Pikaia: upside down or rightside up?
Short one today
as the question of Pikaia’s orientation arises again. Here (Fig 1) are the comparables: the lancelet, Branchiostoma, which has similar muscle arrangements and buccal cirri alongside the more primitive nematode, Enoplus, which has antennae = sensory organs here and lacks a notochord.
Judge for yourself and read the paper.
Figure 1. The ROM IP 61236 specimen of Pikaia shown upside down and right side up compare to the extant lancelet, Branchiostoma, and the nematode, Enoplus.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/pikaia-romip-61236-588.jpg?w=137″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/pikaia-romip-61236-588.jpg?w=468″ tabindex=”0″ role=”button” class=”size-full wp-image-87882″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/pikaia-romip-61236-588.jpg” alt=”Figure 1. The ROM IP 61236 specimen of Pikaia shown upside down and right side up compare to the extant lancelet, Branchiostoma, and the nematode, Enoplus. ” width=”584″ height=”1277″ />
Figure 1. The ROM IP 61236 specimen of Pikaia shown upside down and right side up compare to the extant lancelet, Branchiostoma, and the nematode, Enoplus.
Mussini et al omitted
Enoplus in analysis and for that reason + following tradition (Fig 2), made simpler but derived taxa (Fig 3) primitive to Pikaia. The authors identified new structures on top of the head of Pikaia, but those are spread out buccal cilia attached below the ventral oral cavity. Note the arrangement of the myomeres = fish muscles with a small portion atop a tall portion. Note the natural downturn of the snout. Note the extension of the notochord from anteriormost to posteriormost while oral to anal distance on Branchiostoma is shorter and ventral, not dorsal.
Figure 1. From Romer 1971 the origin of chordates.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2020/08/chordate_originsromer-1971-588.jpg?w=215″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2020/08/chordate_originsromer-1971-588.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-47455″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2020/08/chordate_originsromer-1971-588.jpg” alt=”Figure 1. From Romer 1971 the origin of chordates.” width=”584″ height=”814″ />
Figure 2. From Romer 1971 the origin of chordates.
Figure 3. Chordate evolution, changes to Romer 1971 from Peters 1991. Here echinoderms have lost the tail and gills of the free-swimming tunicate larva.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2020/08/chordate_originspeters-1991-588-1.jpg?w=215″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2020/08/chordate_originspeters-1991-588-1.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-47466″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2020/08/chordate_originspeters-1991-588-1.jpg” alt=”Figure 3. Chordate evolution, changes to Romer 1971 from Peters 1991. Here echinoderms have lost the tail and gills of the free-swimming tunicate larva.” width=”584″ height=”814″ />
Figure 3. Chordate evolution, changes to Romer 1971 from Peters 1991. Here echinoderms have lost the tail and gills of the free-swimming tunicate larva.
Fossil interpretation is not always easy.
Sometimes professionals get it wrong (eg Yi qi and Ambopteryx). Often I get it wrong, too. I re-present my case above with new data. I’ll let you know when I change my mind.
References
Bardack D and Richardson ES Jr 1977. New aganathous fishes from the Pennsylvanian of Illinois. Fieldiana Geology 33(26):489–510.
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
Lacalli T 2024. The Cambrian fossil Pikaia, and the origin of chordate somites. BMC EvoDEvo 15:1.
Mallatt J and Holland ND 2013. Pikaia gracilens Walcott: stem chordate or already specialized in the Cambrian? Journal of Experimental Zoology, Part B, Molecular and Developmental Evolution 320B: 247–271.
Mussini et al (9 co-authors 2024. A new interpretation of Pikaia reveal the origins of the chordate body plan. Current Biology. 34:2980–2989.
Peters D 1991. From the Beginning – The story of human evolution. Wm Morrow.
Walcott CD 1911. Cambrian geology and paleontology. Middle Cambrian annelids. Smithsonian Miscellaneous Collections 57(5):109–144.
Pikaia, the Middle Cambrian lancelet, poorly revisited in 2024
Source: https://pterosaurheresies.wordpress.com/2024/07/30/pikaia-upside-down-or-rightside-up/
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