Musser and Clarke 2020 present
a new Eocene bird, Nahmavis, (Figs. 1, 2) preserved with feathers. The forelimbs (= wings + pectoral girdles and sternum) were taphonomically lost. Originally Nahmavis was considered close to the traditional, but invalid shore bird clade Charadriformes (genus: Charadrius) and the traditional, but invalid stork clade, Gruiformes (genus: Grus). The authors report in their results section, “This analysis recovered a paraphyletic Gruiformes with respect to Charadriiformes, with Nahmavis grandei and Scandiavis mikkelseni being placed as the sister-taxa of all included Charadriiformes.”
As you’ll see
(below), such a nesting is about as general as one can get considering the wide variety of birds traditionally considered members of each clade. The Musser and Clarke gene-based cladogram bears no resemblance to the LRT.
Here
in the large reptile tree (LRT, 1752+ taxa; subset Fig. 3) Nahmavis nests with the less aquatic kory bustard (Ardeotis), a taxon not mentioned by Musser and Clarke. These two are close to an extremely aquatic taxon tested by Musser and Clarke, Heliornis (Fig. 5), which they consider a member of the Gruiformes.
Figure 1. Nahmavis fossil (FMNH PA778) overall. Images from Musser and Clarke 2020.
Traditional Charadriiformes include:
gulls, terns, plovers and other shorebirds. (murre, sandpiper, yellow legs, woodcock, skimmer, auk, phalarope, gull, jacana, oystercatcher, snipe, puffin, killdeer, wry-bill, noddy, razorbill, thick-knee, skuas, jaegers. In the LRT (Fig. 3) these taxa are scattered widely.
Traditional Gruiformes include:
limpkins, seriemas, sunbitterns cranes, finfoots, bustards, trumpeters, coots, rails and the kagu. In the LRT (Fig. 3) these taxa are scattered widely.
Figure 2. Nahmavis skull traced and reconstructed using DGS.
From the Musser and Clarke 2020 abstract:
“The stem lineage relationships and early phenotypic evolution of Charadriiformes (shorebirds) and Gruiformes (rails, cranes, and allies) remain unresolved. It is still debated whether these clades are sister-taxa.”
After testing in the LRT (subset Fig. 3) Charadrius nests only three nodes apart from Grus. Nahmavis does nest outside this clade, but so do crows and jays nesting between them. Membership in the traditional clades Charadriformes and Gruiformes need to be reconsidered in light of results recovered by the LRT.
Musser and Clarke concede in their introduction:
“There is a lack of consensus among large-scale phylogenetic studies of Neoaves concerning the relationships of, and early phenotypic evolutionin, Charadriiformes or Gruiformes.”
Birds are difficult. Having a ready library of colorized skulls and other data helps immensely. Sometimes the cladogram tells you where the mistakes are.
Figure 3. Subset of the LRT focusing on birds. Nahmavis is highlighted in yellow. Color clades include extant taxa. This portion of the LRT is fully resolved.
Musser and Clarke 2020 abstract continues:
“New phylogenetic analyses incorporating Paleogene fossils have the potential to reveal the evolutionary connections of these two speciose and evolutionarily critical neoavian subclades. Although Gruiformes have a rich Paleogene fossil record, most of these fossils have not been robustly placed. The Paleogene fossil record of Charadriiformes is scarce and largely consists of fragmentary single elements. Only one proposed Eocene charadriiform-like taxon, Scandiavis mikkelseni of Denmark, is represented by a partial skeleton.”
Though not yet tested, Scandiavis appears to be a close match to Charadrius.
Figure 4. Kori bustard (Ardeotis) in vivo.
Musser and Clarke 2020 abstract continues:
“Here, we describe a new species from the early Eocene Green River Formation of North America comprising a partial skeleton and feather remains. Because the skeleton lacks the pectoral girdle and forelimbs as in S. mikkelseni, only features of the skull, axial skeleton, and hind limb are available to resolve the phylogenetic placement of this taxon. These anatomical subregions initially showed features seen in Charadriiformes and Gruiformes. To assess placement of this taxon, we use a matrix consisting of 693 morphological characters and 60 taxa, including S. mikkelseni and the oldest known charadriiform taxa represented by single elements. These more fragmentary records comprise two distal humeri from the earliest Eocene Naranbulag Formation of Mongolia and the early Eocene Nanjemoy Formation of Virginia.”
The LRT avoids testing fragmentary records, especially when it comes to birds. A single, fully resolved tree results using a third as many multi-state traits.
Fig. 5 The sun grebe (Heliornis) nests close to the bustard, Ardeotis, and the new Eocene taxon, Nahmavis, in the LRT.
Musser and Clarke 2020 abstract continues:
Our phylogenetic analyses recover the new taxon and S. mikkelseni alternatively as a charadriiform or as a stem-gruiform; placement is contingent upon enforced relationships for major neoavian subclades recovered by recent molecular-based phylogenies.
The LRT does not confirm this rather tentative and/or generalized phylogenetic position. Taxon exclusion may be the problem, since Ardeotis (Fig. 4) was excluded from the Musser and Clarke analysis. Deletion of Ardeotis from the LRT still nests Nahmavis with Heliornis (Fig. 5). Surprised that Musser and Clarke did not come up with the same conclusion, but then, they did not attempt a reconstruction (Fig. 1).
“Specifically, when constraint trees based on results that do not recover Charadriiformes and Gruiformes as sister-taxa are used, the new taxon and S. mikkelseni are recovered within stem Gruiformes. Both Paleogene fossil humeri are consistently recovered within crown Charadriiformes. If placement of these humeri or the new taxon as charadriiforms are correct, this may indicate that recent divergence time analyses have underestimated the crown age of another major crown avian subclade; however, more complete sampling of these taxa is necessary, especially of more complete specimens with pectoral elements.”
The LRT nests all extant birds in a single clade. That clade arises from an Early Cretaceous taxon, Archaeornithura and a Latest Cretaceous taxon, Vegavis. Several other Early and Late Cretaceous birds, many with secondarily evolved teeth (Fig. 6), nest with crown birds, so their fossil record already extends deep into the Mesozoic.
Figure 6. Click to enlarge. Toothed birds of the Cretaceous to scale.
With the addition of Nahmavis to the LRT
the bird subset of the LRT (Fig. 3) came under review. Several scores were changed. The basic tree topology remains the same. Some taxa moved around. These will be discussed in the next few days.
References
Musser G and Clarke JA 2020. An exceptionally preserved specimen from the Green River Formation elucidates complex phenotypic evolution in Gruiformes and Charadriiformes. Frontiers in Ecology and Evolution – Paleontology https://doi.org/10.3389/fevo.2020.559929