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Why did mammals evolve to have two testes?

Why did mammals evolve to have two testes?


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What makes mammals tend to evolve to have two testes?


EvoDevo is not my field, but I will try to give you some pointers.

Mammals as vertebrates, start out as fertilized oocytes, transform into a rapidly dividing lump of cells with eventual polarity through numerous gradients of numerous chemical messengers, and form a neural tube. The forming body is ever more segmented through the use of gradients and thresholds (see french flag model) and (dorsoventral) axial symmetry is maintained throughout development, and as such two testes represent the default development.

Therefore, it may be more interesting to ask for singular organs which require more information, and thus maintenance throughout evolution of this information. But when considering the energy requirements, the need for synchronization, the need for a separate blood supply and thus the introduction of additional faults, it is easy to see why we only have one heart.

Wikipedia; author: Zephyris, title: "The first few weeks of embryogenesis in humans", DOR: 25/09/2012

See:

I hope this helps as a starting point.


Elephant DNA Reveals the Evolutionary Reason (Most) Mammal Balls Drop

Puberty is full of ups and downs — sometimes literally. The moment when testicles descend is a pivotal one in the male life cycle, but as scientists point out in a PLOS Biology study published Thursday, we shouldn’t take it for granted. There was a point in evolutionary history when mammals branched into two groups: those who drop, and those who do not.

In the paper, the team from the Max Planck Institute of Molecular Cell Biology and Genetics shows, using DNA extracted from 71 species, why some male members of the “placental mammal” family — that includes us, and any other animal born from a womb — don’t experience testicular descent. Those animals belong to the Afrotherian family, a group of modern African mammals including manatees, elephants, and some insectivores. Instead of dropping their balls at puberty, these animals keep them close. For a long time, it’s been unclear when and why these animals evolved this trait.

It’s weird because testicle descent has an established biological purpose: keeping sperm factories cool. Testicles are primarily the site of human sperm production, and they function best at a temperature slightly lower than that of the body. Hanging in the scrotum, a few inches below the body, balls stay cool. But not, apparently, those belonging to elephants and their Afrotherian kin. Their balls hang out inside the body where they originally formed, somewhere around the kidneys.

It’s difficult to pinpoint the moment in mammalian evolutionary history their ball-retaining trait developed because it’s not easy to get any information about soft tissues, like testicles, from fossils. But at some point in history, the researchers write, there had to be a common ancestor of all mammals, and finding out whether that ancestor’s balls dropped would clarify when the Afrotherians’ weird characteristic occurred. So they set out to find it.

Since they didn’t have ancient soft tissue to work with, they had to use a roundabout way of determining which animals hung low and which didn’t. They turned to two genes, RXFP2 and INSL3, which are known to spur the development of an organ called the gubernaculum, the ligament that tugs the balls downward during puberty. Afrotherians and their kin, the team hypothesized, wouldn’t have these genes, or at least no working versions of them

Sure enough, their analysis of the genomes extracted from the 71 placental mammals showed that the hypothesis was true for four Afrotherians, namely the tenrec, cape elephant shrew, cape golden mole, and manatee. These animals lack functional versions of these genes, the researchers discovered, but what they do have are remnants of those genes, suggesting that the ancestor of all placental mammals did indeed have testicular descent. In other words, ball dropping was always the default, and it’s the Afrotherians that are a bit weird.

“These ‘molecular vestiges’ show that testicular descent was already present in the placental ancestor and was subsequently lost in Afrotheria,” the team writes.

Comparing the genes of the Afrotherian group to those of the other mammals showed that the split into ball-droppers and ball-keepers happened about 100 million years ago, when the Afrotherian group split from the ancestor of placental mammals. The mutations that made the gubernaculum genes nonfunctional in the four species, the team writes, occurred around 20 to 80 million years ago, which is fairly recent in evolutionary time. Furthermore, they found, the mutations in each of these species occurred independently. (Strangely, these genes are intact in elephants and rock hyrax, so there’s still no explanation for why their balls don’t drop.)

Now that we know that the balls of the ancestor of all placental mammals did drop, two big questions remain: What benefit did testicle retention confer on the four Afrotherian animals affected by the genes, and how are they still able to make sperm? Maybe it’s because the tenrecs and cape golden moles have a body temperature slightly lower than most (35 ̊C as opposed to 37 ̊C), but the same can’t be said for elephants and elephant shrews, who are just about as warm as we are. This aspect of their undescended testes remain a mystery, but we can safely assume one thing: They don’t have to be worried about getting kicked in the nuts.


Evolution of the mammalian vagina

The title and that little picture to the left ought to be hint enough, but if not, read on.

A: The vagina. Aren't we lucky?

There's an old joke going around about poor design: what kind of designer would route the sewer pipes right through the center of the entertainment center? It's a good point. It doesn't make sense from a design standpoint to have our reproductive and excretory systems so intimately intermingled, but it does make a heck of a lot of sense from a purely historical point of view. In a sense, reproduction is an excretory function—we are shedding gametes produced internally, and we already have a perfectly good set of pipes running from our insides to the outside, so why not use them? It's just that in our lineage, which has specialized in giving great care to our gametes and zygotes, that plumbing has become increasingly elaborate, and that part of the system that was once just a convenient throughway has become a destination and a long-term residence in its own right.

Development tells us part of the story. The reproductive and urinary tracts are all tangled together in early development, arising together from two pairs of ducts, the Müllerian and Wolffian ducts, which are modified in complex ways to form a series of kidneys (we keep only the last one, the metanephros), one set of pathways for the male testes, and yet another set for the female ovaries.

In non-therian mammals, all of these complicated pipes have one common destination, a single outlet to the external world: the cloaca. "Cloaca" is Latin for sewer, and it is appropriately named. The terminus of the large intestine is here, as well as the ends of the ureters from the kidneys and the ducts from the ovaries or testes. Everything gets dumped in to the cavity of the cloaca, making a nice stew of feces, urine, and sperm or eggs. Mmm-mmm. The cloaca is the grey cylinder at the bottom of figure A, below, in the first three organisms, amphibians, birds/reptiles, and monotremes (my apologies for the murkiness of the image it's the best copy I have).


(Click for larger image)
Evolution of the tetrapod reproductive system. (A) Female urogenital system from major tetrapod lineages. Inf,
infundibulum Ov, ovary Ovd, oviduct Ut, uterus (or shell-producing region in non-therian animals) Vg, vagina Kd, kidney
Ud, urinary duct Rc, rectum Ub, urinary bladder Cl, cloaca. (B) Tetrapod egg. *, the shell coat of birds and some reptiles
is highly calcified. MPS, marsupial-specific mucopolysaccharide layer. (C) Tetrapod phylogeny showing major transitions
in mammalian reproduction. Divergence of amphibians and amniotes (A). Divergence of birds/reptiles and mammals
(B). Divergence of monotremes and therians (C). Divergence of marsupials and placentals (D).

The fundamental organization of the reproductive part of the vertebrate urogenital tract is straightforward: it's a tube with a funnel at one end that captures eggs released by the ovary, and conducts them to an external orifice. Along the way, cells lining the tube secrete useful products like albumin and yolk, and deposit a shell, and may act to temporarily store the egg before its final release.

Marsupial and placental mammals have dispensed with most of those functions, and expanded on others. One part of the oviduct has acquired a richly vascularized epithelium and specializations for investing and nurturing a resident embryo, becoming a uterus. That's an amazing and innovative function in itself, but in addition, it has formed a new, separate channel, the vagina. The vagina is an entirely new structure, which has no homolog in amphibians or reptiles.

That is an interesting observation. It's a wholly original structure that arose sometime after the monotreme-marsupial split, an evolutionary novelty. How did that happen? How can we study a unique event that occurred over 150 million years ago?

Wagner and Lynch have a proposal to answer both questions. The general mechanism for generating novel structures is evo-devo orthodoxy:

  1. An epigenetic side effect of other evolutionary changes in the body leading to a novel physical structure in the organisms.
  2. The genetic consolidation and individuation of the novel structure.

(Note that this proposes phenotype before genotype, which is somewhat heretical for neodarwinism. It shouldn't trouble the evo-devo gang in the slightest, of course.)

How to study such a process from the past?

The basic assumption of a molecular evolutionary approach to the study of evolutionary novelties
is that changes in developmental regulation have
left traces in the molecular structure of the
genome and a comparative study of genomic
structures should be able to identify genetic
changes coincidental with a phenotypic novelty. (emphasis mine)

That process of consolidation and individuation would have left detectable scars in the genome—the genes involved would have acquired changes necessary to fix the phenotype in the population. Again, as we'd expect from the evo-devo perspective, those changes would have been made to the regulatory genes that control tissue-specific gene expression. What genes should we examine? Let's look at the therian organs of interest, and here are some likely candidates: the HoxA genes that have region-specific domains in the female reproductive tract.


Hox gene expression pattern and the evolution of
the female reproductive tract. (a) HoxA-13 to HoxA-9 are
located at the 5' end of the HoxA cluster and are expressed in
the same regions in the adult as in the embryo: HoxA-13
(green), HoxA-11 (yellow), HoxA-10 (orange) and HoxA-9
(blue). (b) Tetrapod phylogeny showing representative female
reproductive systems from each group (amphibian ovaries
shown only on the left).

The HoxA-9 through HoxA-13 genes are expressed in order along the length of the embryonic Müllerian duct, and also continue to be expressed in adulthood so the cells of the vagina are all expressing HoxA-13, while the cells of the cervix all have HoxA-11 turned on (for some reason, I find that to be a wonderful piece of knowledge, and I just have to say…Hooray for HoxA-13! It has just become my favorite Hox gene.)

So the question is whether there is any evidence that these particular Hox genes have signs of any set of changes that are associated with particular transitions in vertebrate evolution—in particular, are there differences that can be traced to the transition between monotremes and the theria, and between the placentals and marsupials. The answer seems to be yes: the diagram to the right is a measure of the number of synonymous to nonsynonymous changes in HoxA-11, which is an indicator of the selective pressures that have shaped the gene.

Furthermore, they've identified where these changes have occurred, and they are not in the homeodomain (the part of the protein that binds to specific sequences in the DNA, but in the amino terminal end.

The 3-D models below show where the relevant amino acids (in yellow) end up in the folded protein. The interesting thing here is that regulatory proteins don't just interact with each other, but also with other regulatory proteins that are simultaneously binding. It's a whole chain of interactions—regulatory proteins binding to the DNA, and also binding between each other in a complex called the enhancersome—that determines the level of expression of a particular gene.


HoxA-11 protein structure. This three-dimensional protein model was calculated by comparative modeling as part of
the MODBASE project. (A) Model shown as ribbons. (B) Model rendered with a molecular surface. The DNA-binding
homeodomain is shown in red. The carboxy-terminal region of exon 2 is shown in blue. Residues identified as being under
directional (positive) selection in the stem lineage of eutherians are shown in yellow. Residues replaced in the stem lineage of
therians but not identified under selection are shown in green. Note that all of these amino acid sites affect amino acids that are
predicted to be placed on the surface of the molecule as expected if selection is driven by novel protein-protein interactions.

There is a great deal left to be done. Hox genes are rather high up the chain of regulatory genes, so there are many more genes downstream that have to be puzzled out. We also are a long ways from figuring out how these patterns of gene expression define the morphogenetic processes that create this lovely novel structure, the vagina. The important thing, though, is that there are these questions waiting to be answered—the investigators have a research program.

We propose that a research program to explain
evolutionary novelties has to focus on the question
of whether novel characters arise through the
evolution of novel regulatory links among developmental genes. We further propose that a
detailed analysis of the evolution of developmental
genes involved in the development of a derived,
novel character can reveal molecular changes that
could be causally involved in the origin of evolutionary novelties. The case study presented here
suggests that the statistical methods of molecular
evolution are strong enough to provide specific
hypothesis for experimental test. The success of
this research program will depend on the ability to connect the patterns of molecular evolution with
the functional role of these molecular changes.

That's the cool thing about evolutionary biology: exciting questions, titillating ancestors, and the promise of tools to answer more.

Lynch VJ, Roth JJ, Takahashi K, Dunn CW, Nonaka DF, Stopper GF, Wagner GP (2004) Adaptive evolution of HoxA-11 and HoxA-13 at the origin of the uterus in mammals. Proc Biol Sci. 271(1554):2201-7.

Wagner GP, Lynch VJ (2005) Molecular evolution of evolutionary novelties: the vagina and uterus of therian mammals. J Exp Zoolog B Mol Dev Evol. [Epub ahead of print]


Monogamy mystery

This observation clashes with the fact that men are significantly larger than women. This suggests our evolutionary background involved a significant degree of polygynous, rather than exclusively monogamous, mating. This is supported by anthropological data showing that most modern human populations engage in polygynous marriage. Anthropologists Clellan Ford and Frank Beach in their book Patterns of Sexual Behaviour suggested that 84% of the 185 human cultures they had data on engaged in polygyny.

Primates with simpler penises tend to be monogamous like cotton top tamarins (a) or polygynous like gorillas (g). Alan F. Dixson, Primate Sexuality

However, even in these societies most people remain monogamous. Polygynous marriages are usually a privilege reserved only for high status or wealthy men. It is worth noting that hunter-gathers around the world practice only monogamy or serial-monogamy which suggests that our ancestors may have used this mating system.

At first sight, however, it would seem sensible for males to reproduce with as many females as possible. Human monogamy has long puzzled anthropologists, and lots of effort has gone in to working out what keeps males hanging around.

Three main theories have been put forward. First is the need for long-term parental care and teaching, as our children take a long time to mature. Second, males need to guard their female from other males. Third, our children are vulnerable for a long time and infanticide could be a risk from other males. So to ensure that children are able to reach maturity the male is likely to stay to protect them, both socially and physically. This may be why males have maintained their larger relative size.

Hamadryas baboons have unusually long penises. المُصوّر: مُعتز توفيق إغباريّة, CC BY-SA

If we view the evolution of monogamy mating systems in humans through the lens of human society it is clear that it takes a huge amount of social effort to maintain and protect more than one mate at a time. It is only when males have access to additional resources and power that they can protect multiple females, usually by ensuring other males protect them. So monogamy seems to be an adaptation to protect one’s mate and children from other males. This monogamy is reinforced by the high social cost and stress of attempting to do this for multiple partners, and it has become supported by cultural norms.

So when living in complex human societies the largest and most important sexual organ is the brain. Somewhere in our evolutionary past how smart and social we are became the major control on our access to sexual partners – not how big or fancy a male’s penis is.


Answer

We posed this question to Dr. Sebastian Shimeld from the University of Oxford and Dr. Robert Whitaker from the University of Cambridge.

Sebastian - Now that's a really interesting question and it's really got two answers - one of which is how we develop in the womb, but there's also an evolutionary explanation to this which is how we got to be in this situation in the first place. It's not just us that are bilaterally symmetrical. All vertebrates are - be they birds, reptiles, frogs, or fish. In fact, not just vertebrates but almost all other animals are bilaterally symmetrical as well. This includes worms and flies. This is because bilateral symmetry evolved a very long time ago, at least 500 or 600 million years ago. Our body plan has been locked into bilateral symmetry since that point. This brings me onto the last part of the question which is - could it change? I think given that we've been locked into this body plan for such a long period of time it's unlikely that it's going to change. I wouldn't say completely impossible because there are one or two organisms, or one or two animals, which have managed to change this. A really good example of this is the octopus which not only has a major heart but has managed to evolve two ancillary hearts as well to help its blood flow. So, unlikely to change I think, but perhaps not completely impossible given enough time and the right selection. Diana - And the developmental point of view from Dr. Robert Whitaker at Cambridge University. Robert - The obvious first reaction of many people would be to just suggest that multiple identical organs are simply there for spare parts, but I do not believe that this is the correct explanation. I'd like to look at the conundrum from a developmental point of view. The early embryo has an outer layer, a single midline tube passing from mouth to anus to become the gut. From the single and simple midline tube, is developed the intestines. However many other organs develop from it by a system of budding from the tube. Such organs include the lungs, the liver, the pancreas. and whether these become a single organ or two organs depends on whether the bud that grows from tube stays as a single bud or divides to grow more than one. The liver for instance is a single organ whereas the lung comes from two buds to give the organs that we see in the developed child. So what about the kidneys I hear you ask? Well, they develop not as a single tube, as with the gut, but on either side of the body quite separately. There's a fundamental difference between there being two parts to a single organ, for example the lungs and the brain which all develop from a single outgrowth, as opposed to two separate organs with identical functions such as the kidneys, the ovaries, the testicles, which all develop on separate sides of the body.

Diana - So it's a combination of midline symmetry inherited from our fishy ancestors, a result of our developmental processes as budding embryos and as with many things, it's like that because it works. In some cases, it's always good to carry a spare.


RELATED ARTICLES

Researchers at the Max Planck Institute in Dresden, Germany, investigated the evolution of testicles using a new DNA technique.

Tissues break down over time, making them difficult to study in ancient species, but the new method gets around this by looking at the DNA of modern animals to work out the protein structure of long-extinct creatures.

Scientists looked at the genomes of 71 placental mammal species to find out whether their common ancestor had descended or ascended testicles.

Most modern mammals keep their testicles in a scrotum to stop them from overheating inside the body, which would kill the sperm. But a group of modern mammals known as the Afrotherians, which includes the manatee (pictured), keep their testicles inside their bodies

Experts were unsure whether an ancient mammal ancestor passed ascended testicles (right image) to Afrotherians like the African elephant (left image) around 100 million years ago, or if the group developed the abnormal biology later on

They focussed on two key genes known to trigger the development of a ligament that helps pull the testes downward during development.

In four Afrotherian species - the tenrec, cape elephant shrew, cape golden mole, and manatee - the two genes were lost around 100 million years ago.

This happened around the time these species split from the common ancestor they share with the rest of the placental mammals.

The genetic mutation that blocked the genes is different in each of the four species, suggesting they developed it recently - likely between 20 and 80 million years ago.

This shows that the common ancestor of all placental mammals had descended testicles - modern animals with tests inside their bodies evolved them later on.

In four Afrotherian species - the tenrec (file photo) cape elephant shrew, cape golden mole, and manatee - two genes involved in descended testicle were lost 100 million years ago. This showed a key common mammal ancestor had descended testicles

Strangely, the two genes were functional in two Afrotherian species - elephants and rock hyrax - and the causes of their testicular retention remain unknown.

Researchers said their new technique could help to resconstruct the evolutionary history of other animal body parts in future.

'Molecular vestiges offer an alternative strategy to investigate character ancestry,' study coauthor Dr Michael Hiller said.

'Instead of investigating a soft-tissue structure directly, one can trace the evolution of genes that are crucial for the development of this structure.'

The study was published Thursday in PLOS Biology.

WHY ARE TESTICLES KEPT OUTSIDE OF THE BODY?

It might seem counter-intuitive to keep an important male reproductive organs exposed to the elements outside of the body, however scientists have multiple theories about why testicles are not kept safely tucked away inside the body.

Temperature control is the most obvious answer.

Sperm production is at its most effective at 35° Celsius, which is two degrees below the temperature maintained inside the human body.

Organs that perform best at 37°C are shielded by bones inside the cavity of the body, including the brain and the kidneys.

However, there is some disagreement within the scientific community about the temperature thesis.

It’s unclear whether the testes descended because they needed to be cooler than the rest of the body, or whether the organs evolved to perform at that temperature because they were external.

Scientists have multiple theories about why testicles are not kept safely tucked away inside the body

The cooling thesis originated at Cambridge University in the 1890s.

Scientist Joseph Griffiths experimented on dogs, pushing their testicles back into their abdomens and stitching them in place.

Less than a week later, Griffiths discovered the organs had degenerated, with the tubules where sperm production occurs constricted, and sperm virtually absent.

He attributed this to the higher temperatures inside the body, spawning the theory.

The research was picked up in the 1920s by Carl Moore at the University of Chicago.

Building on the work of Charles Darwin, Moore argued that when mammals had transitioned from cold to warm-blooded animals, the internal temperatures severely hampered sperm production.

Males who were born with testicles outside of their body became the most successful breeders and passed on their genetic material, perpetuating the trait.

However, those who oppose the theory point to mammals that still keep their testicles inside the body and continue to successfully reproduce.

Many mammals with internal testicles, like elephants and birds, have a higher core temperature than human beings and primates – which some say discredits the temperature theory.

Some academics believe the cooler temperatures might be to stop DNA from mutating, while others believe keeping sperm below body temperature allows the warmth of a vagina to function as an activating signal for the swimmers.

It is possible external testicles can be explained by the so-called handicap theory, which posits that if a female has to choose between two male suitors who have bested all other competition, she would choose the one who had to surmount the greatest odds – as this hints at an even greater strength.

For example, climbing Mount Everest is impressive, but climbing to the summit with one hand tied behind your back is even more impressive, right?

The controversial theory goes some way to explaining a number of problematic evolutionary phenomena, like male birds’ colourful plumage and songs that seem designed to attract predators.

If the handicap theory is correct, the genes for scrotums were passed along because being able to function with these crucial organs suspended on the outside of the body impressed potential mates.


Why did mammals evolve to have two testes? - Biology

Once multicellular organisms evolved and developed specialized cells, some also developed tissues and organs with specialized functions. An early development in reproduction occurred in the Annelids. These organisms produce sperm and eggs from undifferentiated cells in their coelom and store them in that cavity. When the coelom becomes filled, the cells are released through an excretory opening or by the body splitting open. Reproductive organs evolved with the development of gonads that produce sperm and eggs. These cells went through meiosis, an adaption of mitosis, which reduced the number of chromosomes in each reproductive cell by half, while increasing the number of cells through cell division.

Complete reproductive systems were developed in insects, with separate sexes. Sperm are made in testes and then travel through coiled tubes to the epididymis for storage. Eggs mature in the ovary. When they are released from the ovary, they travel to the uterine tubes for fertilization. Some insects have a specialized sac, called a spermatheca, which stores sperm for later use, sometimes up to a year. Fertilization can be timed with environmental or food conditions that are optimal for offspring survival.

Vertebrates have similar structures, with a few differences. Non-mammals, such as birds and reptiles, have a common body opening, called a cloaca, for the digestive, excretory and reproductive systems. Coupling between birds usually involves positioning the cloaca openings opposite each other for transfer of sperm. Mammals have separate openings for the systems in the female and a uterus for support of developing offspring. The uterus has two chambers in species that produce large numbers of offspring at a time, while species that produce one offspring, such as primates, have a single uterus.

Sperm transfer from the male to the female during reproduction ranges from releasing the sperm into the watery environment for external fertilization, to the joining of cloaca in birds, to the development of a penis for direct delivery into the female’s vagina in mammals.


Why did mammals evolve to have two testes? - Biology

The first amniotes evolved from amphibian ancestors approximately 340 million years ago during the Carboniferous period. The early amniotes diverged into two main lines soon after the first amniotes arose. The initial split was into synapsids and sauropsids. Synapsids include all mammals, including extinct mammalian species. Synapsids also include therapsids, which were mammal-like reptiles from which mammals evolved. Sauropsids include reptiles and birds, and can be further divided into anapsids and diapsids. The key differences between the synapsids, anapsids, and diapsids are the structures of the skull and the number of temporal fenestrae behind each eye (Figure 1).

Figure 1. Compare the skulls and temporal fenestrae of anapsids, synapsids, and diapsids. Anapsids have no openings, synapsids have one opening, and diapsids have two openings.

Temporal fenestrae are post-orbital openings in the skull that allow muscles to expand and lengthen. Anapsids have no temporal fenestrae, synapsids have one, and diapsids have two. Anapsids include extinct organisms and may, based on anatomy, include turtles. However, this is still controversial, and turtles are sometimes classified as diapsids based on molecular evidence. The diapsids include birds and all other living and extinct reptiles.

The diapsids in turn diverged into two groups, the Archosauromorpha (“ancient lizard form”) and the Lepidosauromorpha (“scaly lizard form”) during the Mesozoic period (Figure 2). The lepidosaurs include modern lizards, snakes, and tuataras. The archosaurs include modern crocodiles and alligators, and the extinct ichthyosaurs (“fish lizards” superficially resembling dolphins), pterosaurs (“winged lizard”), dinosaurs (“terrible lizard”), and birds. (We should note that clade Dinosauria includes birds, which evolved from a branch of maniraptoran theropod dinosaurs in the Mesozoic.)

The evolutionarily derived characteristics of amniotes include the amniotic egg and its four extraembryonic membranes, a thicker and more waterproof skin, and rib ventilation of the lungs (ventilation is performed by drawing air into and out of the lungs by muscles such as the costal rib muscles and the diaphragm).

Figure 2. This chart shows the evolution of amniotes. The placement of Testudines (turtles) is currently still debated.

In the past, the most common division of amniotes has been into the classes Mammalia, Reptilia, and Aves. However, both birds and mammals are descended from different amniote branches: the synapsids giving rise to the therapsids and mammals, and the diapsids giving rise to the lepidosaurs and archosaurs. We will consider both the birds and the mammals as groups distinct from reptiles for the purpose of this discussion with the understanding that this does not accurately reflect phylogenetic history and relationships.

Practice Question

Members of the order Testudines have an anapsid-like skull with one opening. However, molecular studies indicate that turtles descended from a diapsid ancestor. Why might this be the case?


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