Pelecaniformes

The Pelecaniformes /pɛlɪˈkænɪfɔːrmiːz/ are an order of medium-sized and large waterbirds found worldwide. As traditionally (but erroneously) defined, they encompass all birds that have feet with all four toes webbed. Hence, they were formerly also known by such names as totipalmates or steganopods. Under this obsolete definition, the Fregatidae (frigatebirds), Sulidae (gannets and boobies), Phalacrocoracidae (cormorants and shags), Anhingidae (darters), and Phaethontidae (tropicbirds) were included in the Pelecaniformes. Subsequent molecular and morphological studies indicate they are in fact not close relatives to "true" Pelecaniformes, and they are now placed in their own orders, Suliformes and Phaethontiformes, respectively.

Pelecaniformes as currently defined comprise the pelicans, shoebill and hamerkop, which form a clade within the order (suborder Pelecani), along with herons (Ardeidae) and ibises and spoonbills (Threskiornithidae); the relationships between Pelecani and the other two families are still being debated by taxonomists.[1]

## Contents

* 1 Systematics and evolution
* 1.1 Fossils
* 2 References
* 3 Further reading
* 4 External links

## Systematics and evolution

Classically, bird relationships were based solely on morphological characteristics. The Pelecaniformes were traditionally, but erroneously, defined as birds that have feet with all four toes webbed (totipalmate), as opposed to all other birds with webbed feet where only three of four were webbed. Hence, they were formerly also known by such names as totipalmates or steganopodes. The group included frigatebirds, gannets, cormorants, anhingas, and tropicbirds.[2]

Research from the beginning of the 21st century strongly suggested that the similarities between this traditional definition of Pelecaniformes are the result of convergent evolution rather than common descent, being adaptations that were converged upon by birds living in and near aquatic environments; the traditional definition of the group would thus be polyphyletic.[3]

Sibley and Ahlquist's landmark DNA–DNA hybridisation studies (now known as the Sibley–Ahlquist taxonomy) led to them placing the families traditionally contained within the Pelecaniformes together with the grebes, cormorants, ibises and spoonbills, New World vultures, storks, penguins, albatrosses, petrels, and loons together as a subgroup within a greatly expanded order Ciconiiformes, a radical move which by now has been all but rejected: their "Ciconiiformes" merely assembled all early advanced land- and seabirds for which their research technique delivered insufficient phylogenetic resolution.

Morphological study had suggested pelicans are sister to a gannet-cormorant clade, yet genetic analysis groups them with the hamerkop and shoebill, though the exact relationship between the three is unclear.[4] Mounting evidence pointed to the shoebill as a close relative of pelicans.[2][needs update] This also included microscopic analysis of eggshell structure by Konstantin Mikhailov in 1995, who found that the shells of pelecaniform eggs were covered in a thick microglobular material; importantly, the eggs of shoebills have these strucures, but not those of the tropicbirds.[5] Reviewing genetic evidence to date, Cracraft and colleagues surmised that pelicans were sister to the shoebill with the hamerkop as the next earlier offshoot.[6] Ericson and colleagues sampled five nuclear genes in a 2006 study spanning the breadth of bird lineages, and came up with pelicans, shoebill and hamerkop in a clade.[7] Hackett and colleagues sampled 32 kilobases of nuclear DNA and recovered shoebill and hamerkop as sister taxa, pelicans sister to them, and herons and ibises as sister groups to each other, with the heron and ibis group a sister to the pelican/shoebill/hamerkop clade.[8] The phylogenetic tree below illustrates this situation:

Pelecaniformes |  | Ardei |  |  |

Threskiornithidae – ibises, spoonbills (36 species)

|

Ardeidae – herons, bitterns (72 species)

Pelecani  |  |  |

Pelecanidae – pelicans (8 species)

|

|  |  |

Scopidae – hamerkop

|

Balaenicipitidae – shoebill

The current International Ornithological Committee classification has pelicans grouped with the shoebill (Balaenicipitidae), hamerkop (Scopidae), ibises and spoonbills (Threskiornithidae), and herons, egrets and bitterns (Ardeidae). The IOC considers Threskiornithidae and Ardeidae to from a clade, citing Hackett et al. 2008.[8][9]

Another hypothesis is that Threskiornithidae is sister to the rest of Pelecaniformes, and Ardeidae and Pelecani form a clade,[10][11] as can be seen below:

Pelecaniformes |  |  |

Threskiornithidae – ibises, spoonbills (36 species)

|

|  |  |

Ardeidae – herons, bitterns (72 species)

|

Pelecani  |  |  |

Pelecanidae – pelicans (8 species)

|

|  |  |

Scopidae – hamerkop

|

Balaenicipitidae – shoebill

These controversies have been explained by analyses of rare genomic changes, which suggested that after the divergence of the pelican (Pelecanidae) and ibis (Threskiornithidae) lineages, interspecies hybridization led to gene flow causing the heron lineage (Ardeidae) to exhibit genetic signatures indicating affinity to both the pelican and ibis lineages.[12]

### Fossils

All families in the traditional Pelecaniformes (except the Phalacrocoracidae) have only a few handfuls of extant species at most, but many were more speciose in the Early Neogene. The pelecaniform lineages appear to have originated around the end of the Cretaceous, appearing to belong to a close-knit group of "higher waterbirds" which also includes groups such as penguins and Procellariiformes. Quite a lot of fossil bones from around the Cretaceous–Paleogene boundary cannot be firmly placed with any of these orders and rather combine traits of several of them. This is, of course, only to be expected, if the theory that most if not all of these "higher waterbird" lineages originated around that time is correct. Of those apparently basal taxa, the following show some similarities to the traditional Pelecaniformes:

* † Lonchodytes (Lance Creek Late Cretaceous of Wyoming, US)
* † Torotix (Late Cretaceous)
* † Tytthostonyx (Late Cretaceous/Early Palaeocene)
* † Cladornis (Deseado Early Oligocene of Patagonia, Argentina)
* † "Liptornis"—a nomen dubium

Fossil genera and species are discussed in the respective family or genus accounts; one little-known prehistoric Pelecaniforme, however, cannot be classified accurately enough to assign them to a family; "Sula" ronzoni from Early Oligocene rocks at Ronzon, France, which was initially believed to be a sea-duck, is a possible ancestral Pelecaniform. The proposed Elopterygidae—supposedly a family of Cretaceous Pelecaniformes—are neither monophyletic nor does Elopteryx appear to be a modern bird.[13]

## References

1. ↑ Jarvis, Erich D. et al. (2014). "Whole-genome analyses resolve early branches in the tree of life of modern birds". Science 346 (6215): 1320–1331. doi:10.1126/science.1253451. PMID 25504713. Bibcode: 2014Sci...346.1320J.
2. ↑ 2.0 2.1 Hedges, S. Blair; Sibley, Charles G. (1994). "Molecules vs. morphology in avian evolution: the case of the "pelecaniform" birds". PNAS 91 (21): 9861–65. doi:10.1073/pnas.91.21.9861. PMID 7937906. Bibcode: 1994PNAS...91.9861H.
3. ↑ Mayr, Gerald (2003). "The phylogenetic affinities of the Shoebill (Balaeniceps rex)". Journal für Ornithologie 144 (2): 157–175. doi:10.1007/BF02465644. Bibcode: 2003JOrni.144..157M. https://www.senckenberg.de/wp-content/uploads/2019/09/balaeniceps.pdf.
4. ↑ Mayr, G. (2007). "Avian higher-level phylogeny: Well-supported clades and what we can learn from a phylogenetic analysis of 2954 morphological characters". Journal of Zoological Systematics and Evolutionary Research 46: 63–72. doi:10.1111/j.1439-0469.2007.00433.x.
5. ↑ Mikhailov, Konstantin E. (1995). "Eggshell structure in the shoebill and pelecaniform birds: comparison with hamerkop, herons, ibises and storks". Canadian Journal of Zoology 73 (9): 1754–70. doi:10.1139/z95-207. Bibcode: 1995CaJZ...73.1754M.
6. ↑ Cracraft, Joel; Barker, F. Keith; Braun, Michael J.; Harshman, John; Dyke, Gareth J.; Feinstein, Julie; Stanley, Scott; Cibois, Alice; Schikler, Peter; Beresford, Pamela; García-Moreno, Jaime; Sorenson, Michael D.; Yuri, Tamaki & Mindell, David P. (2004): Phylogenetic Relationships Among Modern Birds (Neornithes): Toward an Avian Tree of Life. In: Cracraft, J. & Donoghue, M.J. (eds.): Assembling the Tree of Life: 468–489. Oxford University Press, New York. ISBN 0-19-517234-5 PDF fulltext
7. ↑ Ericson, P. G. P.; Anderson, C. L.; Britton, T. et al. (2006). "Diversification of Neoaves: integration of molecular sequence data and fossils". Biology Letters 2 (4): 543–547. doi:10.1098/rsbl.2006.0523. PMID 17148284.
8. ↑ 8.0 8.1 Hackett, Shannon J.; Kimball, Rebecca T.; Reddy, Sushma et al. (2008). "A Phylogenomic Study of Birds Reveals Their Evolutionary History". Science 320 (5884): 1763–68. doi:10.1126/science.1157704. PMID 18583609. Bibcode: 2008Sci...320.1763H.
9. ↑ International Ornithological Committee (2 January 2012). "Ibises to Pelicans & Cormorants". IOC World Bird Names: Version 2.11. WorldBirdNames.org. http://www.worldbirdnames.org/n-ibises.html.
10. ↑ Reddy, S.; Kimball, R.T.; Pandey, A. et al. (2017). "Why do phylogenomic data sets yield conflicting trees? Data type influences the avian tree of life more than taxon sampling". Systematic Biology 66 (5): 857–879. doi:10.1093/sysbio/syx041. PMID 28369655.
11. ↑ Kuhl, H.; Frankl-Vilches, C.; Bakker, A. et al. (2021). "An unbiased molecular approach using 3′-UTRs resolves the avian family-level tree of life". Molecular Biology and Evolution 38 (1): 108–127. doi:10.1093/molbev/msaa191. PMID 32781465.
12. ↑ Kuramoto, T.; Nishihara, H.; Watanabe, M.; Okada, N. (2015). "Determining the Position of Storks on the Phylogenetic Tree of Waterbirds by Retroposon Insertion Analysis". Genome Biology and Evolution 7 (12): 3180–3189. doi:10.1093/gbe/evv213. PMID 26527652.
13. ↑ Mortimer, Michael (2004). "Phylogeny of taxa". The Theropod Database. Archived from the original on 2013-05-16. https://web.archive.org/web/20130516175802/http://archosaur.us/theropoddatabase/Phylogeny of Taxa.html.

## Further reading

* Bourdon, Estelle; Bouya, Baâdi & Iarochene, Mohamed (2005): Earliest African neornithine bird: A new species of Prophaethontidae (Aves) from the Paleocene of Morocco. J. Vertebr. Paleontol. 25(1): 157–170. DOI: 10.1671/0272-4634(2005)025[0157:EANBAN]2.0.CO;2 HTML abstract

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