[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Hanson 2006, Mortimer, Baeker response

Mike Taylor wrote:

[In Homer Simpson voice:] Ah, beer! Is there _anything_ it can't do?

The cause of, and solution to, all of life's problems.

Somehow we are still talking past each other here. If a specimen is morphologically very different from its closest known relative, then that alone makes it interesting (to me, anyway). So if we mean anything at all by "family", then surely what we mean is something like "group that is morphologically similar within itself, but different to other things".

That's the traditional way the term "family" was used. In fact, that's the traditional way *any* Linnaean group was used: to emphasize morphological distinctiveness, and try and quantify the level of distinctiveness by assigning ranks (phylum, class, order, family, etc....).

(And, yes, I know perfectly well that this is not cladistically sound or any of that stuff. That's fine. I am very fond of clades, and recognise their utility in discussing evolution, but I see no reason why they should be the _only_ groups we acknowledge).

Then therein lies the problem of... well, whatever it is we're talking about. A disciple of cladistics would respond to your statement by saying "No!!! It's clades or nothing!" Phylogenetic taxonomy defines clades by shared common ancestry, not by perceived different-ness. If a group is demonstrated to be polyphyletic or paraphyletic, then it gets thrown on the scrapheap.

The other issue is whether "morphologically distinctive" taxa should be granted a monotypic family in order to highlight this distinctiveness. Some authors erect families solely for this reason - such as Tendaguriidae, Agustiniidae, Huabeisauridae, Velocisauridae, etc. Other authors prefer not to erect monotype families, even when it is clear that the taxon being described is quite distinct and/or cannot be accommodated in any existing family - _Eoraptor_, _Incisivosaurus_, _Bagaaratan_, _Jobaria_, _Saturnalia_, etc. This appears to be a matter of personal choice. But phylogenetic taxonomy regards as redundant any clade ("family" or otherwise) that includes only one genus (=monotypic).

Ah, now I think you've confused yourself by tying together Linnaean and phylogenetic principles. In suggesting that Tenduguriidae couldn't be considered a "family" because of orthographical accident of falling inside another group with a name that ends "-idae", you are taking us down a path that I'm sure neither of us wants to take!

I'm only confused because the system itself is confused. In the changeover from Linnaean hierarchy to phylogenetic taxonomy some unattractive Linnaean features have carried over. We've effectively abandoned ranks, yet the hierarchial system persists in the way that names with certain suffixes require a descending level of inclusiveness. For example, we cannot have one -idae inside another -idae (like Agustiniidae inside Titanosauridae), or have -inae above -idae (such as Dromaeosauridae inside Microraptorinae). Other clades can change in content relative to each other, but these family-level coordinated clades ("family", "subfamily", "tribe" - idae, -inae, -ini) have to stay in a hierarchial sequence. This is the "baggage" you are referring to.

In short, I think we bothe know what you and I are talking about, Mike. But we're both trying to come to grips with the different Linnaean and cladistic universes. The ICZN and PhyloCode need to get together in a smoke-filled room and sort this mess out.

No, no, you've gone all ICZN-y on me again :-)

It's not just the ICZN who gets uptight about this. Phylogenetics doesn't do this either. When one family is sunk into another family (such as Dryptosauridae into Tyrannosauridae, or Agustiniidae inside Titanosauridae), the sunken family is history - unless it's kept on as a less inclusive taxon (like Dryptosaurinae or Agustiniinae, which hasn't happened in these particular cases.)

The reason I don't feel the mandatory surge of outrage whenever I hear the word "family" is that I want a way to say the following kind of thing, which is actually pretty common:

        Other sauropod remains from the Hastings Beds Group
        represent basal Titanosauriformes, Titanosauria and
        Diplodocidae; the new taxon brings to four the number
        of sauropod 'families' represented in this unit.

I'm totally fine with this. However, while "family" is useful for categorizing diversity, it does not quantify diversity in any meaningful way. In the above sentence you could replace 'families' with 'taxa'; this would convey the same meaning, given that the first clause already clues us in about the level of phylogenetic diversity.

The knee-jerk reaction seems to be to say "the number of clades", but a moment's consideration shows that this is nonsense.

Yep, I agree.

Since clades nest, the number of clades can be equally legitimately considered to be one (Sauropoda), two (Titanosauriformes and Diplocodoidea), or a dozen (Chordata, Amniota, Reptilia, etc.) Interestingly, the one number that _does_ seem hard to support is three, since one of the
three groups we want to discuss ("basal titanosauriforms") is not a clade at all, but a paraphyletic assemblage that is nevertheless worth discussing as it is morphologically distinct.
I suppose the bottom line of what this fragment is trying to say is "brings to four the number of morphologically distinct sauropod groups represented, where degree of morphological distinctiveness is understood in the context of overall sauropod diversity and evaluated as is typical for group". That is what I am using "family" as shorthand for.

Yes, the term "basal titanosauriform" is useful. But a reviewer might choose to be pedantic and say, "So four sauropod families are represented in the Hastings Beds Group? Which families are those? Name them." If your basal titanosauriform cannot be assigned to any known family, then you've painted yourself into a corner. This isn't a criticism on my part; I'm just trying to think what somebody else might say about your choice of wording.