Proposal (596) to South American Classification Committee
Treat Pluvianellidae as a subfamily of Chionidae
Effect on SACC: This would demote a family, the Pluvianellidae, to subfamily rank, Pluvianellinae, in the family Chionidae (and thus also add the subfamily Chioninae).
Background: Pluvianellus socialis is a very odd bird. Long considered an aberrant plover (Charadriidae) because of superficial similarities, genetic data have confirmed what morphological and behavioral data had already suggested, namely that it is no closely related to plovers. We currently treat it in a monotypic family Pluvianellidae (as do many other sources, e.g. Livezey 2010, Mayr 2011), with the following Note:
1. Pluvianellus was formerly placed in the Charadriidae, but Jehl (1975) elucidated its many unusual characters that indicated that it was not a plover. Treated as a subfamily within Charadriidae in Wiersma (1996). Strauch (1978) and Chu (1995) placed Pluvianellus closer to Chionidae than to its typical position in Charadriidae, based on analysis of morphological characters; genetic data (Paton et al. 2003, Paton & Baker 2006, Baker et al. 2007) support this relationship and thus also the treatment of Pluvianellus as a monotypic family separate from Charadriidae. The supertree of Thomas et al. (2004) also placed Pluvianellus as sister to Chionis. SACC passed to change linear sequence. Family rank is also supported by analysis of phenotypic characters (Livezey 2010), although Livezey’s analysis specifically ejected the sister relationship to Chionidae. Reduced to a subfamily of Chionidae by Cracraft (2013). SACC proposal needed.
New information: Cracraft (2013) treated Pluvianellus as a subfamily, Pluvianellinae, in the Chionidae. Although a sister relationship to sheathbills is consistent with all recent data, treatment of Pluvianellus as a subfamily of Chionidae is a novel treatment as far as I know, and Cracraft did not provide any rationale or justification for this change (and a source of contention between Joel and me during H&M4 production).
Analysis: The fundamental problem is that there is no set of objective criteria to determine taxon rank at these higher levels of classification. Much of what we have is based on traditional ranks. Hopefully, some guidelines with some objective criteria, such as estimated lineage age, will eventually be adopted. In the meantime, all we can do is appeal to tradition and subjective, comparative criteria.
Concerning such subjective, comparative criteria, groups ranked at the family level in the nonpasserines typically have a suite of distinctive morphological and behavioral characters. Using this standard, few “groups” have more distinctive characters than does Pluvianellus. Although now known to be the sister taxon to Chionis, the sheathbills, they bear little external resemblance to them. Pluvianellus rested, albeit somewhat uncomfortably, in the Charadriidae (and still bears the English name “plover”) for perhaps a century before its previously unsuspected relationship to Chionis was proposed on the basis on internal characters detected by the analyses of Strauch and Chu (although later rejected by Livezey).
Here are some additional characters in addition to the lack of any superficial resemblance between the two groups:
• Pluvianellus carries food to its chicks in a crop (Sheathbills carry food in their bills) and also uses the crop in display (unique in Charadriiformes?).
• Pluvianellus digs with its feet into sand for food (unique in Charadriiformes?)
• Pluvianellus has slightly asymmetric bill, twisted to right or left.
Other than their similar sub-Antarctic distribution, I actually can’t find any similarities between the two groups, certainly nothing that they share to the exclusion of other charadriiform groups.
Discussion and Recommendation: I recommend a NO on this. The sister relationship between the two groups is signified by their adjacent placement in the sequence, and ought to be signified at a higher level with a superfamily designation, but under current standards of family designations, both groups merit treatment as separate families. I can’t think of any other group ranked at the family level that contains birds as divergent in morphology and behavior as these two.
BAKER, A. J., S. L. PEREIRA, AND T. P. PATON. 2007. Phylogenetic relationships and divergence times of Charadriiformes genera: multigene evidence for the Cretaceous origin of at least 14 clades of shorebirds. Biology Letters 3: 205–209.
CHU, P. C. 1995. Phylogenetic reanalysis of Strauch's osteological data set for the Charadriiformes. Condor 97: 174-196.
CRACRAFT, J. 2013. Avian higher-level relationships and classification: nonpasseriforms. Pp. xxi-xliii in The Howard and Moore Complete Checklist of the Birds of the World, 4th Edition, Vol. 1. Non-passerines (E. C. Dickinson & J. V. Remsen, Jr., eds.). Aves Press, Eastbourne, U.K.
JEHL, J. R., JR. 1975. Pluvianellus socialis: biology, ecology, and relationships of an enigmatic Patagonian shorebird. Transactions of the San Diego Society of Natural History 18: 26–73.
LIVEZEY, B. C. 2010. Phylogenetics of modern shorebirds (Charadriiformes) based on phenotypic evidence: analysis and discussion. Zoological Journal of the Linnean Society 160: 567-618.
MAYR, G. 2011. The phylogeny of charadriiform birds (shorebirds and allies) – reassessing the conflict between morphology and molecules. Zoological Journal of the Linnean Society 161: 916-934.
STRAUCH, J. G. 1978. The phylogeny of the Charadriiformes (Aves): a new estimate using the method of character compatibility analysis. Transactions Zoological Society London 34: 263-345.
THOMAS, G. H., M. A. WILLS, & T. SZÉKELY. 2004. A supertree approach to shorebird phylogeny. BMC Evolutionary Biology 4: 28.
WIERSMA, P. 1996. Species accounts. Family Charadriidae (plovers). Pp. 410-442 in "Handbook of the Birds of the World, Vol. 3. Hoatzin to auks." (J. del Hoyo et al., eds.). Lynx Edicions, Barcelona.
Van Remsen, October 2013
Comments from Stiles: “NO. Given the large differences in morphology and behavior, I feel happier retaining Pluvianellus in its own family, while placing it next to Chionidae (Chionididae?) in the sequence.”
Comments from Zimmer: “NO. I can’t see this species as being placed in the same family as sheathbills. That would be more extreme than moving Pittasoma into Conopophagidae (which we did), and I was opposed to that, so NO.”
Comments from Pacheco: “NO. Considero igualmente Pluvianellus e Chionis muito divergentes para vê-los compartilhando uma mesma família. “
Comments from Robbins: “NO. It doesn’t matter how we designate the uniqueness of Pluvianellus, however, I’m fine with giving it family status. So, a no vote.”
Comments from Pérez-Emán: “NO. It is not possible to evaluate this proposal as no rationale was provided for inclusion of Pluvianellus into Chionidae. However, if we want to evaluate if Pluvianellus deserve family or subfamily status we face the problem, as Van pointed out, that we do not have criteria to define these taxonomic categories. Paton et al. (2003), in their molecular study, found Pluvianellus and Chionis to be sister taxa, with a genetic divergence lower than that found among members of Scolopacidae or Charadriidae. However, morphological and behavioral differences have been suggested to make this potential family highly variable. How do we weight these characters to make an informed taxonomic decision? How generalizable could it be among different groups? Working on defining these criteria is a difficult challenge but it would be very helpful to work on this higher-level classification categories.”
Comments from Jaramillo: “NO – Definitely sister groups, but treating Pluvianellidae as a subfamily of the sheathbills is extreme. The two groups are incredibly different, certainly as much as an oystercatcher vs. a plover!”