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Metaspriggina: Vertebrate Fish Found in Cambrian Explosion

Metaspriggina_and_Maripolia.jpg

Now that some months have passed since the discovery of another rich trove of Cambrian fossils 26 miles from the Burgess Shale, scientists are starting to publish findings from the new Marble Canyon site. One amazing find just published by Simon Conway Morris and Jean-Bernard Caron is putting more bang in the Cambrian explosion.

Not so long ago, evolutionists emphasized that no vertebrates existed in the Cambrian. They knew that vertebrates were too advanced for that first appearance of multicellular body plans. Primitive chordates, maybe -- but nothing like fish till many millions of years later.

Metaspriggina (originally named after an Ediacaran species Spriggina but later determined to be unrelated) was earlier thought to be a primitive chordate -- an ancestor of vertebrates. Now, Conway Morris and Caron have examined a hundred more fossils of Metaspriggina and compared them with similar fossils from China and the Burgess Shale. The greater detail seen in the Marble Canyon specimens (thought to be earlier than the Burgess Shale) confirms that this animal was far more than a chordate: it was a vertebrate fish, right there in the Lower Cambrian! Imagine a vertebrate fish, with a skeleton, binocular vision, muscles, nerves, gut and blood vessels: it is so complex compared to what came before, it makes the suddenness and explosive increase in complexity undeniable.

Just as remarkable is the range of this species. Since it correlates with specimens in China's Chengjiang deposits, it's clear this fish was already "cosmopolitan" (Conway Morris's term) when it was buried in Canada -- it spanned the globe! The abstract in Nature catalogs the surprises as the authors "redescribe" Metaspriggina:

Knowledge of the early evolution of fish largely depends on soft-bodied material from the Lower (Series 2) Cambrian period of South China. Owing to the rarity of some of these forms and a general lack of comparative material from other deposits, interpretations of various features remain controversial, as do their wider relationships amongst post-Cambrian early un-skeletonized jawless vertebrates. Here we redescribe Metaspriggina on the basis of new material from the Burgess Shale and exceptionally preserved material collected near Marble Canyon, British Columbia, and three other Cambrian Burgess Shale-type deposits from Laurentia. This primitive fish displays unambiguous vertebrate features: a notochord, a pair of prominent camera-type eyes, paired nasal sacs, possible cranium and arcualia, W-shaped myomeres, and a post-anal tail. A striking feature is the branchial area [gills] with an array of bipartite bars. Apart from the anterior-most bar, which appears to be slightly thicker, each is associated with externally located gills, possibly housed in pouches. Phylogenetic analysis places Metaspriggina as a basal vertebrate, apparently close to the Chengjiang taxa Haikouichthys and Myllokunmingia, demonstrating also that this primitive group of fish was cosmopolitan during Lower-Middle Cambrian times (Series 2-3). However, the arrangement of the branchial region in Metaspriggina has wider implications for reconstructing the morphology of the primitive vertebrate. Each bipartite bar is identified as being respectively equivalent to an epibranchial and ceratobranchial. This configuration suggests that a bipartite arrangement is primitive and reinforces the view that the branchial basket of lampreys is probably derived. Other features of Metaspriggina, including the external position of the gills and possible absence of a gill opposite the more robust anterior-most bar, are characteristic of gnathostomes and so may be primitive within vertebrates. (Emphasis added.)

The cladogram shows Metaspriggina right on the same branch as the Chinese specimens, suggesting that they were "close to" each other in time and traits, even though found on opposite sides of the globe. Conway Morris says the Chinese specimens are "slightly older," but from his descriptions, they are similar to Metaspriggina in most important respects. Whether these creatures had bony or cartilaginous skeletons is not clear.

This relationship strengthens the identification of the Chinese species as vertebrate fish. Wikipedia had reservations about that description, saying of Myllokunmingia (announced in 1999) that it is "thought to be a vertebrate, although this is not conclusively proven," and of Haikouichthys (found in 2004), that it has been "popularly characterized as one of the earliest fishes...but does not possess sufficient features to be included uncontroversially even in either stem group" of craniates or chordates. Well, now it's essentially proven.

Another surprise is that Metaspriggina has a bipartite gill structure "characteristic of gnathostomes" -- the jawed vertebrates. Gnathostomes were thought to be further down the evolutionary timeline, but here are gnathostome-like traits found at the time of the Cambrian explosion. This means (in evolutionary terms) that the gill arrangements of lampreys (jawless fish) are "derived" rather than intermediate to the gnathostomes.

Needless to say, a creature that has "a pair of prominent camera-type eyes" and paired nasal sacs show this to be a sophisticated animal. Conway Morris does not hesitate to call it a fish and a vertebrate. The drawing in the paper shows "possible blood vessels" and a mouth. Fins were not preserved, making it look a bit like a tapering tonguefish, but the lack of fins could be an artifact of preservation.

Fins notwithstanding, Metaspriggina was a good swimmer, based on its muscle structures called myomeres. These are the W-shaped sheets of muscle you see on store-bought salmon filets; they allow fish to bend their bodies in wave-like motions to swim. Metaspriggina was apparently more advanced than Pikaia, an eel-like animal found in 1911 by Charles Walcott at the Burgess Shale: "The myomeres, totalling at least 40, are considerably more acute than in Pikaia and, in contrast to this chordate, Metaspriggina was evidently an effective swimmer."

All these traits show that Metaspriggina was not a primitive chordate intermediate to lampreys or other extinct Cambrian swimmers, but was in fact more "derived" (advanced) in some respects than some of the alleged descendants. The Editor's Summary agrees, stating clearly that vertebrate fish are now unquestionably part of the early Cambrian:

The Cambrian Burgess Shale of Canada has produced some of the most intriguing and spectacular fossils of early animal life, although fossil vertebrates have been rare to non-existent. New exposures close to the classic locality have remedied that deficiency with many spectacular fossils of the hitherto enigmatic fossil Metaspriggina, revealed in this study -- by Simon Conway Morris and Jean-Bernard Caron -- as one of the earliest known and most primitive fishes, basal to extant vertebrates whether jawed or jawless. The structure of the gills of Metaspriggina are revelatory, showing a simple structure that presages that of the jawed vertebrates in many ways, suggesting that the branchial basket seen in modern jawless vertebrates such as lampreys is a highly derived structure.

A vertebrate swimming fish with camera eyes, blood vessels, digestive system, muscular swimming, and gills in the Lower Cambrian: for Darwinists, it should hardly be more surprising to find than a Precambrian rabbit.

Image source: Wikipedia.