The animals (Kingdom Animalia), otherwise known as the Metazoa, represent one of the two groups to be considered in this website, the other being the Land Plants. The phylogenetic context of the animals is illustrated in the following time tree:
Figure 1. Phylogenetic time tree showing first appearance of total-group clades within the Unikonta
The above tree illustrates the large amount of time (at least a billion years) that elapsed between the appearance of the eukaryotes and that of the Metazoa.
The phylogenetic relationships within the metazoans are illustrated in the following tree:
The phylogenetic relationships within the metazoans are illustrated in the following tree:
Figure 2. Summarized phylogenetic tree of the Metazoa
The crown node of the animals is shown as a black dot. The phylogenetic tree shown above is only one of several that are currently under consideration in the literature, given that there is still no consensus over the phylogenetic relationships between the component clades of the Metazoa. One of the unresolved uncertainties concerns whether the sponges (Porifera) are basal to the comb jellies (Ctenophora) or vice versa (Redmond and McLysaght, 2021).
On this page we will consider only the transition to the metazoan crown group as no literature pertaining to stem-Opisthokonta or stem-Unikonta has been found in the research carried out for this website.
All the animal groups on the right hand side of the above tree, with the exception of the Cephalochordata, will be considered in the following sections of the website. (The Cephalochordata are excluded because of their very limited fossil record.) These groups belong to several higher categories:
Of the groups on the right hand side of the tree, only three are phyla: Ctenophora, Porifera and Cnidaria. The rest represent higher or, in the case of the Chordata, lower groups:
On this page we will consider only the transition to the metazoan crown group as no literature pertaining to stem-Opisthokonta or stem-Unikonta has been found in the research carried out for this website.
All the animal groups on the right hand side of the above tree, with the exception of the Cephalochordata, will be considered in the following sections of the website. (The Cephalochordata are excluded because of their very limited fossil record.) These groups belong to several higher categories:
- Eumetazoa: all animals except the sponges (Porifera); in some classifications, ctenophores are excluded from the eumetazoans
- Bilateria: all animals with a body form exhibiting bilateral symmetry
- Deuterostomia: bilaterans in which the anus forms before the mouth during development of the embryo
- Chordata: having at some point in their life cycle several unique features, including a notochord (a stiff cartilaginous rod supporting, the nerve cord)
- Olfactores: all chordates except the Cephalochordata.
Of the groups on the right hand side of the tree, only three are phyla: Ctenophora, Porifera and Cnidaria. The rest represent higher or, in the case of the Chordata, lower groups:
- Protostomia: bilaterans in which the mouth forms before the anus during development of the embryo; includes the arthropods, worms and molluscs
- Ambulacraria: living representatives comprise the echinoderms and the hemichordates
- Cephalochordata: soft-bodied chordates without true vertebrae; living representatives are the lancelets
- Tunicata: chordates with a polysaccharide exoskeleton
- Vertebrata: chordates with vertebrae.
Origin of the animals
The fossil record is less useful for determining the origin of animals than it is for later transitions. There are two main reasons for this. One is that the early animals are generally soft-bodied, and thus tend not to be well preserved as fossils. The other issue is the difficulty of relating the earliest fossils of multicellular organisms to the extant taxa of animals, as well expressed by Budd (2001):
“Useful stem-group taxa are unfortunately difficult to assign, as they lack diagnostic features of crown groups. Further, unlike stem groups that lie within extant phyla, which often have their basal nodes well known, fossils that lie within the stem-groups of phyla, which have virtually unknown basal nodes, cannot be assigned to that place except on the basis of shared similarities with the crown group. As a result, very basal stem-group forms may be completely unrecognized.”
The above statement was made in the context of defining synapomorphies for the stem groups of the phyla or clades within the Metazoa, but it applies with even more force to the search for fossils that might represent the metazoan stem group.
A further problem is that in some cases it is not clear that these fossils even represent animals at all (for discussion of a non-metazoan multicellular fossil, see Agić et al, 2019).
The reality is that there is still no consensus assignment of any single fossil to the animal stem group. However, there is growing agreement that at least some of the macrofossils of the Ediacaran, the "Ediacaran macrobiota", belong to the stem-Eumetazoa and thus represent crown-Metazoa (Dufour and McIlroy, 2017; Hoyal Cuthill and Han, 2018; Runnegar, 2021).
“Useful stem-group taxa are unfortunately difficult to assign, as they lack diagnostic features of crown groups. Further, unlike stem groups that lie within extant phyla, which often have their basal nodes well known, fossils that lie within the stem-groups of phyla, which have virtually unknown basal nodes, cannot be assigned to that place except on the basis of shared similarities with the crown group. As a result, very basal stem-group forms may be completely unrecognized.”
The above statement was made in the context of defining synapomorphies for the stem groups of the phyla or clades within the Metazoa, but it applies with even more force to the search for fossils that might represent the metazoan stem group.
A further problem is that in some cases it is not clear that these fossils even represent animals at all (for discussion of a non-metazoan multicellular fossil, see Agić et al, 2019).
The reality is that there is still no consensus assignment of any single fossil to the animal stem group. However, there is growing agreement that at least some of the macrofossils of the Ediacaran, the "Ediacaran macrobiota", belong to the stem-Eumetazoa and thus represent crown-Metazoa (Dufour and McIlroy, 2017; Hoyal Cuthill and Han, 2018; Runnegar, 2021).
The metazoan crown group
The oldest known members of the metazoan crown group are macrofossils found in the Ediacaran Drook Formation in the Mistaken Point Ecological Reserve, Newfoundland, Canada (Liu et al, 2012; Matthews et al, 2021). Key members of this assemblage are illustrated below (for a larger view, click on image):
Figure 3. Some Ediacaran fossils
The nature of these animals is helpfully discussed in Butterfield (2020) and Runnegar (2021).
Based on the age of the Drook Formation in which the above fossils are found, the crown group of the Metazoa appeared no later than 572 million years ago.
In addition to the Ediacaran biota, several other fossil groups have been assigned to the crown-Metazoa without consensus as to their affinities within the crown group. These are the Vetulicoans and the Yunnanozoans. Both were discovered first in the Early Cambrian Chengjiang Lagerstätte of Yunnan Province, China. The Vetulicoans have been variously interpreted as stem-Deuterostoma, stem-Chordata, stem-Tunicata or even as Protostomia (Vinther et al, 2011), while the Yunnanozoans have been assigned to stem-Ambulacraria, crown-Ambulacraria, stem-Cephalochordata, stem-Olfactores or stem-Vertebrata (Benton, 2015). Examples of fossils of these groups are shown below:
Based on the age of the Drook Formation in which the above fossils are found, the crown group of the Metazoa appeared no later than 572 million years ago.
In addition to the Ediacaran biota, several other fossil groups have been assigned to the crown-Metazoa without consensus as to their affinities within the crown group. These are the Vetulicoans and the Yunnanozoans. Both were discovered first in the Early Cambrian Chengjiang Lagerstätte of Yunnan Province, China. The Vetulicoans have been variously interpreted as stem-Deuterostoma, stem-Chordata, stem-Tunicata or even as Protostomia (Vinther et al, 2011), while the Yunnanozoans have been assigned to stem-Ambulacraria, crown-Ambulacraria, stem-Cephalochordata, stem-Olfactores or stem-Vertebrata (Benton, 2015). Examples of fossils of these groups are shown below:
Figure 4. Early Cambrian Vetulicolian Nesonektris aldridgei
Figure 5. Early Cambrian Yunnanozooan Yunnanozoon lividum
Figure 6. Early Cambrian Yunnanozooan Haikouella lanceolata
One other fossil of uncertain affinity is Pikaia gracilens, described from the Middle Cambrian Burgess Shale of Canada. There is consensus that this fossil is a chordate (Morris and Caron (2012), Mallat and Holland (2013), Benton (2015)), but its position within the total group remains unresolved. A fossil image and a life restoration are shown below:
Figure 7. Fossil and life restoration of Pikaia gracilens (Middle Cambrian)
Given the uncertainty in phylogenetic position of the above fossils, we will now focus our attention on fossils that can be assigned with confidence to one of the terminal groups shown in the phylogenetic tree above.
References
Agić, H., Högström, A. E., Moczydłowska, M., Jensen, S., Palacios, T., Meinhold, G., ... & Høyberget, M. (2019). Organically-preserved multicellular eukaryote from the early Ediacaran Nyborg Formation, Arctic Norway. Nature scientific reports, 9(1), 1-12.
Benton, M. J. (2015). Vertebrate Palaeontology - Fourth edition. John Wiley & Sons, 468 pages.
Benton, M. J., Donoghue, P. C., Asher, R. J., Friedman, M., Near, T. J., & Vinther, J. (2015). Constraints on the timescale of animal evolutionary history. Palaeontol Electron, 18(1), 1-106.
Budd, G. E. (2001). Tardigrades as ‘stem-group arthropods’: the evidence from the Cambrian fauna. Zoologischer Anzeiger-A Journal of Comparative Zoology, 240(3-4), 265-279.
Budd, G. E., & Jensen, S. (2017). The origin of the animals and a ‘Savannah’ hypothesis for early bilaterian evolution. Biological Reviews, 92(1), 446-473.
Butterfield, N. J. (2020). Constructional and functional anatomy of Ediacaran rangeomorphs. Geological Magazine, 1-12.
Dufour, S. C., & McIlroy, D. (2017). Ediacaran pre-placozoan diploblasts in the Avalonian biota: the role of chemosynthesis in the evolution of early animal life. Geological Society, London, Special Publications, 448(1), 211-219.
Fedonkin, M. A., & Waggoner, B. M. (1997). The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism. Nature, 388(6645), 868-871.
García-Bellido, D. C., Lee, M. S., Edgecombe, G. D., Jago, J. B., Gehling, J. G., & Paterson, J. R. (2014). A new vetulicolian from Australia and its bearing on the chordate affinities of an enigmatic Cambrian group. BMC evolutionary biology, 14(1), 1-14.
Hoyal Cuthill, J. F., & Han, J. (2018). Cambrian petalonamid Stromatoveris phylogenetically links Ediacaran biota to later animals. Palaeontology, 61(6), 813-823.
Ivantsov, A., Nagovitsyn, A., & Zakrevskaya, M. (2019). Traces of locomotion of Ediacaran macroorganisms. Geosciences, 9(9), 395.
Liu A.G., McIlroy D., Matthews J.J., Brasier M.D. (2012). A new assemblage of juvenile Ediacaran fronds from the Drook Formation, Newfoundland. J. Geol. Soc. London 169, 395–403. (doi:10.1144/0016-76492011-094).
Liu, A. G., Matthews, J. J., Menon, L. R., McIlroy, D., & Brasier, M. D. (2014). Haootia quadriformis n. gen., n. sp., interpreted as a muscular cnidarian impression from the Late Ediacaran period (approx. 560 Ma). Proceedings of the Royal Society B: Biological Sciences, 281(1793), 20141202.
Mallatt, J., & Holland, N. (2013). Pikaia gracilens Walcott: stem chordate, or already specialized in the Cambrian?. Journal of Experimental Zoology Part B: Molecular and Developmental Evolution, 320(4), 247-271.
Matthews, J. J., Liu, A. G., Yang, C., McIlroy, D., Levell, B., & Condon, D. J. (2021). A chronostratigraphic framework for the rise of the Ediacaran macrobiota: new constraints from Mistaken Point Ecological Reserve, Newfoundland. Bulletin, 133(3-4), 612-624.
Morris, S. C., & Caron, J. B. (2012). Pikaia gracilens Walcott, a stem‐group chordate from the Middle Cambrian of British Columbia. Biological Reviews, 87(2), 480-512.
Redmond, A. K., & McLysaght, A. (2021). Evidence for sponges as sister to all other animals from partitioned phylogenomics with mixture models and recoding. Nature communications, 12(1), 1-14.
Runnegar, B. (2021). Following the logic behind biological interpretations of the Ediacaran biotas. Geological Magazine, 1-25.
Vinther, J., Smith, M. P., & Harper, D. A. (2011). Vetulicolians from the Lower Cambrian Sirius Passet Lagerstätte, North Greenland, and the polarity of morphological characters in basal deuterostomes. Palaeontology, 54(3), 711-719.
Benton, M. J. (2015). Vertebrate Palaeontology - Fourth edition. John Wiley & Sons, 468 pages.
Benton, M. J., Donoghue, P. C., Asher, R. J., Friedman, M., Near, T. J., & Vinther, J. (2015). Constraints on the timescale of animal evolutionary history. Palaeontol Electron, 18(1), 1-106.
Budd, G. E. (2001). Tardigrades as ‘stem-group arthropods’: the evidence from the Cambrian fauna. Zoologischer Anzeiger-A Journal of Comparative Zoology, 240(3-4), 265-279.
Budd, G. E., & Jensen, S. (2017). The origin of the animals and a ‘Savannah’ hypothesis for early bilaterian evolution. Biological Reviews, 92(1), 446-473.
Butterfield, N. J. (2020). Constructional and functional anatomy of Ediacaran rangeomorphs. Geological Magazine, 1-12.
Dufour, S. C., & McIlroy, D. (2017). Ediacaran pre-placozoan diploblasts in the Avalonian biota: the role of chemosynthesis in the evolution of early animal life. Geological Society, London, Special Publications, 448(1), 211-219.
Fedonkin, M. A., & Waggoner, B. M. (1997). The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism. Nature, 388(6645), 868-871.
García-Bellido, D. C., Lee, M. S., Edgecombe, G. D., Jago, J. B., Gehling, J. G., & Paterson, J. R. (2014). A new vetulicolian from Australia and its bearing on the chordate affinities of an enigmatic Cambrian group. BMC evolutionary biology, 14(1), 1-14.
Hoyal Cuthill, J. F., & Han, J. (2018). Cambrian petalonamid Stromatoveris phylogenetically links Ediacaran biota to later animals. Palaeontology, 61(6), 813-823.
Ivantsov, A., Nagovitsyn, A., & Zakrevskaya, M. (2019). Traces of locomotion of Ediacaran macroorganisms. Geosciences, 9(9), 395.
Liu A.G., McIlroy D., Matthews J.J., Brasier M.D. (2012). A new assemblage of juvenile Ediacaran fronds from the Drook Formation, Newfoundland. J. Geol. Soc. London 169, 395–403. (doi:10.1144/0016-76492011-094).
Liu, A. G., Matthews, J. J., Menon, L. R., McIlroy, D., & Brasier, M. D. (2014). Haootia quadriformis n. gen., n. sp., interpreted as a muscular cnidarian impression from the Late Ediacaran period (approx. 560 Ma). Proceedings of the Royal Society B: Biological Sciences, 281(1793), 20141202.
Mallatt, J., & Holland, N. (2013). Pikaia gracilens Walcott: stem chordate, or already specialized in the Cambrian?. Journal of Experimental Zoology Part B: Molecular and Developmental Evolution, 320(4), 247-271.
Matthews, J. J., Liu, A. G., Yang, C., McIlroy, D., Levell, B., & Condon, D. J. (2021). A chronostratigraphic framework for the rise of the Ediacaran macrobiota: new constraints from Mistaken Point Ecological Reserve, Newfoundland. Bulletin, 133(3-4), 612-624.
Morris, S. C., & Caron, J. B. (2012). Pikaia gracilens Walcott, a stem‐group chordate from the Middle Cambrian of British Columbia. Biological Reviews, 87(2), 480-512.
Redmond, A. K., & McLysaght, A. (2021). Evidence for sponges as sister to all other animals from partitioned phylogenomics with mixture models and recoding. Nature communications, 12(1), 1-14.
Runnegar, B. (2021). Following the logic behind biological interpretations of the Ediacaran biotas. Geological Magazine, 1-25.
Vinther, J., Smith, M. P., & Harper, D. A. (2011). Vetulicolians from the Lower Cambrian Sirius Passet Lagerstätte, North Greenland, and the polarity of morphological characters in basal deuterostomes. Palaeontology, 54(3), 711-719.
Image credits - Animals
- Header: Hippopotamus in the river near St.Lucia, by Petter Lindgren [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons.
- Figure 2 (Trepassia wardae): Peter Trusler, CC0, via Wikimedia Commons
- Figure 2 (Charnia masoni): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 2 (Life restoration of Charnia masoni): Nobu Tamura (http://spinops.blogspot.com/), CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2 (Hiemalora stellaris): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 2 (Charniodiscus sp.): Daderot, CC0, via Wikimedia Commons
- Figure 3: From Open Access article García-Bellido, D. C., Lee, M. S., Edgecombe, G. D., Jago, J. B., Gehling, J. G., & Paterson, J. R. (2014). A new vetulicolian from Australia and its bearing on the chordate affinities of an enigmatic Cambrian group. BMC evolutionary biology, 14(1), 1-14.
- Figure 4: Zhu Maoyan, professor, Copyrighted free use, via Wikimedia Commons
- Figure 5 (fossil): Vassil, CC0, via Wikimedia Commons
- Figure 5 (restoration): Apokryltaros, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 6 (fossil): Jstuby at English Wikipedia, Public domain, via Wikimedia Commons
- Figure 6 (restoration): Nobu Tamura email:nobu.tamura@yahoo.com http://spinops.blogspot.com/ http://paleoexhibit.blogspot.com/, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons