[Taxacom] African origin of Malagasy chameleons

[John Grehan]

To those interested in tracking aspects of biogeography (i.e., for those
for whom biogeographic debate is painful, please do not read), I came
across the following title:

“The only complete articulated early Miocene chameleon skull (Rusinga
Island, Kenya) suggests an African origin for Madagascar’s endemic
chameleons” (Scientific Reports 2020).

Its quite an eye catching title, essential for maximizing publication
exposure. But of course one is immediately intrigued as to how an
individual fossil skull can suggest a geographic origin that is narrower
than the geographic range of the modern taxon. A reasonable question would
you not think?

So of course I started reading the paper to find out just how a fossil
could provide this evidence of a narrow geographic origin (essentially
conforming to Darwin's 'center of origin' concept.

The authors note in their abstract that the fossil represents 'one' of the
oldest chameleon fossils ever recovered, and they confidently in the genus
Calumma which is currently endemic to Madagascar. They state that this
fossil uniquely demonstrates the existence of Calumma on continental Africa
in the past. I would most certainly agree with that. Obviously. Excellent
empirical observation.

Now we get to the nitty gritty. They state that their results “challenge
the long-held view that chameleons originated on Madagascar and dispersed
over water to Africa” They don't say here why this is so, but later in the
paper.

They also assert that the fossil provides a strong evidence of an African
origin for some Malagasy lineages. Interesting. This the key assertion,
again not explained in the abstract. They also indicate that the 'evidence'
for an African origin is applicable for 'some' (i.e. more than one)
lineages.

They then state that “The Oligocene–early Miocene dispersal to Madagascar,
using oceanic currents that favoured eastward dispersal at that time, is a
highly supported scenario matching the suggested dispersal of lemurs to
this island. This is consistent with a previously suggested hypothesis
based on molecular data.”

So here we have it, that the postulated dispersal is a 'highly supported'
scenario, and this scenario is the same as suggested for Madagascar lemurs.
But really, how is it 'supported'? What is the evidence.

So next in the paper is the introduction.

They note that about half of all chameleon species occur in Madagascar and
this is why the island has been seems as a centre of diversity of the clade
from where it likely radiated via oceanic dispersal. This is the center of
diversity criterion for a center of origin (one of many, often
contradictory, criteria for finding a center of origin as noted by Camp in
the 1940's).

But they then note that the molecular phylogeny of Tolley et al. suggested
that the family most probably originated in Africa, with two separate
oceanic dispersal events to Madagascar during the Palaeocene and the
Oligocene. OK, so here a new criterion, that of phylogeny, is introduced to
find the postulated center of origin. They do not state how a phylogeny
constitutes evidence of geographic movement for the origin of allopatry.

They also note that molecular data suggest a Cretaceous origin while the
oldest known fossil record of crown members only dates back to the early
Miocene (so is the Cretaceous 'unreasonable?).

In their discussion on the biogeographic history of the Malagasy chameleons
gets a little muddy.

They first state that the separation of Madagascar already occurred in the
early Cretaceous, and that a Malagasy lineage in continental Africa during
the early Miocene is similar to Madagascar’s other endemic terrestrial
extant mammals. They then state that Malagasy taxa with a sister group in
Africa each each result from a single colonization event. But what is the
evidence for that? They do not say. Muddy waters indeed.

They then state that 'likewise', recent palaeontological discoveries
suggest that two lemur lineages likely dispersed from Africa to Madagascar
across the Mozambique Channel independently, and thus have an African
origin. Again, what is the actual empirical evidence for fossils providing
this scenario remains opaque to the reader's gaze.

They think is is also worth noting that some other squamate lineages such
as pythons and varanids did not manage to follow this dispersal route.
Again, no actual evidence. Just a series of assertions.

Next we get to “The origin of chameleons?”

First the note that Raxworthy et al. suggested that chameleons originated
on Madagascar and dispersed over water at least three times to Africa,
while Tolley et al. suggested that the family originated in Africa, with
two separate oceanic dispersal events to Madagascar during the Palaeocene and
the Oligocene. Bu no mention is made of the actual evidence.

Then they say that the early Miocene Calumma benovskyi (from Kenya)
supports an African continental origin for Malagasy chamaeleonine lineages,
at least for the Calumma clade. OK. But what is the evidence!!

They next say that the the African origin of Calumma is also supported by
the distribution of members forming a clade [Chamaeleo + Trioceros +
Kinyongia + Furcifer + Calumma]; where only the latter two taxa are found
in Madagascar. How this clade supports an 'African' origin is not stated!
They then note that according to Tolley et al.4, Calumma and Furcifer are
sister taxa and the split of these two lineages occurred in the Eocene -
but no evidence is specified!

Within these genera, species-level divergence is said to have occurred
during the Oligocene and Miocene. But no evidence is stated. If the split
is older than the Miocene fossil, then the last common ancestor of these
two lineages most likely lived on the continent, which is seem to suggest a
continental origin for Calumma and Furcifer. But why? Again, what is the
evidence?

They end by saying that an “out of Madagascar” dispersal scenario would
have required the existence of fully terrestrial land bridge between the
late Oligocene and the early Miocene and this would have enabled any taxon,
even large animals (e.g., elephants), to disperse. The strange thing about
this assertion is that there are plenty of geographically contiguous areas
that for which many taxa are not distributed throughout. A land connection
in of itself does not necessarily result in range expansion.

So altogether I found this a rather vacuous paper. After reading it through
I could never find the explicit empirical basis for their dispersal model.
To me, that failure marks a very, very, very poor account. Or have I missed
the statement of evidence? Its possible. No doubt someone will point it out
as it would seem inconceivable for a scientific paper not to make explicit
statements of evidence. After all, that is the hallmark of science.

John Grehan