Naked Science Forum
Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: GlentoranMark on 19/04/2012 17:36:51
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And how do we know?
A Rhinocerous (is that how you spell it :))looks pretty similar to a Tricerotops so how do we know that the Tricerotops wasn't a mammal?
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No dinosaurs were reptiles. Just like no mammals are reptiles. We know they were their own family because the bones are different. Dinosaur bones are hollow like bird bones are. Mammal skulls are covered in bone for protection. Dinosaur skulls have holes in them and are much lighter, again like birds. Now, had you asked if all dinosaurs were birds, the answer would have still been no, but some of them became birds.
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"Where all dinosaurs reptiles?"
Yes they were and the Rhinoceros, regardless of how much you may think it resembles a Triceratops, is a mammal, they are certainly not descendents of the dinosaurs.
Perhaps your question should have been, "Where all dinosaurs dinosaurs?" Or even "Where all reptiles dinosaurs?" The fact is that we rather tend to use the term to describe anything which lived in the Jurassic and Cretaceous periods. But there were other animals around during these periods, including mammals and non dinosaur reptiles.
It is often said that the Tortoises and Crocodilians are 'surviving dinosaurs'. In fact, neither can be described as dinosaurs and the Tortoises were around long before any dinosaur, having first evolved in the Triassic period or possibly even the latter part of the Permian.
There are a number of features which determine whether a reptile was or was not a dinosaur, but perhaps the single most obvious feature is that the dinosaurs stood erect on their limbs, unlike the Tortoises, Crocodilians and modern lizards, where the body is low slung between limbs which are splayed out to the sides.
As for birds, it is pretty much accepted that these are the descendants of Theropod dinosaurs.
As to how we know that dinosaurs were reptiles, their fossilised remains bare very close similarities to the modern reptiles. Most laid eggs, though a few retained the eggs within the body until hatched. These eggs, however, were not similar to those of the mammals. The developing foetus was nourished by a yolk, as with other reptiles and the birds, not by any placental connection.
For more characteristics of the reptiles, take a look here (http://biology-today.com/general-zoology/reptilia-classification/).
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The list of positive indicators for classification as a dinosaur is as follows:
A detailed assessment of archosaur interrelations by S. Nesbitt[20] confirmed or found the following 12 unambiguous synapomorphies, some previously known:
in the skull, a supratemporal fossa (excavation) is present in front of the supratemporal fenestra
epipophyses present in anterior neck vertebrae (except atlas and axis)
apex of deltopectoral crest (a projection on which the deltopectoral muscles attach) located at or more than 30% down the length of the humerus (upper arm bone)
radius shorter than 80% of humerus length
fourth trochanter (projection where the caudofemoralis muscle attaches) on the femur (thigh bone) is a sharp flange
fourth trochanter asymmetrical, with distal margin forming a steeper angle to the shaft
on the astragalus and calcaneum the proximal articular facet for fibula occupies less than 30% of the transverse width of the element
exocciptials (bones at the back of the skull) do not meet along the midline on the floor of the endocranial cavity
proximal articular surfaces of the ischium with the ilium and the pubis separated by a large concave surface
cnemial crest on the tibia (shinbone) arcs anterolaterally
distinct proximodistally oriented ridge present on the posterior face of the distal end of the tibia
Nesbitt found a number of further potential synapomorphies, and discounted a number of synapomorphies previously suggested.
When I was in school (the dark ages) the only features taught were the skull holes - the fossa. These are the most distinctive features, I think.
WIkipedia -
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Well, yes. We know they had big hooters (That's noses!! For goodness sake try to control yourself) but were they warm or cold-blooded?
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Thanks for listing those defining anatomical features JB. I would agree that the fossa would be the most obvious anatomical feature of the skeletal remains we see today. I was rather thinking more on the lines of if you saw a few prehistoric reptiles taking a strole around 5th Ave., how you would tell which was which.
Well, yes. We know they had big hooters (That's noses!! For goodness sake try to control yourself) but were they warm or cold-blooded?
Big hooters!!! You must be refering to the Dolipartonasaur.
I think it can be safely said that most dinosaurs were ectothermic, but some are suggested to have evolved, to a degree (pun intended), to be endothermic. Fossils found in the colder latitudes, such as Alaska, can be explained by the fact that during the Mesozoic era the climate was warmer than it is today. But some of these fossils date from the end of the era (the late cretaceous) when temperatures would not have been high enough to support reptilian life at such latitudes. Could these have been migratory dinosaurs or were they the early stages of endothermy in the dinosaurs? Whichever the case may be, the KT event put an end to them. Had this event not happened, perhaps the dinosaurs would have evolved into two distinct lines, the ectothermic and the endothermic and life on Earth might look very different today.
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I can't include dinosaurs as reptiles. The evidence is fairly clear that dinos were warm blooded or at least lukewarm blooded. Just because they laid eggs and had scales is no reason to call them reptiles. Birds lay eggs and some have scales on their legs. Their bone structure was different and some dinos had feathers. Show me a reptile with feathers.
Dinosaurs were their own family separate and distinct from the reptiles. Morphological similarities caused them to be called reptiles early on, but we know so much more know and it is past time the association was ended.
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OokieWonderslug;
Yours is a very rash statement. Where is your evidence? Birds may be endotherms, but that is no evidence that their ancestors were. It may even be that the earliest avians were ectothermic.
You might like to read this (http://www.ucmp.berkeley.edu/diapsids/endothermy.html).
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The clincher for me is feathers. Feathers are a hindrance for a cold blooded animal. They insulate the animal and prevent heat from entering the body. There are no cold blooded animals with fur or feathers. Dinosaurs had feathers. Not all of them, but enough to get the picture that these creatures needed to conserve heat not collect it from outside.
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Yes, some dinosaurs had feathers and among these were undoubtedly the early ancestors of the birds. Both the avian and non avian dinosaurs would probably have had a common ancestor. But these avian dinosaurs may have split from the non avian dinosaurs as far back as the Triassic period.
There is some evidence that the avian dinosaurs evolved non feathered quills in the first place, with the actual feather filaments evolving later. But since feathers do not fossilise readily, this can only be a hypothesis.
Just what the purpose of these feathers was is open to debate. I think it can be ruled out that they were specifically for flight, though later in their evolution, they may well have given the gliders an edge. But the first feathers could have been for insulation. However, my suggestion is that this was not to conserve heat, but rather a shield from heat.
The 30 or so feathered dinosaurs which have been discovered are all relatively small. In the warmer climate of the Mesozoic era, these reptilians may have overheated if exposed to the sun for long periods. A layer of feathers would help them to remain in the sun for a little longer. However, toward the end of the Mesozoic era, the late cretaceous, temperatures begin to fall. The feathers would now act in reverse, conserving heat, as the avian dinosaurs shift to endothermy.
As for the larger non avian dinosaurs, endothermy would have presented a considerable problem with overheating. A dinosaur the size of Triceratops or Iguanadon and the much larger T. Rex and the sauropods would have had extreme difficulty regulating the core temperature of a body of such great mass. Again, remember that ambient temperatures were quite high. A large endothermic dinosaur might have reached maximum core temperature very quickly. Conversely, an ectothermic large dinosaur would be capable of remaining active for longer, without the need to seek shelter from the sun.
Solar panels on some dinosaurs, such as triceratops and stegosaurus are also an indicator of their ectothermic nature. Turned full on to the sun, these bony plates would help the animal reach working temperature fairly quickly and turned laterally to the sun would help reduce body temperature.
Whatever caused the KT event we can never know for sure, but it seems most likely that it was a celestial collision. This threw up much debris which would have had an effect on the ectothermic dinosaurs ability to regulate their body temperature. Hence they begin to die out. But the feathered avian dinosaurs, whether endo or ectothermic, would have had an advantage in that their feathers gave them the ability to either stay warm through their endothermy or retain for long periods heat absorbed whenever possible if ectothermic. Perhaps the shift from ecto to endothermy had already taken place, or was well under way at this time. In which case, those more advanced into endothermy would stand a better chance of survival than those (including the non avians) still reliant on the sun for heat.
I think the fact that only avian dinosaurs survived the KT event is a pointer to thermic ability in the dinosaurs and it points to ectothermy.
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There are no cold blooded animals with fur or feathers.
Spiders are cold-blooded, and many spiders, in fact, have fur.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fi2.asntown.net%2F4%2F_41779544_tarantula416.jpg&hash=aea1ba3f77dbeba9002913601134f247)
Some of the fur is for sensory purposes, but it isn't a big leap between sensory fur and insulating fur.
There are also furry lobsters, although I can't imagine it would be very insulating in an aqueous environment.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.mediatinker.com%2Fblog%2Fimages%2Fkiwahirsuta-thumb.jpg&hash=e7a24b5847056088ece087db6f90338b)
Other insects, such as flies also have hair or fur.
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As you rightly say, Clifford, there are many furry ectotherms. Bees, Butterflies, Moths.
Also, just picking up on OokieWonderslug's claim that the bone structure of dinosaurs points to them not being reptilian, the fibro-lamellar bone I think you are referring to exists in extant rapid growing reptiles, such as Turtles, crocodilians and lizards.
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... Feathers are a hindrance for a cold blooded animal. They insulate the animal and prevent heat from entering the body. There are no cold blooded animals with fur or feathers. Dinosaurs had feathers. Not all of them, but enough to get the picture that these creatures needed to conserve heat not collect it from outside.
This is the most unrehearsed and incorrect statement, set as an assertion, I have seen on this forum in a long time.
1. It is certain some dinosaurs were warm blooded.
2. Few, if any, dinos had feathers
3, feathers more often conserve heat by keeping it within the body that generated it rather than gathering it in. How else would a penguin survive the Antarctic water?
4. geckos have keratin hairs on their toes.
5. Many, many reptiles need to collect heat to function - rely ALL of them. Why do you think that lizards and other reptiles sun themselves in the early mornings? \\
(Oh, by the way, #5, above, is contradicted in the earlier part of O.W.'s post "Feathers are a hindrance for a cold blooded animal." The whole argument of these post of his is logically indefensible.)
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You must have read someone else's post and attributed it to me. You agree there were some warm blooded dinosaurs. You also agree that feathers are insulators. They insulate the wearer. I read today they found a dino in China that has furry feathers and lived in a cooler climate than it's non feathered cousins. Insulators insulate both ways. Not only do they keep the heat in, they also keep it out. A cold blooded creature would spend it's entire day trying to warm up in a coat of feathers. Where is the flaw? Why is there so much opposition to the idea that dinos were warm blooded? It was obvious from the first time I saw a dinosaur hip that they were birds or birds come from them. Birds are warm blooded. How could the logic that same hip, has feathers, equals warm blooded be indefensible? You might want to have a talk with Jack Horner over this.
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In a nature show yesterday, it suggested that size of snakes was related to environmental temperature.
The larger snakes (and larger lizard relatives) are located in the warmest regions.
Super-sized dinosaurs may have only been possible on a warmer planet due to passive temperature control mechanisms.