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Spec Dinosauria: Enantiornithes

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INTRODUCTION & HISTORY

In our home timeline, of all dinosaurs only a few species of Neornithes survived the end of the Cretaceous. Those few species all seem to have lived in Antarctica or perhaps its surrounding landmasses of South America, Madagascar-India and Aotearoa. Therefore it is no wonder that Home-Earth’s birds are all so similar – all our birds have descended from one small handful of species that has survived the end-Cretaceous apocalypse.

In Spec, where the Earth ticked through the Cretaceous-Cenozoic boundary with only the mildest of hiccups, most of the other bird clades still exist. Most notable among these are the Enantiornithes, which have enjoyed a global distribution more or less since the age of their oldest fossil representative, some 135 million years ago. These “opposite-birds”, so called for some skeletal features which are built the opposite way compared to Euornithes, make up most of Spec’s landbird diversity, with a few water-loving forms thrown in just to disturb the picture.

BIOLOGY

Both internally and externally, Enantiornithes truly deserve their moniker of “opposite birds”, as they approached “avianhood” much differently than Euornithes. The articulation of the scapula/coracoid is inverse to that of Neornithes, with the facet of the scapula being concave and disc shaped and the coracoid’s a convex knob, while the opposite happens in Neornithes. The metatarsals are also differently fused, being fused from the ankle to the toes instead of the other way around. Enantiornithes also have a different flight musculature from that of Euornithes: they have a V shaped furcula with a long backward prominence that serves as a flight keel, being attached to an unique set of flight muscles, and they also have well developed deltoid crests as pterosaurs and flying mammals. This rendered their sternum much shallower than in Euornithes, with a small keel, though it also posses strange antler-like structures that support wing musculature as well as air sacs.

Externally, although several opposite-birds have converged closely with Neornithes, from HE or Spec, they still have traits that betray their nature. For instance, most lack the familiar tail feathers that Euornithes have. When not simply lacking tail feathers beyond down or contour feathers, Enantiornithes have strange, ribbon-like tail feathers similar to those of the long gone confuciusornithids and scansoriopterygids, which in several species form fans similar to those of the retrices in Euornithes, forming an airfoil surface just as efficient or even more so, as they require less maintenance and are more repairable. Like all flying dinosaurs, opposite-birds have allulas/bastard wings, which help them to steer in the air. Many species still retain formidable wing claws, used in climbing – particularly so for tailess species, in which the forelimbs are used to aid in landing – as well as defense and prey capture, though more derived forms have very reduced wing claws just like more familiar birds. Many species retain toothed jaws, often even completly covered in feathers, forming fuzzy muzzles, though most modern taxa have toothless, beaked jaws, though opposite-bird rhamphothecae are generally less mineralised and more plastic than “true bird” ones.

All opposite-birds (with the exception of pseudoraptorids) are extremely precocial, meaning they can fly and leave the nest just a few days or even hours after hatching. At first sight a serious drawback against the much faster-breeding “true” birds, these traits have allowed opposite-birds to evolve some features that “true” birds could hardly afford. Most conspicuous among those is the poison of the tweety-birds and the gondwanaviformes. Less spectacularly, large opposite-birds like flankers go through several different ecological niches in their life cycle. By strongly reducing intraspecific competition, this allows those species to have higher population densities than those that comparable species in our timeline can sustain. It is almost certainly the reason for the small number of species in all opposite-bird clades.

All modern Enantiornithes are divided in five clades whose individual origins stretch well into the Cretaceous. Other forms were appearently already rare or absent by the Eocene, either outcompeted or having died in the PETM. Combined all together, Enantiornithes make up for over a third of all living avian taxa.

LAURASIAVIFORMES 

Throughout the history of the Cenozoic, the story of bird life in North America has been one of conquest. Invaders from Eurasia or South America have, again and again, swept over the neararctic, replacing the natives and then going native themselves, only to be replaced, in turn, by the next wave. There are, however, always survivors from prior ages, living fossils that continue to exist under the shadow of the new regime by dint of isolation or extreme specialization.

One of these relic groups is Laurasiaves, a once-mighty enantiornithian clade now relegated to a few endemic species in the warmer parts of North and Central America.Laurasiavians are a group of enantiornithians and thus branched off from true birds quite early in their evolution, either in the Jurassic or early Cretaceous. As such, laurasiavians resemble true birds, but their internal anatomy is so fundamentally strange that one is at a loss when considering where to begin their description. Perhaps their ankles would be best.

Laurasiavians are easily identifiable by their ankles (the namesake of Enantiornithes or 'opposite birds') in which the three metatarsal bones are not fused bottom-to-top as in true birds, but top to bottom. This feature, common to all enantiornithian clades, is a fundamental difference between these birds and the euornithians.

(Picture by Daniel Bensen)

Metatarsus

Right metatarsus of green bunglebird (Xenosornis veridigenalis). Note fusion of bones at top, rather than base. (Picture by Daniel Bensen)

In most respects of their skeletal systems, laurasiavians are typical enantiornithians, but for their heads and their hands. The skull of a laurasiavian is heavily built, with a crushing, toothless beak. Indeed, on first glance, a laurasiavian could almost be mistaken for one of the long-extinct confuciusorniforms.

Laurasiavian2

(Picture by Daniel Bensen) Skull of typical xenosornid, the green bunglebird. Note the primitive, robust construction.

Another feature of the laurasiavians distinctive to the clade is their distinctive hand structure. Laurasiavians have gone through some of the same adaptations as the true birds, in their wing anatomy, fusing digits two and three and retaining digit one (the thumb) as a movable airfoil, or bastard-wing. However, while the euornithians possess only a spur of bone to which alula feather is attached, laurasiavians retain a complete first digit whose enlarged and flattened claw (the aileron) serves the same purpose. Some fossil laurasiavians sported enormous ailerons, presumably to increase agility or for display, but those of extant species are small.

Laurasiavianhead

(Picture by Daniel Bensen) Left aileron of green bunglebird

In the rest of their anatomy, laurasiavians are typical enantiornithians, with swept-back (though still keeled) sterna, relatively long tails, and primitive hips with the pubis and ischium fully separated.

Laurasiaviformes was quite diverse during the Eocene and in modern Eurasia they occupy a number of clades, but in North America, only one branch of the tree is left, the xenosornids.

Green Bunglebird (Xenosornis veridigenalis)

Laurasiavian

Green Bunglebird, Xenosornis veridigenalis (Southern United States)

The green bunglebird (Xenosornis veridigenalis) is an enigmatic member of an enigmatic family.  One of the last remaining North American laurasiavians, the bunglebird feeds exclusively upon fruits and nuts.  Locally common in the marshes of Florida and parts of Louisiana, this bird is reluctant flyer, prefering to hitch its way along stout branches, ripping apart the tough-skinned fruits with its powerful beak. Bunglebirds nest in large, carefully constructed nests high above the forest floor.

GONDWANAVIFORMES

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