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Six fossil birds from Bavaria's Hammerschmiede pit add four fish-eaters to a Miocene river
Gerald Mayr's team named two genera new to science, including the eagle Mioaetus pforzenensis, at a site where only one bone find in 40 is a bird. The additions bring the fish-eating birds level with the fish-eating mammals.
The Scientist · Science desk
What happened
- A team led by Gerald Mayr of Senckenberg Frankfurt has described six more bird species from the Hammerschmiede clay pit in Bavaria, among them cormorants, a loon and an eagle, plus two genera new to science.
- The eagle went into a new genus, Mioaetus, with the species name pforzenensis given in honour of the community and inhabitants of Pforzen.
- A swift identified as Apus cf. gaillardi, smaller than any living swift species, is the 161st vertebrate species recovered from the clay pit.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- capability A reconstruction of the river beside the ape site can now put four bird predators into a fish-eating guild that mammals, turtles and salamanders had been carrying on their own.
- constraint At 2.5 percent of bone finds, with swifts almost never preserved, the site's bird tally is bounded by what fossilises, so any density comparison against modern rivers is a floor and not a measurement.
- decision Anyone entering Mioaetus into a food-web model is buying an inference drawn from the shape of one lower-leg bone, and the loon in the same list rests on a single humerus.
Four of the six birds in this paper ate fish: two cormorants, a loon and the new eagle [2][6][8]. The large darter Anhinga pannonica was already turning up frequently in the pit [10]. That gives five fish-eating bird species against the five fish-eating mammals from earlier work, three otters and two seals [18][3]. Madelaine Böhme puts the whole guild at "at least 13 previously identified fish predators" fed by 10 other fish species [20], which works out at 1.3 predator species per prey species [2]. The descriptions are in the Swiss Journal of Palaeontology [3].
Birds were on the Hammerschmiede list before this: darters, a giant crane and several waterfowl, including Allgoviachen tortonica and a dwarf duck [5]. They were scarce. "Birds are among the rarest fossil finds in the Hammerschmiede. Only 1 in 40 bone finds is from this class of vertebrates; today, they make up the greatest number of species on Earth," excavation leader Thomas Lechner said [4]. One bone in 40 is 2.5 percent [1].
The thing that fraction does not measure is how many birds were alive. It counts bones that survived and were found. On the swift in the new set, an insect-eater that lives on the wing [14], Böhme said, "Because they live constantly in flight, it is very rare to find fossils of swifts" [15].
Bavaricarbo brevirostris, one of the two new genera, is the most recent known representative of the extinct original stem group of cormorants [6]. Gerald Mayr said, "In Bavaricarbo, the bones of the humerus, which make up part of the wing, and the tarsometatarsus are formed differently from those of modern cormorants. Therefore, Bavaricarbo could presumably fly better than modern cormorants but probably could dive only in shallow waters such as the river by the Hammerschmiede" [7].
The eagle came from a single element. "We discovered a lower-leg bone of a hawk that differs in essential details from present-day species and is vaguely reminiscent of an osprey," Mayr said [11]. From the form of that tarsometatarsus, Böhme reads a wingspan of more than one metre and a bird that presumably caught fish in its talons [13]. Cormorant material is plentiful; the loon so far is one humerus [9]. On that combination Böhme said, "However, if we combine these with the similarly frequent finds of the large darter Anhinga pannonica, it is clear that the rivers of the Hammerschmiede area were dominated by fish-eating diving birds" [10].
Her wider comparison is the claim worth watching: "The number of fish-eating predators in the Miocene ecosystem is far higher than in European inland waters today" [17]. Every one of those predators comes out of the same 11.6-million-year-old river sediments [1], and bone counts from a deposit do not show that all 13 hunted the same stretch of water in the same season. Böhme's proposal is that the fish-based food chains explain why the pit has produced 161 vertebrate species at all [22][16].
What to watch
- Whether the second cormorant, currently unnamed and pygmy-cormorant-sized, gets a formal description and where it falls relative to Bavaricarbo.
- More loon material from the pit would show how much weight the single humerus can carry.
- A published estimate of how long the fossil-bearing sediments took to accumulate would test whether all 13 fish predators could have overlapped.