Have we been looking at Pikaia upside-down?

[Halichoeres]

Pikaia has been regarded as a possible early chordate since at least the 1970s, but there are some odd topological features that arise from the typical interpretation, like the position of the gut relative to the notochord. Turn it upside-down and you solve some problems, although that creates its own oddities: the tentacles now point downward, and the feathery gill-like structures now point upward. They also claim that there is no notochord at all, but there is a dorsal nerve cord, which is interesting because

I'll be interested in seeing the responses to this. I'm a decent anatomist, but I don't know enough to be able to tell how good this line of argument is. The authors argue that what they interpret as the dorsal nerve cord is associated with small nerves in the head, which, you know, makes sense.

A visual summary of their hypothesis and reconstruction:


Open access: https://doi.org/10.1016/j.cub.2024.05.026
And a write-up in the New York Times: https://www.nytimes.com/2024/06/11/science/pikaia-vertebrate-evolution.html?unlocked_article_code=1.zE0.Vyzm.ktoFhp3XTZjR&smid=url-share (this should get around the paywall, but I'm not certain of that)

[Bowhead Whale]

Well, that is indeed an interesting question. Although, I don't think we have been looking at the animal upside down. According to my own experience, invertebrates like worms, slugs or onychophoras have a head that is smaller and thinner then their behinds. Not only that, but none of them has what you called a "post-anal tail"; that characteristics seems to be exclusive to the vertebrated animals. Slugs and Onychophoras have their antennas on their heads, thos appendices being pedonculed eyes. Why would is be different for the pikaia? The "antennas" can be very useful on a head, but I have no idea why those appendices would be put on a behind, if you see what I mean. What do you think?

[Halichoeres]

Well, that is indeed an interesting question. Although, I don't think we have been looking at the animal upside down. According to my own experience, invertebrates like worms, slugs or onychophoras have a head that is smaller and thinner then their behinds. Not only that, but none of them has what you called a "post-anal tail"; that characteristics seems to be exclusive to the vertebrated animals. Slugs and Onychophoras have their antennas on their heads, thos appendices being pedonculed eyes. Why would is be different for the pikaia? The "antennas" can be very useful on a head, but I have no idea why those appendices would be put on a behind, if you see what I mean. What do you think?

Well, this study posits that we've been interpreting it upside-down, but not backward. I agree that the head end is still the head end, with its tentacles and everything. It just forces the reinterpretation of the various lengthwise smears of color that could be nerve cords, guts, notochords, etc., and puts the tentacles facing the ground rather than facing the sky (when the animal is oriented laterally).

[bmathison1972]

Sorry I just saw this; I also posted on this today in a new thread. I looked but I must have missed your thread, Tim.

[stargatedalek]

The tentacles pointing down makes sense if they were sensory organs does it not?

[Halichoeres]

Sorry I just saw this; I also posted on this today in a new thread. I looked but I must have missed your thread, Tim.

No worries, I've done the same thing!

The tentacles pointing down makes sense if they were sensory organs does it not?

It doesn't not make sense, but I can imagine them being useful in either orientation.