before ear-bones evolved. All modern animals (egg-laying, pouched, placental) have all three - ear-bones, milk, fur. The elucidation of the origin of mammals remains a fascinating area of research. Hopefully we will have more answers in the near future.
