In biological science, a lazarus taxon is a species of animal that mysteriously reappears after a long gap within the fossil record. An example is the coelacanth, a type of fish that was thought to have gone extinct about eighty million years ago. Interestingly enough, tortoises are not usually considered to be lazarus taxa, because despite first appearing in the fossil record about three hundred million years ago, examples of tortoises are plentiful right up until the modern day.

This, of course, has not stopped Chicago post-rock band Tortoise from coopting the term as the title for this, a rather extensive collection of rare and obscure material from the band. The collection consists of three CDs, a DVD containing a number of the band's videos as well as a selection of live performances, and the requisite booklet that actually contains very little of interest.

There is a tendency, when faced with a volume of music as thick and impenetrable as this to simply dismiss it as a cash-in and an attempt to wring a few more quid out of the various offcuts and detritius that litters up most band's back catalogues. Particularly when a band is as challenging as Tortoise, who are probably best described not as "post-rock", but as "postmodern rock", the temptation is rife to write the whole thing off as a dense self-indulgent waste of time.

And, to be honest, I'd probably be able to justify making that sort of a declaration. Difficult to listen to at the best of times for the uninitiated, "A Lazarus Taxon" is Tortoise at their abstract, impermeable best. If you couldn't sit still all the way through Djed, then you're unlikely to be particularly impressed by the delicate finger picked guitar that opens of Gamera, the eleven minute track which has been placed at the beginning of the first CD of the set. Like a typical Tortoise track, Gamera swells and grows, with a pulsating baseline and trills of heavily processed guitar, before the bassline and beat slowly fade away towards the end of the song, finishing things up in a bath of warm-sounding feedback.

Other tracks are also typically oblique, such as Whitewater, which is built around gentle humming noises while an indefinable noise warbles in the background, and Autumn Sweater, which is nothing but a series of autumnal drones with the faintest hint of percussion hidden somewhere up at the back. Cliff Dweller Society is a sprawling, mishmash of a track that moves through about a dozen different styles and themes.

At times however, the band's experimentalism begins to grate, especially in the various remixes and faux-IDM tracks that litter this collection. A couple of minimalist songs cobbled together from various glitchy beats and reverberating hums are alright, but too many of them distract from the worthier tracks on offer here.

The inclusion of a collection of remixes here is also baffling, while they use Tortoise songs as their base, they seem to be a lot more representative of the style of the remixer, be that Autechre or Nobukazu Takemura. It's all very well to say that they sound cool, but if I wanted to listen to Autechre, I wouldn't be listening to Tortoise. The third disc, the out of print remix album "Rhythms, Resolutions, and Clusters", while probably included for the sake of completeness, also sounds a bit dated, and probably won't be of interest except to collectors and completists (who probably already have the original, anyway).

"A Lazarus Taxon" has clearly been designed for the dedicated Tortoise fan, by eschewing their more accessible material in favour of material which is more difficult and challenging. Working on this music for long enough to "get" it is something that will take a while, but it's a journey that you will find incredibly rewarding. The fact that this collection is extra affordable compared to other similar collections is icing on the cake, lowering the barrier to entry considerably. This Tortoise isn't going to be leaving gaps in the fossil record any time soon.