A little bit mole-ish in the Miocene

After the last two posts on unicellular organisms, I'm going to bravely leap to another end of phylospace and cover a mammal. Necrolestes patagonensis was a small fossorial animal from the Miocene of Patagonia that has always held a certain appeal for me, both because of its somewhat morbid genus name (it translates as "robber of the dead") and because of its enigmatic phylogenetic position (recent review by Asher et al., 2007).

Necrolestes was described by Florentino Ameghino in 1891. Ameghino seems to have left a few conundrums in his wake - he originally described the giant carnivorous bird Phorusrhacos (sometimes spelt Phororhacos - I'll have to explain that sometime) as a toothless mammal, and mistakenly described the "wingless" penguin Palaeoapterodytes (see here for an explanation). Ameghino seems to have favoured an association of Necrolestes with the African golden moles (Chrysochloridae) - a not unreasonable suggestion for the time. Since then, probably the majority of authors have felt that Necrolestes was a marsupial, but it has also been compared with edentates or suggested as a late survivor of an equally enigmatic group of South American fossil mammals called Gondwanatheria (see here). I recall seeing a nice little cartoon in one paper doubting a marsupial affinity for Necrolestes (I think it was Archer, 1984 but I'm not certain) showing a little Necrolestes being drop-kicked by an anthropomorphised borhyaenid out the door of a gathering of marsupial representatives (borhyaenids were a family of dog-like marsupial carnivores).

After a detailed redescription of the available material (which, among other things, introduced me to the glorious-sounding term schmelzmuster, which refers to the spatial arrangement of different enamel types within a tooth), Asher et al. (2007) attempt to shed some light on the position of Necrolestes by trying to match its characters with previously optimised trees for other mammals. This proves to be quite tricky - Necrolestes has a rather oddball combination of primitive and derived characters, and any suggested position requires a certain amount of convergence. Asher divide the possibilities into three main options - a position outside the Theria (the marsupials + placentals clade), a position close to or within marsupials (metatherians), and a position close to or within placentals (eutherians).

In regards to a position outside Theria, Necrolestes has an atlas (the first cervical vertebra after the skull) with the left and right halves not fused to each other, something unlike any adult therian. It also lacks many of the tooth apomorphies associated with Theria, though this may just be due to the simplified teeth of Necrolestes. However, Necrolestes does have a coiled cochlea, an astragalar neck and lacks a septomaxilla, so Asher et al. conclude it is most likely a therian. As Gondwanatheria is often regarded as non-therian, Asher et al. suggest that Necrolestes is probably not a gondwanatherian, but I feel that the non-therian nature of Gondwanatheria has not really been demonstrated.

In regards to whether Necrolestes is a metatherian or eutherian, Asher et al. don't really come to a firm conclusion. Patterson (1958) claimed that Necrolestes possessed epipubic bones, which are a primitive character retained in marsupials but absent from modern placentals (thought they were present in some stem eutherians). Asher et al., however, found no sign of epipubic bones. It also has a non-inflected mandibular angle, which is generally a eutherian character, but is also found in some derived marsupials. Necrolestes does share a number of characters with metatherians, most of them "absence" characters - lack of a stapedial artery sulcus, lack of a labial mandibular foramen, etc. It agrees with metatherians in having three premolars, but seems to have one too few molars (three instead of four), and shares a ball-shaped distal process on the ulna and transverse canal foramina on the basisphenoid with crown marsupials.

Characters shared with eutherians are a posteriorly small zygomatic process on the squamosal and small incisive foramina, as well as the aforementioned non-inflected mandible and lack of epipubic bones.

Overall, Asher et al. feel that a metatherian affinity for Necrolestes is most likely, which is appealing on biogeographical grounds (most South American insectivores and such at the time being marsupials). However, they admit that a eutherian affinity can't be ruled out, and I would certainly like to see this possibility further investigated. In particular, if Gondwanatheria are related to edentates (another South American group) as some authors have apparently suggested, the idea that Necrolestes is a late survivor of them may yet be reborn.

REFERENCES

Ameghino, F. 1891. Nuevos restos de mamiferos fosiles descubiertos por Carlos Ameghino en el Eoceno inferior de la Patagonia austral. Especies nuevas adiciones y correcciones. Revista Argentina de Historia Natural 1: 289–328.

Archer, M. 1984. Origins and early radiations of marsupials. In Vertebrate Zoogeography and Evolution in Australasia (M. Archer & G. Clayton, eds.) pp. 585–626. Carlisle: Hesperian Press.

Asher, R. J., I. Horovitz, T. Martin & M. R. Sanchez-Villagra. 2007. Neither a rodent nor a platypus: a reexamination of Necrolestes patagonensis Ameghino. American Museum Novitates 3546: 1-40.

Patterson, B. 1958. Affinities of the Patagonian fossil mammal, Necrolestes. Breviora Museum of Comparative Zoology 94: 1–14.

Complexity of organisms

I came across this at P. Z. Myers' Pharyngula page: Step away from that ladder. By the assumption of increased DNA = increased complexity, the most complex organism in the world would be Amoeba. I don't think so....

Pseudo-worms and such

Yesterday I started with dinoflagellates, and showed a fairly typical example. While the typical dinoflagellates are fairly neat in their own right, this post will deal with some far less typical dinoflagellates - the parasitic members of the orders Blastodiniales and Syndiniales.

In the Fensome, Taylor et al. (1993) classification of dinoflagellates, Blastodiniales were a group of extracellular parasites with a dinokaryon (the distinctive dinoflagellate nucleus - see the previous post) for only part of their complicated life cycles. Blastodiniales were a diverse group, and Fensome et al. made no secret of its probable polyphyly. Most families of Blastodiniales start out as a trophont (feeding stage) attached to the host by rhizoids, a peduncle or a stylet (the exception is Blastodinium, in which the trophont is not actually attached but resides within the gut of copepods). The trophont may produce spores while attached and feeding (palisporogenesis), or may detach first before producing spores (palintomy). The spores are eventually released as motile dinospores, that have been recorded fusing to form gametes in at least some taxa, or give rise directly to the new generation of trophonts. Plastids are present in Blastodininium and Protoodinium, while other Blastodiniales are non-photosynthetic.

In Cachonella (Cachonellaceae), the trophont is attached to its siphonophore host by rhizoids. When it is finished feeding, it detaches and develops long tubular processes within the host's gut (the illustration in Fensome et al. has definite B-grade sci-fi appeal). After being passed from the host, the ex-trophont produces coccoid aplanospores (non-motile spores) that in turn remain attached to each other while shedding a series of cyst membranes, to form a branching structure with a spore at the end of each branch. Eventually the non-motile aplanospores give rise to motile dinospores.

Haplozoon is a very distinctive form that Fensome et al. assigned to Blastodiniales (though Leander et al. (2002) disputed this position, as Haplozoon appears to possess a dinokaryon throughout its life-cycle). Haplozoon initially attaches to its host (a larvacean or annelid) as a unicellular trophont by a stylet. It then undergoes multiple cell divisions to give rise to a flat worm- or ribbon-like (apparently) multicellular form with a single trophocyte (feeding cell), multiple rows of gonocytes (dividing cells) and a distal row of sporocytes (spore-producing cells). The single nuclei of the sporocytes become four, and individual sporocytes are released as cysts (probably eventually releasing four dinospores, but this doesn't appear to have actually been observed). While the mature stage of Haplozoon has generally been interpreted as multicellular (or colonial), Leander et al. found that SEM images of H. axiothellae appeared to show a single continuous membrane covering all "cells", and so interpreted Haplozoon as forming a unique compartmentalised syncytium (multinucleate single cell).

The non-photosynthetic Syndiniales possess dinoflagellate-like flagella, but do not possess a dinokaryon. In the Syndiniaceae, the trophont is a multinucleate plasmodium. In the Sphaeriparaceae, the trophont produces a long chain of aplanospores that are eventually released as dinospores.

Amoebophrya is a member of Syndiniales whose trophont develops a large conical cavity, the mastigocoel, in which multiple flagella are formed before the entire structure flips inside-out to give rise to a long worm-like, multinucleate, multiflagellate, mobile stage, the vermiform. The vermiform then undergoes multiple cleavages to form hundreds of individual dinospores.

REFERENCES

Fensome, R. A., F. J. R. Taylor, G. Norris, W. A. S. Sarjeant, D. I. Wharton & G. L. Williams. 1993. A classification of fossil and living dinoflagellates. Micropaleontology, Special Publication 7: i-viii, 1-351.

Leander, B. S., J. R. Saldarriaga & P. J. Keeling. 2002. Surface morphology of the marine parasite Haplozoon axiothellae Siebert (Dinoflagellata). European Journal of Protistology 38: 287-297.

Little whirling photosynthetic (and not so photosynthetic) thingies

I spent long and hard thinking of what I should make the subject of my first post (honestly, it took minutes!) but I eventually decided to write something on a subject I've spent a little time on recently but know precious little about - dinoflagellates (Dinoflagellata).

Dinoflagellates are unicellular protists, generally accepted to form a clade called Alveolata with ciliates and sporozoans (the image at left is a generalised dinoflagellate from Andrew MacRae's Dinoflagellates [http://www.geo.ucalgary.ca/~macrae/palynology/dinoflagellates/anatomy.html]). They are most familiar to the general public as the main culprit behind toxic algal blooms. The main distinguishing feature of dinoflagellates is that they possess two distinct flagella - a fairly straight one that sticks out from the cell, and a wavy one that wraps around the cell, usually in a groove. Most dinoflagellates also have a distinctive nucleus that lacks histones, the proteins that DNA wraps around in other eukaryotes, and with chromosomes that don't decondense between divisions. According to Fensome, Taylor et al. (1993), about half of dinoflagellates are photosynthetic, while the other half are mostly parasitic (some are both).

I got onto the subject of dinoflagellates because I was organising my records on dinoflagellate taxonomy (I try and cover the taxonomy of all organisms, and I am completely happy in the knowledge that this is probably an impossible task for one person - you can see a number of my efforts, varying from some I'm quite proud of to the truly tragic, at http://www.palaeos.org/). Dinoflagellates have perhaps the worst taxonomy of any group of organisms - worse than fossil plants, worse than South American harvestmen, worse than hominins. You may be aware that there are separate taxonomic codes for plants and animals. There are organisms that are neither plants nor animals, but because the codes were developed before this was understood, protists are assigned to either the botanical or zoological codes depending on which they were traditionally regarded as. Photosynthetic protists are covered by the botanical code, mobile protists are zoological. Problem is, some protists are both photosynthetic and mobile. Dinoflagellates are probably the largest group of organisms that have been regarded by different workers as under different taxonomic codes. As a result, the literature is full of names for dinoflagellates that are valid under one code but not under the other, and cases different codes require different names for the same thing. Palaeontologists working on dinoflagellates agreed to use the botanical code after 1961, and Fensome, Taylor et al. (1993) suggested the same thing for neontological taxa.

The other major issue with dinoflagellates is reconciling the fossil and living taxa. Many dinoflagellates form resistant vegetative cysts at some stage in the life-cycle, and these are the only stage that can be fossilised. Fossil taxonomy, therefore, is based on these. Neontological taxonomy, however, is generally based on the motile stage of the cycle. As a result, two separate taxonomies have developed in parallel, and there are relatively few cases where a cyst can be connected with a motile form. The worst case of this problem involves the fossil genus Spiniferites Mantell 1850, which has long been known to represent the cyst of the living genus Gonyaulax Diesing 1866. Unfortunately, because both of these genera are quite large and involve a lot of species, most workers have turned something of a blind eye to this point, and no real solution has been developed.

Coming up later, a few examples of the odder dinoflagellates, and the boundary between unicellularity and multicellularity. Unless, of course, I get distracted and cover something else.