The animals are one of the two groups to be considered in this website. In most modern classifications, such as that of the Tree of Life web project (http://tolweb.org/Animals/2374), animals are synonymous with the Metazoa. The phylogeny of the animals, based on the Open Tree of Life (https://tree.opentreeoflife.org) is illustrated below:
All the nodes shown in the above figure, shown as black dots, represent crown nodes of the successive crown groups into which the metazoan crown group has divided.
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:
Of these groups, only the vertebrates have been completed to date.
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 ctenophores and the sponges (Porifera); in some classifications, ctenophores are included in the eumetazons
- 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.
Of these groups, only the vertebrates have been completed to date.
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 (Budd and Jensen, 2017). In some cases, it is not clear that the fossils even represent animals at all (for discussion of a non-metazoan multicellular fossil, see Agić et al, 2019).
Despite these difficulties, some fossils have been assigned to stem groups and crown groups of extant taxa.The following figure depicts the appearance times of the metazoan crown clades shown above
Despite these difficulties, some fossils have been assigned to stem groups and crown groups of extant taxa.The following figure depicts the appearance times of the metazoan crown clades shown above
Owing to the lack of fossils for many taxa, there is quite a lot of uncertainty in the timing of crown-group appearances. However, it demonstrates clearly that most, if not all, of the extant taxa appeared before the middle of the Cambrian. Furthermore, most crown groups appeared after the beginning of the Cambrian. This relatively short time interval of around 40 million years represents the Cambrian Explosion (Morris, 2000).
Some of the fossils used to calibrate the above plot are illustrated below:
Stem-Metazoa
Some of the fossils used to calibrate the above plot are illustrated below:
Stem-Metazoa
These are the most difficult to identify, and in fact there is no consensus assignment of any single fossil to the animal stem group. However, there is general agreement that the macrofossils of the Ediacaran, the "Edicaran macrobiota", do represent total-group Metazoa (Wood et al, 2019). This implies the strong likelihood that some are stem metazoans. Some examples are shown below (click on any image for a larger view):
* after name indicates that the image represents a life restoration.
Basal crown-Metazoa
The following images illustrate some (i.e. those for which public-domain images are available) of the earliest known fossil representatives of the stem and crown groups that comprise the basal crown-Metazoa (click on any image for a larger view):
* after name indicates that the image represents a life restoration.
Photo credit
- 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.
- Charnia masoni By Smith609 (Transferred by AdmiralHood) [CC BY 2.5 (https://creativecommons.org/licenses/by/2.5)]
- Charnia masoni* By Nobu Tamura (http://spinops.blogspot.com/) [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Dickinsonia costata By Verisimilus at English Wikipedia [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]
- Dickinsonia costata* By Nobu Tamura (http://spinops.blogspot.com/) [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Aspidella sp. By Phoebe Cohen, Attribution-NonCommercial-ShareAlike 2.0 Generic (CC BY-NC-SA 2.0)
- Fractofusus misrai By Phoebe Cohen, Attribution-NonCommercial-ShareAlike 2.0 Generic (CC BY-NC-SA 2.0)
- Charniodiscus arboreus By Experiencing Life Trips, Attribution-NonCommercial-ShareAlike 2.0 Generic (CC BY-NC-SA 2.0)
- Bradgatia linfordensis By Phoebe Cohen, Attribution-NonCommercial-ShareAlike 2.0 Generic (CC BY-NC-SA 2.0)
- Bradgatia linfordensis* By MUSE [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Maotianoascus octonarius By Shallowell [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Maotianoascus octonarius* By Apokryltaros [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Paleoctenophora brasseli* By Apokryltaros [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Haootia quadriformis* By Apokryltaros [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Embryos of Olivooides sp. By Gabreila mela [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Kimberella quadrata By Masahiro miyasaka [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Kimberella quadrata* By Nobu Tamura (http://spinops.blogspot.com) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Shankouclava anningense* By Apokryltaros [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Haikouichthys ercaicunensis* By Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license