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Cited by 69Developmental Biology and Gene RegulationMarine Invertebrate Physiology and EcologyPaleontology and Evolutionary Biology

New perspectives on the evolution of protochordate sensory and locomotory systems, and the origin of brains and heads.

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Lacalli, Thurston C.·2001·Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences
DOI 10.1098/rstb.2001.0974
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Abstract

Cladistic analyses generally place tunicates close to the base of the chordate lineage, consistent with the assumption that the tunicate tail is primitively simple, not secondarily reduced from a segmented trunk. Cephalochordates (i.e. amphioxus) are segmented and resemble vertebrates in having two distinct locomotory modes, slow for distance swimming and fast for escape, that depend on separate sets of motor neurons and muscle cells. The sense organs of both amphioxus and tunicate larvae serve essentially as navigational aids and, despite some uncertainty as to homologies, current molecular and ultrastructural data imply a close relationship between them. There are far fewer signs of modification and reduction in the amphioxus central nervous system (CNS), however, so it is arguably the closer to the ancestral condition. Similarities between amphioxus and tunicate sense organs are then most easily explained if distance swimming evolved before and escape behaviour after the two lineages diverged, leaving tunicates to adopt more passive means of avoiding predation. Neither group has the kind of sense organs or sensory integration centres an organism would need to monitor predators, yet mobile predators with eyes were probably important in the early Palaeozoic. For a predator, improvements in vision and locomotion are mutually reinforcing. Both features probably evolved rapidly and together, in an 'arms race' of eyes, brains and segments that left protochordates behind, and ultimately produced the vertebrate head.

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  • Vision in vertebrates dates to the Palaeozoic, and well-developed eyes are a key feature for distinguishing fossil vertebrates from non-vertebrate chordates.

    “Vision in vertebrates dates to the Palaeozoic, and well-developed eyes are in fact a key feature for distinguishing fossil vertebrates from non-vertebrate chordates (Shimeld and Holland, 2000; Lacalli, 2001).”

    From The function(s) of consciousness revisited: insights from a modular/constitutive model using vision as a test case (2026) · cites this paper as Lacalli, 2001

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Authors
Lacalli, T. C.
Affiliations
University of Saskatchewan (CA)
Corresponding author
Lacalli, Thurston C.
ORCID
Lacalli, Thurston C.
Venue
Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences
Publisher
The Royal Society
Published
2001-10-29
Volume
356
Issue
1414
Pages
1565-1572
ISSN
  • 0962-8436 (ISSN-L)
  • 1471-2970
DOI
10.1098/rstb.2001.0974
OpenAlex
W2141959073
Cited by
69
References
50
Citations by year
2026: 1 · 2023: 3 · 2021: 1 · 2020: 1 · 2019: 3 · 2018: 3 · 2017: 1 · 2016: 2 · 2015: 4 · 2014: 1 · 2013: 1 · 2012: 3
FWCI
0.39
Type
journal article
Language
en
Open access
Licence
royalsociety.org/journals/ethics-policies/data-sharing-mining/ (tdm)
Locations
  • Philosophical Transactions of the Royal Society B Biological Sciences (publishedVersion)
  • Europe PMC (PubMed Central) (publishedVersion)
  • PubMed (publishedVersion)
  • PubMed Central (publishedVersion)
Topics
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Developmental Biology and Gene Regulation
  • Physical Sciences › Earth and Planetary Sciences › Paleontology › Marine Invertebrate Physiology and Ecology
  • Physical Sciences › Earth and Planetary Sciences › Paleontology › Paleontology and Evolutionary Biology
Keywords
protochordates, tunicates, cephalochordates, amphioxus, sensory organs, locomotion, central nervous system evolution, chordate evolution, evolution of vision, predator-prey arms race, homology, Paleozoic
MeSH terms
Animals, Brain, Central Nervous System, Chordata, Nonvertebrate, Biological Evolution, Head, Locomotion, Neurons, Afferent, Urochordata

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