Are anthracotheres alive and well and living on Sulawesi? No, but it was a nice idea. Babirusas, part II

Verbreitungsgeschicht

-e.

Interesting critter, is that inner set of tusks actually anchored in the lower jar and grown through the upper jaw?

I hate to seem like a molecule snob, but what about the DNA sequence data for babirusas? I see plenty in GenBank, but I don't have any of the publications on hand.

What about O'Leary & Gatesey 2008, Cladistics 24:397-442?

http://www.boneclones.com/images/bc-251-lg.jpg

A quick search for images of 'babirusa skull' on Google confirms what should be obvious. The upper tusks are not from the lower jaw.

To me the babirusa is the closest extant match to the cetiartiodactyle last common ancestor, as much as Indohyus or hippos anyway. Dolphins have multi-chamber stomachs. Have any of the zoo hybrids reproduced?

So babirusas can hybridise with domestic pigs: doesn't this mean that they should be classified as a species of Sus?

...we should remember that successful hybridisation can occur between species that are only distantly related, so it actually doesn't mean much.

Well I give it some meaning. But I am unsettled on just how much. If babirusas were anthracotheres I would be shocked at their successful mating with pigs. In fact, given the lack of a babirusa fossil record, those little piglets are the best evidence that they are suids and not anthracotheres. For me anyways. Has anyone tried to mate a pygmy hippo with a large pig, and how did that work out?

Among tetrapods, which are the most "distantly related" species that have successfully reproduced? And are they known to be fertile or not? I have heard of many hybrids but none that really seem to be from truly "distantly related" species.

Distant hybrids: There are quite a few known, apparently fertile hybrids between various species of ducks (Anatinae) and geese (Anserinae), which diverged at least 30 million years ago. How's that?

Steve P:

So babirusas can hybridise with domestic pigs: doesn't this mean that they should be classified as a species of Sus?

There is no definition for what a 'genus' constitutes. Ideally the concept should be used to express diversity, and all hybridization tells us is that through some evolutionary quirk no reproductive barriers evolved. A cursory glance through phylogenetic data suggests Babyrousa is nowhere near Sus.

AnJaCo:

Among tetrapods, which are the most "distantly related" species that have successfully reproduced?

Discussed several times on these forums. Here are a few (and there are a lot more):
Puma x Leopard
Camel x Llama
False Killer Whale x Bottlenose Dolphin
Turkey x Chicken

This page has a lot more, mostly feline.

is that inner set of tusks actually anchored in the lower jar and grown through the upper jaw?

No, it's the upper canines that grow upwards instead of downwards. Fascinating and unique, but actually not much more bizarre than wisdom teeth growing in all manner of transverse directions in humans.

The outer set consists of the lower canines which grow upward as usual.

all hybridization tells us is that through some evolutionary quirk no reproductive barriers evolved

Or, alternatively, reproductive barriers were lost. I'm not aware of any cases where this has been demonstrated for post-mating isolation methods but it has been for pre-mating methods. For instance, lacewing species in North America have different courtship songs from other closely related species in North America, but may have identical songs to more distantly related species in Asia. Presumably, when the chance of individuals of two separate populations to encounter each other is pretty much nil, there's no reason for selective pressure to stop them converging on the same characters through drift.

One of the best examples of the non-linear nature of reproductive isolation is probably Aix sponsa (the Carolina wood duck) versus Aix galericulata (the mandarin duck). Despite their being (as far as I know) undoubted sister taxa, wood ducks seem to have the usual duck ability to hybridise with almost any other duck (a Google search brings up hybrids with mallard, shoveler and hooded merganser) while mandarin ducks seem to be incapable of producing hybrids (with possible contentious exceptions of hybrids with wood ducks).

I found this page with a few neat pictures of hybrids between domestic birds, including peacock X guinea fowl. In fact, Galloanserae as a whole seem to think that reproductive isolation is something other people do.

You're overstating slightly. While mandarin hybrids are rare, they aren't unknown. There are known hybrids between mandarin and gadwall, long-tailed duck, Laysan teal, and redhead. Probably more, but that's all I found in a quick search.

Damn, I'm gonna have to find another example.

T-shirt slogan: Interfertility is symplesiomorphy.

Was the babirusa/pig hybridization intentional or, y'know, opportunistic?

Poster #15: I hope the "q" in your name is a typo!

Thanks to Cameron and Chris Taylor for the observations and links. I remembered the messybeast.com page from an earlier post on big cats, and I hadn't followed the non-cats links. Just did. Neat stuff.

Both links provide info informing the observation that "successful" hybridisation (which I define here as producing a living 'healthy' adult) is a graded phenomenon: from being a high probability event (e.g. mules) to a very-low-to-zero probability event (e.g. ~1% turkey-chicken [from Chris' link]).

OK, so can one say which species 'wins' in answer to my original question of "which are the most 'distantly related' species that have successfully reproduced?".

Can one objectively rank a chicken-pheasant hybrid as a more or less distant pairing than say, a camel-llama cross? Would the time of divergence be the sole factor? Would the degree of morphological divergence - regardless of when - play a part in the ranking?

Just askin'...

The problem, I think, would be coming up with a meaningful way of measuring 'morphological divergence' that would allow for comparisons. Even overall genetic divergence may not be directly informative because it has been shown that in at least some cases reproductive isolation may be dependent on a single gene.

I also can't resist mentioning the case of Wolbachia infection in insects where two individuals may be "cured" of reproductive isolation.

Poster #15: I hope the "q" in your name is a typo!

No, and neither is the u behind it. The second h is.

OK, so can one say which species 'wins' in answer to my original question of "which are the most 'distantly related' species that have successfully reproduced?".

You need to define "distantly related" first.

Darren:

in theory we should expect to find babyrousines going back to the Oligocene

Molecular data do not support such a contention. Randi et al. (1996)* estimated that the Babyrousa lineage diverged from other suids as recently as 10-19 MYA, i.e., in the Miocene.

* 1996 was a while ago but I'm not aware of any more recent molecular studies of babyrousine origins; if anyone is, please tell.

Reference:

Randi, E., Lucchini, V. & Diong, C.H. 1996. Evolutionary genetics of the Suiformes as reconstructed using mtDNA sequencing. Journal of Mammalian Evolution 3, 163-194.

Is it known how babirusas ended up with their aberrant upper canines? Did they somehow rotate upwards (passing through a stage of pointing out horizontally), or are we looking at a hopeful-monster type mutation?

Thanks for another pic of hybrids! I find it very strange that two look like white piglets, but the third is brown... can any zootechnology guru explain why?

BTW - there are hybrids of chicken and several cracid genera - which old Sibley classified in different orders!

You need to define "distantly related" first.

My follow-on question (above) was a [perhaps weak] attempt to elicit a way to quantify how distantly related 2 taxa are, and a way to compare that to the distance between 2 other taxa.

Is time of divergence any more or less important than morphological divergence? And how about DNA base-pair divergence?

The World of Cladograms (a place I am obliged to visit but not reside) is loaded with language such as 'taxon A consists of organisms closer to X than to Y'. I take the word "closer" to be the opposite of "more distantly related". And since phylogenetic analyses are presumably time-free, then would not a cladist quantify the distance of a relationship solely on morphological distance/divergence regardless of when it happened?

Another way to illustrate the dilemma:
2 genera diverged in the Cretaceous, but their morphology remains very similar.
2 other genera diverged in the Miocene, and their morphology differs radically.
Which of these pairs is more distantly related? I presume that the cladist would say the latter without blinking. Am I wrong on that? And I would suggest that the time of divergence has to play some role in assessing distance of relationships.

I suppose that all this is just my way of noting that when I find phrases in the literature such as "closer to" or "more distantly related" it is not always clear whether time of divergence or morphological divergence is what is being discussed. Unless its a cladogram. :-)

AnJaCo--
I think I understand what you are saying, and that you are right in your understanding of what "cladists" say.
My impression is that the phrase "more closely related to... than" is typically used in making three-way comparisons than in four-way. That is, usually in saying things like
"X is more closely related to Y than to Z,"
and only very infrequently (I can't off hand recall seeing it in ANY technical literature in recent years) in saying things like
"X is more closely related to Y than Z is to W."
The three-place comparison is the central concept of cladistics, and is defined as meaning that X and Y have a more recent common ancestor than X and Z or than Y and Z. Morphological similarities are taken as at best EVIDENCE for this, and sometimes misleading evidence: witness the standard trout/coelacanth/cow example, meant to show that fins and scales versus legs and hair ISN'T conclusive on the which is nearer to which question.

My sense is that many cladists (and probably non-cladists) would say the four-place comparison is useless if not meaningless. Four-place comparisons of similarity can, I suppose, be useful in practice sometimes, but the useful ones are likely to be just matters of similarity, NOT of relatedness in the phylogenetic sense. Compare linguistic comparisons: it may be useful to say that English is more like German than Estonian is like Hungarian (or vice versa-- I don't know), but any practical interest in this (from the point of view of, say, predicting how easy it is for someone who speaks one language to learn another) is INDEPENDENT of the "phylogenetic" question of whether the diverrgence of the Gerrmanic languages is more or less recent than the divergence of the Finno-Ugrian languages.

Estonian and Hungarian diverged long before English and German.

(The English-German divergence, BTW, isn't the basal one within the Germanic languages - exactly which is is disputed, but English and German both belong to the West Germanic sub-branch.)

While humans and seals have BAT brown fat, 20ma domestic pig and wild boar ancestors (like penguins and marine iguanas) lost the ability to produce thermal brown fat due to mutation (so piglets must shudder or snuggle to get warm), does anyone know if babirusa also lack brown fat? http://www.physorg.com/news75117516.html

Allen Hazen-

You are right about the three-way vs. four-way comparisons. The former are de rigueur in the taxonomic literature while the latter are, well, I can't recall any off hand.

Going back to my original question:

"Among tetrapods, which are the most "distantly related" species that have successfully reproduced?"

This may be impossible to answer objectively.

This pairing of the male babirusa with pigs was apparently the result of zoo keepers responding to perceived "depression" in the babirusa that was being housed alone. Now if only there was a zoo with a lonely pygmy hippo ...

I'd pay cash moneys to see those babies.

My sense is that many cladists (and probably non-cladists) would say the four-place comparison is useless if not meaningless.

Not necessarily meaningless, I would say, but probably too difficult to calculate in a meaningful manner in most cases. I think the big problem is that such comparisons almost always represent a major over-simplification. For instance, if we were comparing two pairs of languages as in your example above, the members of one pair might be more different in their verb formation, but the members of the other pair might be more different in their use of pronouns.

"More closely related" has a simple, even somewhat operational four-taxon meaning: time. If A and B diverged from each other more recently than C and D diverged from each other, then A and B are more closely related than C and D. This of course says nothing about the degree of morphological difference, though it's at least somewhat related to overall genetic difference. The only other problem is in estimating divergence dates.

But I suggest that by this definition, my duck/goose hybrids must be at least close to the vertebrate maximum. Good estimates are hard to come by, but I estimate a minimum of 30 million years divergence. Got anything better?

Not sure if it beats the duck-goose split, but pythons don't seem to care much about miscegenation. A number of viable Morelia x Liasis crosses have been reported, as well as ones between the M. spilota and M. amethistina groups. Putting dates on these splits depends on the exact phylogenetic position of fossils I refer to Morelia from the early Miocene and late Oligocene; Rawlings et al. (2008) would make it about 35 my, but used a dodgy-looking tree and calibration procedure. I strongly suspect it will shorten a bit when the fossils are included explicitly and external calibrations are used as well, but may still be over 20.
There's also folks on the interwebs advertising things like Python m. bivittatus x Python reticulatus, which (because Broghammerus reticulatus is actually closer to the Australian lineage than to Python sensu stricto) is necessarily a deeper split than Morelia-Liasis (more than 45 my according to Rawlings et al.), and could well beat your goose-duck.

Rawlings, LH, DL Rabosky, SC Donnellan and MN Hutchinson. 2008. Python phylogenetics: inference from morphology and mitochondrial DNA. Biol J Linn Soc 93: 603-619.

The hybrid python business is bigger than I realised, with some very decorative products out there (for example, gallery here). They certainly cross the deepest splits in the phylogeny, whichever tree you prefer (e.g. Python regius x Aspidites ramsayi!!!) without looking grossly deformed at all, and I gather that many of the crosses are fertile to some extent. I've long wondered whether some of the problems with phylogeny in the group are caused by hybrid origin of some species (I'm looking at you, Apodora papuana), but I don't think the molecular evidence to test that is out there yet.

John H. and John S.: I see your anatids and your pythons, and raise you sea turtles. Several chelonid species are known to hybridise in the wild (and there are even suggestions that the hybrids themselves may be fertile), and some such hybridisations have occurred between lineages that separated for more than 50 MYA (Karl et al. 1995; James et al. 2004).

Also, Wilson et al. (1974) list a large number of anuran species that supposedly have hybridised under laboratory conditions; some of these hybridisations might conceivably be between members of even more anciently diverged lineages than in the case of the sea turtles (though I haven't yet looked up what the most recent frog and toad phylogenies say).

References:

James, M.C., Martin, K. & Dutton, P.H. 2004. Hybridization between a green turtle, Chelonia mydas, and loggerhead turtle, Caretta caretta, and the first record of a green turtle in Atlantic Canada. The Canadian Field-Naturalist 118, 579-582.

Karl, S.A., Bowen, B.W. & Avise, J.C. 1995. Hybridization among the ancient mariners: characterization of marine turtle hybrids with molecular genetic assays. Journal of Heredity 86, 262-268.

Wilson, A.C., Maxson, L.R. & Sarich, V.M. 1974. Two types of molecular evolution. Evidence from studies of interspecific hybridization. PNAS 71, 2843-2847.

AnJaCo:

This pairing of the male babirusa with pigs was apparently the result of zoo keepers responding to perceived "depression" in the babirusa that was being housed alone.

Desmond Morris wrote in one of his books (can't remember which) of a case where an Old World porcupine and a New World porcupine were housed together in a zoo cage. The animals were of different genders (can't remember which one was which, but that doesn't really matter) and the male tried to mate with the female. That, however, turned out to be a very painful experience for the male, due to a different arrangement of the quills on the other, very distantly related species' back...

Dartian:

1. Youch!
2. Males. Heh heh. Gotta love 'em.

I'd wager that most/all of these fascinating hybridisations were instigated by the males.

- - -

Gratuitous comment for these Flag Waving-Olympic Days:
If you haven't looked at John Scanlon FCD's link to the gallery of hybrid pythons/boas, check out the red, white, and blue hybrid Emerald Tree Boa (Corallus caninus) x Amazon Tree Boa (Corallus hortulanus). (If you are French: check out the bleu, blanche, et rouge hybrid...). (BTW, how do Brits list their colours?). Not your usual color combo in a snake.

Those python and sea turtle hybrids are mind-blowing...

Is time of divergence any more or less important than morphological divergence? And how about DNA base-pair divergence?

You choose.

The World of Cladograms (a place I am obliged to visit but not reside) is loaded with language such as 'taxon A consists of organisms closer to X than to Y'.

Well, definitions of taxon names like this are phylogenetic nomenclature, not cladistics...

I take the word "closer" to be the opposite of "more distantly related". And since phylogenetic analyses are presumably time-free, then would not a cladist quantify the distance of a relationship solely on morphological distance/divergence regardless of when it happened?

Neither nor. If A and B share a common ancestor with each other but not with C, they're more closely related to each other than to C. That's the cladistic definition of "more closely related".

My sense is that many cladists (and probably non-cladists) would say the four-place comparison is useless if not meaningless.

Pretty much.

Choosing absolute time as a criterion only leads back to the question if rates of morphological-or-whatever evolution should be considered...

I think a big part of what makes hybridisation interesting is morphological divergence. So even if we could settle on a different metric for relatedness, we'd come up with a boring winner.

Wow, awesome, very interesting discussion - thanks to all. A few select responses.

Remember that the article at top was written in 2007, and I haven't had time to update it. The whole 'babyrousines may have diverged in the Oligocene' thing (see comments 3 and 20) comes from statements in Thenius (1970) and Groves (1980). However, I should have changed this in view of Randi et al. (1996) and other publications. Will alter article.

Regarding 'long distance' hybrids, the python and sea turtle examples given above are pretty incredible (I was aware of the turtles, but not the more obscure pythons). The example I usually give is also Riversleigh-inspired: it involves hybridisation between the Australian lizards Egernia cunninghami (Cunningham's skink) and the bluetongue Tiliqua scincoides. Both are relatively common species in captivity (Cunningham's is, I'm sorry to say, the only Egernia species I've ever seen). Fossils often said to be of Egernia are known from the Miocene, so are perhaps 15-20 million years old... though whether they're Egernia in the strict sense is now questionable I think (Gardner et al. (2008) showed that Egernia sensu lato represents four clades, and they restricted the name Egernia to the lineage that includes Cunningham's skink). Anyway... a 15-20 Ma divergence that still 'permits' hybridisation is no longer so impressive given some of the other examples discussed here.

Anurans: Wilson et al. (1974) do list a load of anuran hybrids (comment 32), but most are between species in traditional 'genera' like the over-bloated monster that is Bufo sensu lato. Using the new taxonomy (toads have been featured once or twice on Tet Zoo, in case you didn't know), some of these hybrids are between such genera as Anaxyrus and Epidalea, and therefore occur betweeen taxa that have been separated since some time in the Oligocene at least. In other words, a separation of about 30 million years.

Refs - -

Gardner, M. G., Hugall, A. F., Donnellan, S. C., Hutchinson, M. N. & Foster, R. 2008. Molecular systematics of social skinks: phylogeny and taxonomy of the Egernia group (Reptilia: Scincidae). Zoological Journal of the Linnean Society 154, 781-794.

Groves, C. P. 1980. Notes on the systematics of Babyrousa (Artiodactyla, Suidae). Zoologische Mededelingen 55, 29-46.

Randi, E., Lucchini, V. & Diong, C.H. 1996. Evolutionary genetics of the Suiformes as reconstructed using mtDNA sequencing. Journal of Mammalian Evolution 3, 163-194.

Thenius, E. 1970. Zur Evolution und Verbreitungsgeschichte der Suidae (Artiodactyla, Mammalia). Zeitschrift für Säugetierkunde 35, 321-342.

Wilson, A. C., Maxson, L. R. & Sarich, V. M. 1974. Two types of molecular evolution. Evidence from studies of interspecific hybridization. Proceedings of the National Academy of Sciences 71, 2843-2847.

The Egernia cunninghami X Tiliqua scincoides hybrid that Darren mentions actually spans a slightly older divide than the Miocene. Tiliqua fossils are known from late Oligocene / earliest Miocene of Riversleigh. These are much more apomorphic than plain old Egernia so the id is more secure. Note that this fossil is already diagnostic for Tiliqua , meaning that Cyclodomorphus and the Egernia luctuosa groups (which split from the Tiliqua lineage after Egernia s.s.) must have already diverged by the beginning of the Miocene.

Cool, thanks for that Adam. It was thanks to you that I learnt of this stuff in the first place. Hey, I can't remember my phone number, my wife's birthday or which mobile network I'm on, but I can remember a conversation about fossil skinks that happened more than 10 years ago :)

Incidentally, the Egernia luctuosa group is now Lissolepis Peters, 1872... again.

Thats cool Darren, I'm glad I make an impression on someone. However I have to add that what I wrote just minutes ago is actually inaccurate. I said that The Egernia luctuosa group had diverged after Egernia s.s. had diverged from Tiliqua. According to Gardner et al. the luctuosa group (for which they coin a new name) is actually basal to the whole lot. My statement was based on my recollection of a conversation I had with Mark Hutchinson years ago on what was then unpublished work. Whether my memory is simply faulty or whether new data caused the position of the luctuosa group to change dramatically I do not know.

OOPs again! damn, I must read carefully before typing. OK Lissolepis is NOT a new genus, but a ressurection of an old one

If anyone's interested, the Riversleigh Egernia/Lissolepis (originally named Proegernia) were described in...

Martin, J. E., Hutchinson, M. N., Meredith, R., Case, J. A. & Pledge, N. S. 2004. The oldest genus of scincid lizard (Squamata) from the Tertiary Etadunna Formation of South Australia. Journal of Herpetology 38, 180-187.

... and it states therein that the fossils are Late Oligocene, so I was being dumb in mentioning Miocene. Anyway, the divergence is pegged at something like 30 million years ago. John Scanlon's gonna be peeved that he missed all this :)

For the record, neither the q (with its u) nor the h are typos. They're there on purpose.

Thanks to AnJaCo for answering my question!

Sorry to be a party spoiler, but has anybody considered that the female pig was already pregnant when housed with the male babyrusa? Those little piglets look just like little piglets to me! I want proof -some genetic analysis should be easy enough. Things would have been easier to believe if it had been a female babyrusa in the zoo.

Darren, for someone supposed to be too busy to blog...

(I was aware of the turtles, but not the more obscure pythons)

...you seem to be overcompensating for that 'snake guilt' you mentioned a while back. What did I miss to be peeved about? - Of course I knew that Egernia-group skinks hybridise like mad and have some sort of fossil record (not only at Riversleigh; the Etadunna Fm is in South Australia) and calibrated phylogeny, but it slipped my mind because they're only good for snake food.
Actually most are too big and bony-scaled for that, but, well, skinks. I get lots of 'em in the late Oligocene stuff I'm working on, but not Proegernia or Tiliqua, just Eulamprus-like water skinks and a couple of smaller things. Lollies for Nanowana.

Slightly off-topic, but regarding mammalian hybrids, I can't resist mentioning that in Maracay Zoological Park, Venezuela, several hybrid offspring between a male spectacled bear Tremarctos ornatus and a female Asian black bear Ursus thibetanus have been born over the years. That is mentioned here; unfortunately, that site has no pictures of the hybrid bears*. (There is a photo of the spectacled bear and the black bear shagging, though.) What makes this noteworthy is the fact that the spectacled bear and the Asian black bear are quite distantly related; the clade Tremarctini and the clade Ursini diverged from each other in the mid-Miocene, about 12.4-15.6 MYA, according to a recent molecular estimate (Krause et al. 2008). By mammalian standards (if not by reptilian), hybridisation between members of so anciently diverged lineages is pretty remarkable.

* There appears to be a technical paper describing the first of these hybrids (Mondolfi & Boede 1981), but, alas, I've not seen it.

Back to pig phylogeny; Funk et al. (2007), while not primarily investigating the phylogenetic position of Babyrousa, incidentally noted that it is far from certain that it really is the most basal extant suid. In their words, '[u]nderstanding evolutionary relationships between the genera of Suidae will require further sampling and sequencing, especially for Babyrousa and Potamochoerus' (p. 433). (Funk et al. also reported that according to molecular data, inclusion of the pygmy hog in Sus would make this genus paraphyletic, and they thus recommended placing the pygmy hog in a separate genus, Porcula.)

References:

Funk, S.M., Verma, S.K., Larson, G., Prasad, K., Singh, L., Narayan, G. & Fa, J.E. 2007. The pygmy hog is a unique genus: 19th century taxonomists got it right first time round. Molecular Phylogenetics and Evolution 45, 427-436.

Krause, J., Unger, T., Noçon, A., Malaspinas, A.-S., Kolokotrinis, S.-O., Stiller, M., Soibelzon, L., Spriggs, H., Dear, P.H., Briggs, A.W., Bray, S.C.E., OâBrien, S.J., Rabeder, G., Matheus, P., Cooper, A., Slatkin, M., Pääbo, S. & Hofreiter, M. 2008. Mitochondrial genomes reveal an explosive radiation of extinct and extant bears near the MioceneâPliocene boundary. BMC Evolutionary Biology 8(220), doi:10.1186/1471-2148-8-220

Mondolfi, E. & Boede, E.O. 1981. A hybrid of a spectacled bear (Tremarctos ornatus) and an Asiatic black bear (Selenarctos thibetanus) born at the Maracay Zoological Park, Venezuela (Mammalia; Ursidae). Memoria de la Sociedad de Ciencias Naturales La Salle 41, 143â148.

Blackbird (comment 45): the domestic pig was definitely not already pregnant.

John (comment 46): in terms of 'what you missed', I was referring to the whole fossil skink discussion. I wrote my comments as if all of the fossils were from Riversleigh, and forgot that the Etadunna Formation had been important in the story too (however, Egernia sensu lato is still present at Riversleigh).

Dartian (comment 47): thanks for the refs. Interesting, then, that all of the claims made pre-2006 about ancient divergence of babirusas may well be erroneous. And, a reminder to anyone who didn't know: my babirusa articles are recycled from Tet Zoo ver 1, were written in 2006, and have not been updated.

yay, Babirusas!
(Why are the genus and species parts of name Babyrousa babirussa spelt (pronounced?) differently?)
Looking at the skull photo, I wonder whether, developmentally, the upper tusks (or their toothbuds) start out oriented like normal teeth and then rotate out and up.. If so, at what embryonic stage?.. or whether they form directed upward like that from the start.

My sister had to have a tooth removed in her early teens that was growing horizontally in the roof of her mouth, rather than in a tooth-row at all. How rare is that?

Oh, now I see the answer to my second question given in Darren's Part III.

(Why are the genus and species parts of name Babyrousa babirussa spelt [...] differently?)

Because in the 19th century people found it funny to do this to names from "exotic languages". There are a few more such names.

My sister had to have a tooth removed in her early teens that was growing horizontally in the roof of her mouth, rather than in a tooth-row at all. How rare is that?

Happens to wisdom teeth fairly often.