But Brontornis might not actually have been a terror bird at all – it may have instead been a giant cousin of ducks and geese.
The known fossil material is fragmentary enough that it’s still hard to tell for certain, but there’s some evidence that links it to the gastornithiformes, a group of huge herbivorous birds related to modern waterfowl.
If it was a gastornithiform, that would mean it represents a previously completely unknown lineage of South American giant flightless galloanserans. And, along with the gastornithids and the mihirungs, it would represent a third time that group of birds convergently evolved this sort of body plan and ecological role on entirely different continents during the Cenozoic.
Non-tetrapod vertebrates like fish have spines that are much less differentiated, with just body and tail segments. So for a long time multiple distinct spine regions were thought to be something completely unique to tetrapods – a specialization developed early in their evolutionary history that served to better support their weight when moving around on land.
But one little fossil fish makes this idea… problematic.
Tarrasius problematicus lived during the early Carboniferous, about 345 million years ago, in shallow tropical marine waters in what is now southern Scotland. Around 9cm long (3.5″), it was an early type of ray-finned fish with a scaleless body and a long scaled eel-like tail with a single continuous dorsal fin.
Its spine shows five different regions all corresponding to those seen in tetrapods, despite it not being closely related to them. But unlike early tetrapods Tarrasius was no land-walker, with its lack of hind fins indicating it was instead a streamlined fully aquatic fast swimmer.
It’s not clear why this fish developed such an incredibly convergent backbone, but it may have helped to stiffen its body so its more flexible tail could provide more efficient thrust, swimming like a modern tadpole.
It also suggests that a pre-existing genetic basis for regionalization – specific patterns of Hox gene expression – was actually an ancestral trait for all bony fish or jawed vertebrates. Tarrasius and early tetrapods may have just happened to specialize their spines in the same way for different purposes, with only the tetrapods going on to see long-term evolutionary success with it.
They were previously known mainly from isolated teeth and jaw fragments, with some rare full skull material, but Adalatherium is remarkable for being represented by a complete skeleton.
And it’s turned out to be far stranger than anyone expected.
Living in northwestern Madagascar during the Late Cretaceous, about 70-66 million years ago, Adalatherium was one of the larger known Mesozoic mammals at around 60cm long (2′) – although the one known specimen seems to have been a juvenile, so mature individuals were probably slightly larger.
It was probably a marmot-like digging animal, excavating burrows with its large claws and powerful limbs, and since it likely evolved from ancestors that had become isolated on Madagascar over 20 million years earlier it had developed a very unusual mixture of both “primitive” and highly specialized anatomical features. It had more back vertebrae than any other known Mesozoic mammal, upright forelimbs, sprawling hind legs with bowed-out tibias, strong back and leg musculature, and a therian-like pelvis with epipubic bones.
And then there’s the snoot.
The snout region of Adalatherium‘s skull was pockmarked with a large number of foramina, holes that allow the passage of nerves and blood vessels through the bone. It had more of these than any other known mammal, and their presence suggests that it probably had a very sensitive upper lip and whiskery snout. Most mammals with a lot of whiskers just have one very big foramina, but Adalatherium seems to have evolved a different solution to the same problem.
It also had one other bizarre feature – a hole in the top of its nose. A large “internasal vacuity” between its nasal bones is a unique feature not known in any other mammal, and its function is a total mystery.
Since this hole was also surrounded by many foramina it may have supported some sort of soft-tissue sensory structure on top of its nose. So I’ve speculatively depicted it here with a leathery horn-like “shield”.
Aquilarhinus palimentus here was an early hadrosaurid dinosaur known from the Late Cretaceous of Texas, USA, living about 80 million years ago. Around 5m long (16″5″), it had a prominent humped nose that seems to have been an evolutionary prelude to the larger and much more elaborate crests found in later hadrosaurs.
It also had an unusually wide and shovel-like beak, unlike any other known hadrosaur, which was probably a specialization for a different diet than its relatives. Since it lived along coastal marshlands it may have used its broad jaws to scoop up large mouthfuls of soft vegetation – or, much like the “shovel-tusker” proboscideans that were once thought to have a similar lifestyle, it may actually have been doing something else entirely with that beak.
Their two shell valves are also arranged very differently – while bivalve shells originate from the left and right sides of their bodies, brachiopods grow theirs on the top and bottom.
They first appear in the fossil record in the early Cambrian, about 530 million years ago, but they may have actually diverged from a tommotiid-like ancestor as far back as the late Ediacaran. Only around 300 species survive today, but during the Paleozoic they were some of the most abundant filter-feeding and reef-building animals with tens of thousands of fossil species known. Different species tended to have strict habitat and temperature preferences, and so their fossils are also useful indicators of how ancient climates changed over time.
Hyoliths were a group of small shelled animals that first appeared in the fossil record just after the start of the Cambrian, about 536 million years ago. They had conical calcareous shells with a lid-like operculum, and some species also featured long curling spines that made them look like ice-cream cones with mammoth tusks.
They were so odd that for a long time their evolutionary relationships were unknown. They were generally accepted to be lophotrochozoans, but some studies considered them to be part of their own unique phylum while others tended to place them as being closely related to molluscs.
However, this isn’t universally accepted and some recent studies continue to dispute it. The feeding organ of a different hyolith fossil has been interpreted as not being a lophophore, classifying the group as an early lophotrochozoan stem lineage, while an analysis of shell microstructure has instead suggested realigning them with molluscs. I’m grouping them with brachiopods here, but future discoveries might still make this obsolete.
Horseshoe worms, or phoronids, are represented by about 15 living species and are usually considered to be their own phylum, but some analyses classify them as a sub-group of brachiopods instead. Like other lophophorates they have a “crown” of filter-feeding tentacles around their mouths, and similarly to some bryozoans they build protective chitinous tubes around their bodies.
There are no definite body fossils of phoronids at all, although there are a few possible trace fossils of their tubes and the enigmatic fossil hederelloids might be related to them.
But some Cambrian fossils might give us a hint about their evolutionary history.
The only Cambrian fossil species that seems to be closely related to the entoprocts is Cotyledion, but there are several other enigmatic animals that have also been tentatively allied with the group as members of early stem lineages.
Ectoprocts, common known as bryozoans or “moss animals”, are aquatic lophotrochozoans that usually live in colonies made up of many tiny cloned zooids. The exoskeletons they build for their colonies have a range of forms, including gelatinous blobs, chitinous branches, and calcified sheets and coral-like fronds.