Unraveling a radiation
Scott D. Sampson and Mark A. Loewen published a review of ceratopsid distribution through time and space, their taxonomic diversity as well as their evolutionary history. They observed that the number of known ceratopsids has nearly doubled in less than five years and opined that it is likely that many new ceratopsid species will be discovered in the future. Their optimism for the prospects of future discoveries is based on the presence of chronological gaps in the ceratopsid fossil record. Additionally, since ceratopsids are known primarily from "a restricted region of the northern region of the Western Inerior Basin" there may be new discoveries to be made outside it. Regarding chasmosaurine evolution, Sampson and Loewen concluded that Chasmosaurus and Pentaceratops were primitive forms near the base of the chasmosaur family tree. Anchiceratops and Arrhinoceratops were intermediate forms, with the chasmosaur family tree crowned by a Triceratops-Torosaurus clade. The centrosaur family tree was found to have forms like Albertaceratops and other species with long horns over the eyes near the base. More intermediate branches included a Centrosaurus-Styracosaurus clade. The most deeply nested centrosaur clade included Einiosaurus and its relatives, the boss-faced pachyrhinosaurs.
The distribution of ceratopsids through-out the geologic column is now known in "considerable resolution". The clade lasted roughly from 79 to 65 million years ago. Although some previous workers had concluded that ceratopsid diversity within individuuual ecosystems was high, Sampson and Loewen found that in any given environment only one centrosaur and one chasmosaur was present. The low ceratopsid diversity existing at any one time combined with the large number of known species implies that new species of ceratopsids appeared frequently and quickly became extinct. The authors found that, on average, a ceratopsid species survived only for "considerably less" than a million years.
The geographic distribution of ceratopsid species tended to be quite restricted. Most species have only been documented from a single geologic formation. The authors found there to be "strong evidence" for many ceratopsids to be unique to their own environment in a north-south series of separate ecosystems. The presence of environments varying by latitude in western North America during the Cretaceous had been proposed by earlier paleontologists. The limited range of most ceratopsid species implies that they were less likely to participate in long distance migrations than might be expected for an animal of their size. It can be further inferred that they "had low-to-intermediate metabolic rates," access to "abundant food supplies," or both.
Some ceratopsids had the longest skulls of any terrestrial vertebrate.
The evolution of the ceratopsids is easy to study. One reason is that their massive skulls fossilized easily so almost every kind of ceratopsid is known from its skull. Also, ceratopsids had cranial ornamentation unique to each species. Other factors simplifying the study of ceratopsian evolution is their short time span (Campanian to Maastrichtian or 14 million years) and that their geographic distribution was limited to western North America. Sampson and Loewen characterized ceratopsids as having the easiest "tempo and mode" of evolution to study of any dinosaur group.
There are more field crews investigating Late Cretaceous strata of North America now than at any time in history. The investigation of previously under-explored deposits contributes to the increased rate of new discoveries.
In 2004, Dodson and other reaserchers published a review of ceratopsian biodiversity. They found 6 valid genera of centrosaurines with one species each and and 10 species of chasmosaurines within 7 genera. That's a total of 16 ceratopsid species. By the time Sampson and Loewen published their review there were 15 known species of centrosaurines and 17 known species of chasmosaurines. This total of 32 species represent a doubling of known centrosaur diversity in less than 5 years. These discoveries added at least 5 new centrosaurine species in 3 genera. A new taxon from the Wahweap Formation is both smaller and older than other ceratopsids. Its remains date back to the Campanian, over 79 million years ago. A Pachyrhinosaurus-like centrosaur from the Lethbridge Coal Zone of the Dinosaur Park Formation catalogued as TMP 2002.76.1 may also represent a new species. At the time however, the specimen lacks the parietal, making it impossible to be sure that it doesn't belong to Pachyrhinosaurus or Achelousaurus. If it belongs to the latter, Achelousaurus will become one of very few known Campanian ceratopsids from multiple geological formations. Sampson and Loewen observed that both the A. horneri of the Two Medicine Formation and TMP 2002.76.1 of the Lethbridge Coal Zone were found just under marine shales composing the Bearpaw Formation. However, they also note that TMP 2002.76.1 seems to be about 500,000 years older than the Achelousaurus of Two Medicine.
Pachyrhinosaurus was once thought unusual for having a "boss" for facial ornamentation, but in recent times more related forms have been discovered with similar snout ornamentation.
The ceratopsids have been continuously regarded as monophyletic even after the adoption of cladistics by the paleontological community. Sampson and Loewen caution, however, that a detailed analysis of ceratopsids with a large number of basal neoceratopsians hasn't been performed, however. Likewise, their two subfamilies, the centrosaurs and chasmosaurs have each been regarded as monophyletic in all recent research. Within each clade however the placement of individual taxa has been more controversial. In 1996, Lehman performed a cladistic analysis that found different chasmosaurs branching off a single main lineage. Other analyses have found that Chasmosaurus and Pentaceratops form their own clade. In 2004 Sampson and others found that Chasmosaurus was a distinct clade but Pentaceratops was more closeley related to more nested forms of chasmosaurine.
Within the centrosaurines the placement of Avaceratops has been controversial. In 1999 Penkalski and Dodson argued that Avaceratops should be removed from the centrosaurines altogether and was instead the sister taxon to all other ceratopsids.
Different studies have recovered different positions for different kinds of ceratopsid due to the researchers' choices of which characters were being examined but also due to which kinds of ceratopsids were included in each study since most examined only a few different species.
Although there have been controversies, for the most part, ceratopsid family trees have inspired agreement between scientists. Chasmosaurus and Pentaceratops are generally regarded as relatively primitive chasmosaurs. Anchiceratops and Arrhinoceratops are intermediate forms while Triceratops and Torosaurus are among the most nested members of the chasmosaurinae.
Long horned centrosaurs like Albertaceratops are regarded as primitive for the subfamily. Centrosaurus and Styracosaurus form a clade, and Einiosaurus forms a clade with the Pachyrhinosaurus like centrosaurs. Sampson and Loewen note, however that in 2004 Dodson and others' study of relationships with the ceratopsidae, the largest such study in history, they recovered a unresolved polytomy within centrosaurinae consisting of Einiosaurus + (Achelousaurus + Pachyrhinosaurus) + (Styracosaurus + Centrosaurus).
Sampson and Loewen criticized previous research on certopsid evolution as all suffering from biasing their cladistics studies by focusing only on skull characters since some research has indicated there are likely relevant characters present elswhere on the body that could be helpful. The general similarities of the bodies of different kinds of ceratopsids is the source of this bias. Sampson and Loewen also observed that no species level complete phylogenetic analysis of all ceratopsids has been performed as of 2010.
Increasing knowledge of relevant stratigraphy has been very helpful for adding to knowledge about ceratopsids. Laser-fusion Ar40/Ar39 research in volcanic ash bed deposited in the Kaiparowits Formation have produced dates of 76.1 to 74.0 miilion years of age. This upper Campanian age coincides or nearly so with other formations with abundant dinosaur fossils. Similarly aged strata from the north include the Dinosaur Park Formation of Alberta as well as the upper parts of the Montanan Two Medicine and Judith River Formations. This is also true of some strata to the east including the the New Mexican Fruitland Formation and lower Kirtland Formation. The ceratopsid-bearing deposits of the Aguja Formation's Upper Shale Member or Lehman's Kritosaurus zone was once believed to be about the same age as the aforementions strata but is now thought to be more recent than them. The most productive intervals of the Aguja formation probably range from the Latest Campanian to the early Maastrichtian.
Radiometric dates for important stratigraphic units have provided insights into when certain ceratopsian species lived, how long they lived, which ones lived at the same time. The small sample sizes of many species. Centrosaurus apertus specimens are known from an interval of at least 500,000 years.
The earliest known ceratopsian is a centrosaurine is a new species from the Wahweap Formation. From radiometric dates of the Wahweap Formation as well as the stratigraphy of the holotype, this species lived in the early Campanian at about 79 million years ago. Triceratops horridus is the latest occurring ceratopsian and occurs almost right up to the Cretaceous-Paleogene boundry. The last 5 million years of the Campanian is the most thoroughly studied period in ceratopsian evolution. 21 of the 32 known species, or two thirds, are known from this interval. The stratigraphy of this period is understood at a higher resolution often being able to trace successions of closeley related species. For instance, in the Dinosaur Park Formation three successive species of Chasmosaurus occur one right after the other C. russeli, C. belli and C. irvinensis in the late Campanian. More recent sediments in the Two Medicine Formation have produced a similar succession of related centrosaurs; Styracosaurus ovatus, Einiosaurus procurvicornis, and Achelousaurus horneri. The Two Medicine series occurs from 74.9 to 74 million years ago while the Dinosaur Park taxa occur from 76.4 to 74.8 million years ago. Sampson and Loewen cautioned that because the centrosaurs of the Two Medicine formation were known from so few sites it might be hasty to for confident conclusions about their stratigraphic ranges.
There is little chronological overlap between the Two Medicine and Dinosaur Park ceratopsian faunas. However, the Dinosaur Park ceratopsids do overlap with those of the Kaiparowits Formation. Kaiparowits new taxa ABC all occur in the lower part of the middle member, about 75.3 million years old. Kaiparowits lived at the same time as Centrosaurus in the north. B and C were about the same age as Chasmosaurus belli. Torosaurus latus, Triceratops horridus and Diceratopps hatcheri lived at the same time in the late Maastrichtian. Agujaceratops lived at the same time as either Arrhinoceratops brachyops or Anchiceratops ornatus. The coexistence of Centrosaurus apertus and Kaiparowits new taxon A is the only clear cooccurence among centrosaurines, although they did not have overlapping geographic ranges, too.
In 2005, Ryan and Evans reported a possible overlap between Centrosaurus apertus and Styracosaurus albertensis. However, the "Styracosaurus" specimen this was based on was found by Ryan and others in 2007 to be the first process on the caudal parietal of a Centrosaurus apertus. The only other possible co-ocurrence is Avaceratops lammersi with Centrosaurus apertus. However since both the age and classification of Avaceratops. As it now stands there is no evidence for multiple kinds of ceratopsid living in the same time and place within either the northern or southern regions of the Western Interior Basin.
The longest surviving ceratopsid species known in the scientific literature is Pentaceratops sternbergi, which lived for about 2.5 million years From 75 to 72.5 million years ago. The "distant" second is Triceratops horridus whic survived for about 1.5 million years, from 67 to 65.5. However, Sampson and Loewen encouraged critical resppraisal of these alleged durations of these taxa because they are suspicious outliers compared to the generally shorter lived species. They also noted that most ceratopsid species are known from less than five specimens. Those known from larger samples, like Einiosaurus procurvicornis often have those samples concentrated in only a short stratigraphic interval. The best studied stratigraphic ranges are those of Centrosaurus apertus and Styracosaurus albertensis which are both known form more than ten specimens in a wide stratigraphic range in a well-studied geologic unit, the Dinosaur Park Formation. These species lasted for 700,000 and 500,000 years respectively. Because both of these intervals are ended with the appearance of closely related species they may represent the true lifespans of the species. Among chasmosaurines there are also known sequences of short-lived non-overlapping species in the Dinosaur Park formation. There are likewise short-lived non-overlapping sequences of centrosaurines in the Two Medicine Formation. This suggests a general pattern of short lived species (significantly less than a million years) that evolved rapidly. This general pattern of very short lived species casts doubt on the idea that Pentaceratops sternbergi actually lived for 2.5 million years.
Climate, vegetation distributions, landmass size, and physical barriers are all factors that could affect which environments ceratopsians lived in. Evolution is also affected by perturbations in the environment like habitat tracking, extinction, and the formation of isolated populations. This makes the environmental contexts of ceratopsian remains important for understanding their biology and evolution.
Changes in the level of the Western Interior Seaway had profound impacts on ceratopsian. About 100 million years ago the epieric sea flooded the interior of western North America. This formed two partially isolated land masses, known as Laramidia in the west and Appalachia in the east. The Sevier Fault Belt bordered the Western Interior Seaway to the west. Over the next 35 million years the Wester Interior Seaway rose and fell, changing the size of the Western Interior Basin and the kinds of environments found there. Several fossil-bearing rock formations found between the Sevier and the KWIS preserve evidence of environments like a near shore coastal plain, more distal alluvial plain and upland intermontane basins during the Maastrichtian. The last major expansion into norther Laramidia of the Western Interior Seaway was the Bearpaw Transgression. For 3.5 million years of the late Campanian the KWIS migrated 200 miles west, with occasional short regressive fluctuations. During the Maastrichtian, the KWIS retreated to the Northeast, eventually exposing connections between western and eastern North America. A remnant of the KWIS called the Canonball Sea remained in north-central North America into the Paleogene.
Zuniceratops is the siter taxon of the ceratopsids and already existed 90 million years ago, significantly after the Western Interior Seaway came into existence. The earliest known ceratopsid occurs at about 79 million years. The lack of ceratopsids in Appalachia suggests they evolved after the KWIS isolated the two landmasses from eachother. Triceratops is the last known ceratopsid and coincides with the retreat of the canonball sea at the end of the Cretaceous. Triceratops, Torosaurus, and Diceratops are the only known ceratopsids who weren't restricted to the narrow band of alluvial coastal plain sediments lying between the Sevier orogenic belt and the Western Interior Seaway. They lived in the area once occupied by the sea before it began retreating and the two halves of North America were reunited.
After the Dinosaur Renaissance, ceratopsians were viewed as rhino or antelope-like animals living in mixed-sex herds in upland habitats. In 1987, Lehman challenged this view by noting that Triceratops lived in humid lowlands near the coast. Later studies on earlier ceratopsids from the Campanian supported the coastal lowland ideas. In the northern region of the Western Interior Basin, ceratopsian fossils are most abundant when the sea expanded. The clastic wedge that preserves dinosaur fossils in the region includes geologic formations like the Judith River Formation, Dinosaur Park Formation, and Two Medicine. All of these formations are overlain by the marine shales of the Bearpaw Formation. This implies that fossil sites located stratigraphically higher within these formations represent environments closer to the coast. In 1998 Brinkman and others published a study documenting evidence of this same stratigraphic pattern. They also studied sites of the same age geographically varying in placement from east to west within southern Alberta. If ceratopsians really preferred coastal habitats they should be seen in increasing numbers as the sites progressed farther east because those sites would be closest to the coast. The researchers found that this was indeed the case in both macrovertebrate and microvertebrate sites.
In 2007 Eberth concluded that most ceratopsians lived in alluvial to coastal plain habitats with poorly drained soil but a small number lived in better drained alluvial plain settings. In 2007 Butler performed a "relatively coarse analysis" that concluded that ceratopsians preferred drier inland habitats.
Sampson and Loewen surveyed the literature for every known ceratopsian species and found "strong support" for the coastal hypothesis. They found that most ceratopsians are found in strata near coastal environments, just like Brinkman and others did. Deposits preserving ceratopsian fossils tend to be located in close stratigraphic proximity to marine deposits associated with transgression and regression cycles. This is true of Agujaceratops mariscalensis, Cerro del Pueblo new taxon, and Eotriceratops xerinsularis.
The Two Medicine Formation is particularly significant because it's one of the few Campanian deposits in the Western Interior Basin to preserve more arid upland environments. In 2007 Ryan and others speculated that Styracosaurus albertensis of the Dinosaur Park Formation was adapted to more coastal mesic settings while S. ovatus of the Two Medicine Formation was adapted for more inland xeric settings. Most dinosaur remains in the Two Medicine Formation are known from the upper portion 76-74 million years ago. Ceratopsians, however, are known only from the 75-74 million year old uppermost part of the formation. Below this level dinosaur remains are relatively abundant and include animals like the Egg Mountain fauna including Maiasaura, Orodromeus, and Troodon. Sampson and Loewen feel that the absence of ceratopsian fossils below the uppermost Two Medicine reflects and absence in life. They hypothesize that ceratopsians didn't appear in the Two Medicine depositional environment until the Bear Paw transgression had changed the environment to a coastal setting. Since S. ovatus is slightly younger than S. albertensis and lived during a period of rapid transgression it's possible that it too could have been a coastal animal.
In 1998 Rogers concluded that the uppermost Two Medicine strata were deposited in a semi-arid environment, which may be evidence against Sampson and Loewen's hypothesis. They said the coastal hypothesis can be tested with increased sampling of strata lower in the Two Medicine and further study into how its and other formations' environments changed over time. Dinosaurs of the Campanian WIB are only known from west coastal plains and cooler alluvial plains, a third paleoenvironment appears during the Maastrichtian. After the Laramide orogeny began in the early Maastrichtian, semi-arid intermontane basins appeared. These were characterized by a unique fauna of herbivorous dinosaurs that includes the titanosaur Alamosaurus and the chasmosaur Torosaurus. These environments are generally only found in the wouth of the WIB from Utah to Texas. The presence of Torosaurus in upland intermontane basins a significant distance away from the coasts suggests it may have been an ecological outlier that lived in a paleoenvironment significantly different from those preferred by other ceratopsians. Sampson and Loewen encouraged future research into understanding ceratopsian habitat preferences in order to better understand their evolutionary history.
Most Mesozoic biogeography has focused on comparing the lifeforms between and among continents. Because there are many fossils and the area is well studied, Cretaceous western North America can be examined for regional differences. The total combined area of Cretaceous dinosaur-rich habitats in western North America adds up to 4,000,000 kilometers or 16% of the continent, a "remarkabl[e]" figure. Although North America was sometimes linked to Asia during the Late Cretaceous, the lack of species shared between the two continents suggest that there was only limited faunal exchange over this land bridge. The meant that western North America was effectively isolated. Sampson and Loewen called it a "sweepstakes filter". Late Cretaceous wwestern North America is probably the best studied "slice" of space-time of any major Mesozoic land mass.
Some paleontologists have argued that during the Late Cretaceous western North America had distinct northern and southern biomes. Others have challenged thhis hypothesis on the grounds that some geologic formations being compared were of different ages. Recent radiometric dates for different strata have supported the idea of faunas varying by latitude. This is because the Dinosaur Park, Two Medicine, and Judith River Formations all overlap with eacother in times. This is also true of the southern Kaiparowits and Fruitland Formations. The discovery of a new macrovertebrate fauna in the Campanian Kaiparowits Formation of Utah in particular has supported this idea. Sampson and Loewen consider the most remarkable fact about purported latitudinal variations in the fauna of Late Cretaceous Western North America in that so conclusively identified fossils from a single species of dinosaur from the time period is known from both the north and south of the continent. In fact most dinosaurs are known only from a single geologic formation, although Triceratops and Torosaurus were exceptions to that rule.
The narrow geographic ranges of dinosaurs living in Cretaceous western North America contradicts the idea that some of them engaged in long distance migrations the way caribous and wildebeest do. Despite their large body sizes both hadrosaurs and ceratopsids seem to be divided into a northern faunal group including Montana and Alberta, as well as a southern faunal group that included Utah, Colorado, New Mexico and Texas. These separate geographic ranges occur within well-constrained overlapping time intervals, which is strong support for the idea of dinosaur provincialism in western Late Cretaceous North America as advocated by Lehman.
Sampson and Loewen enouraged further testing on the idea of endemism by looking for new kinds of dinosaur, reexamining the identity of known fossils, phylogenetic analyses, increasing stratigraphic resolution and collecting in unexplored formations.
In 1998 Brinkman and others proposed a hypothesis Sampson and Loewen called "intriguing" regarding ceratopsid migration. The observed that the many monodominant ceratopsid bonbeds tended to occur in the middle of the Dinosaur Park Formation. The rock here was deposited in an inland deposit. This is in contradiction to most ceratopsid fossil finds which tend to be closer to the shore of the KWIS. The authors hypothesized that ceratopsids may have gathered into herds seasonally and engage in east-west migrations perhaps in accordance with food availability or mating season. Sampson and Loewen encouraged more research into this idea as more evidence comes to light.
It is currently unclear whether the ceratopsids of the Maastrichtian had the same levels of endemism as those of the late Campanian. Torosaurus latus has been reported from both the north in formations like the Hell Creek of Montana, North Dakota, and South Dakota as well as the Frenchman Formation of Saskatchewan. Torosaurus latus is also known from the south in formations like the North Horn Formation of Utah, McCrae Formation of New Mexico, and Javelina Formation of Texas. Triceratops horridus also has a broad range, although not as broad as Torosaurus latus. Some paleontologists have regarded southern specimens of Torosaurus as belonging to a second species, Torosaurus utahensis. The number of species in Triceratops has also been controversial. Most specimens have been referred to T. horridus. There is some tentative evidence for geographically isolated species. Andrew Farke thinks that northern Triceratops specimens and southern Triceratops specimens may be distinct form each other.
If these dinosaurs really were divided into different species by area, this implies that whatever factors may have led to enedmism in the Campanian was still around in the Maastrichtian. If they weren't divided into different species and instead had larger ranges than the Campanian ceratopsids then that may mean those factors were gone. The disappearance of these factors could have been related to changing climate and the retreat of the KWIS. Also, if they were predominantly coastal animals ceratopsids like Triceratops may have followed after the retreating KWIS.
The discovery of centrosaurine fossil in the Parras Basin of Mexico and reports of Pachyrhinosaurus from Alaska means that centrosaurs spanned all of North America from Mexico to the Arctic. This is a "much larger" range than has been documented in chasmosaurs. The discovery of new centrosaurs in Utah is evidence of previously unrealized diversity in the south of the Western Interior Basin. Zuniceratops is evidence that long eye horns were present in the common ancestor of centrosaurs and chasmosaurs. The presence of centrosaurs with long eye horns like Albertaceratops and the new taxa from the Kaiparowits and Wahweap Formations indicates they were still present in primitive centrosaurs and that these primitive centrosaurs already achieved a wide latitudinal range. Pachyrhinosaurs with facial bosses seem restricted to the northern part of western North America.
The small geographic ranges of most ceratopsids is unusual given their large body sizes. This is because larger animals need broader geographic ranges in order to get enough food to eat. Larger home ranges would also imply lower population densities. Home ranges are expected to be larger for larger bodied animals. This trend would be more extreme among carnivores than herbivores and among endotherms than endotherms. Maximum body size in large bodied predators reflects a quest for balance between keeping populations high enough not to go extinct but not so high as to over-exploit prey items.
There are only a few possible explanations for how large elephant sized herbivores could survive at high population densities in small geographic ranges:
Metabolic rate much lower than those of modern mammals.
Access to larger amounts of high quality food than is avaliable to modern mammals
Sampson and Loewen characterize the fossil record of ceratopsids as relatively dense. There are 32 known species from a 14 million year span of time. However, the quality of that record isn't uniform and there still many gaps. Before the discovery of other centrosaurines with facial bosses Pachyrhinosaurus canadensis was thought to be a "bizarre, odd-ball taxon". The discovery of centrosaurs with long horns over their eyes changed science's understanding about what it meant to be a centrosaur. A new centrosaur from the Wahweap Formation is the oldest known ceratopsid and may help shed light on the trasition from basal neoceratopsians to ceratopsids.
The best studied span of time in ceratopsid research is 76-74 million years ago during th late Campanian. Of the 32 known ceratopsids, almost half are from this time span. Taking into account the late Campanian generally, 76-70.5 million years ago, more than twenty of the thirty two known ceratopsids are accounted for. This means that two thirds of known ceratopsid diversity is known from one third of their lifespan. Late Campanian strata are also more widely studied across a wide geographic range than those of other ages. They are known from Mexico to Alaska. Only for the 76-74 million years ago timespan can distinct northern and southern faunas be perceived.
The best studied region of ceratopsid habitation is the northern region of the Western Interior Basin, with southern Alberta and Northern Montana being the best studied area of that northern region. Most kinds of Maastrichtian or Campanian ceratopsids are known only from this area. Almost half of known ceratopsids are known from southern Alberta and most of those are unknown any place else. Most of these taxa's stratigraphic distribution are well understood. It's also important to note that the fossil-bearing geologic units in the area represent multiple different kinds of environments from nearshore coastal plain to more distal alluvial plain settings.
In some points in time few or no kinds of ceratopsids are known. Very few ceratopsids older than 76 million years are known. In the north only the 77 million year old Centrosaurus brinkmani, the 78 million year old Albertaceratops nesmoi, and Medusaceratops lokii. In the south is the 79 million years old Wahweap new taxon. Only the new taxon from the Wahweap Formation has significant implications for understanding early ceratopsid evolution. Before 79 million years ago is a 10 million year gap before encountering the ceratopsids' sister taxon, Zuniceratops christopheri.
It's not known when the first ceratopsids appeared, if there was a significant radiation of ceratopsids prior to the centrosaur-chasmosaur split. In order to resolve these quandries paleontologists will have to conduct research in strata dating from 90-80 million years ago.
Another major stratigraphic gap in the fossil record of ceratopsids spans the Maastrichtian, the final five million years of the Cretaceous Period, from 70.5 to 65.5 million years ago. Dramatic environmental changes occurred at this time including dropping temperatures and withdrawal of the Cretaceous Interior Seaway. The latter resulted in renewed connection between Laramidia and Appalachia. Known ceratopsians from this time period include relatively well-known dinosaurs like Triceratops horridus and Torosaurus latus from the latest Maastrichtian. Arrhinoceratops brachyops and Eotriceratops xerinsularis date to the latest Campanian or early Maastrichtian.
It's not known whether a major extinction exterminated most ceratopsids at the end of the Campanian leaving mostly chasmosaurine descendants. It's also possible that ceratopsids continued to produce many short-lived species into the Maastrichtian the way they had in the Campanian.
Related to the Maastrichtian gap is the relative ignorance about southern ceratopsids. There have been enough recent finds of new ceratopsids in the south of the Western Interior Basin to suggest the prospect that southern ceratopsid diversity was at a similar level as the north. It's not known if southern ceratopsians experienced the same rapid evolution and replacement of species as the northern ecosystems did. It's also not known if the regional division of faunas continued to exist in the Maastrichtian or if the environments of western North America became more uniform. If these places with unique faunas existed, how long did they last, and what stopped animals from one fauna from moving into another faunal region.
Many paleontologists have thought that the Judithian, or late Campanian was the peak of dinosaur evolution, at least in North America, with large numbers of species both within and between geologic formations. In 2001, Lehman proposed that there was an association between Corythosaurus and Centrosaurus in the Dinosaur Park Formation and a Maiasaura-Einiosaurus fauna in more inland habitats of the same time period in the Two Medicine Formation. Sampson and Loewen felt that stratigraphic research since then had discredited this hypothesis. Centrosaurus and Einiosaurus don't seem to have lived at the same time period and were separated in age by about a million years. There also isn't any evidence for multiple chasmosaurs inhabiting the same latitude. In fact the scientific evidence suggests that at any given time in Late Cretaceous western North America there were one each of centrosaurs and chasmosaurs living in the northern and southern regions of the Western Interior Basin.
Sampson and Loewen noted possible exceptions to this generalization include the co-occurrence of Diceratops hatcheri, Triceratops horridus and Tatankaceratops in the Late Maastrichtian Hell Creek Formation in the northern region. And in the south the co-occurrence of Kaiparowits new species B and C in the Kaiparowits Formation.
They also observed that Diceratops hatcheri's status as a legitimate species has been questioned and it is only known from one specimen. The stratigraphy of the Kaiparowits new taxa is currently under investigation. The current pattern in ceratopsian stratigraphy might change with the acquisition of more data. There might truly be divisions between coastal and inland faunas. However, Sampson and Loewen, called the current level of understanding "striking" and the evidence currently indicates that ceratopsids mostly lived near the coast. There is little evidence supporting the idea that multiple chasmosaurs or multiple centrosaurs inhabited the same place at the same time in either the south or the north of the Western Interior Basin.
The relatively low diversity of ceratopsids living at the same time coupled with their high diversity over time suggests that ceratopsid species turnover rates were very high. The average ceratopsid species probably survived significantly less than a million years. The scarcity of data and wholes in the fossil record prevent scientists from making detailed analysis, however, the existence of documented series of distinct ceratopsid species that don't overlap chronologically offers further support to the idea of short species lifetimes and rapid turnover.
Sampson and Loewen noted that it was possible that the latest Cretaceous only supported a single lineage each of centrosaurs and chasmosaurs, although earlier estimates anticipated higher levels of diversity.
Ceratopsid traits suited for survival underwent very little evolution compared to traits related to reproduction. This phylogenetic conservatism might have resulted in a lack of niche partitioning necessary to allow multiple related species to coexist without overcompetition. Some of the features that distinguish chasmosaurs from centrosaurs may have been adaptations that occurred early in the clade's evolution to prevent them from competing for the same food resources.
Most ceratopsids were social and lived in open environments. This means that camoflauge wouldn't have been a viable way to avoid predators in adult ceratopsids due to the inherent visibility associated with their large body size in the open terrain. As a consequence the elaborate and conspicuous display and signalling structures like horns and frills wouldn't make ceratopsids more likely victims of predation. The only negative effect on the evolutionary fitness these structures might have had would be the extra weight they imparted to the skull. Also, since a ceratopsid's jaw muscles didn't attach to the frill, it wouldn't have negatively impacted their ability to feed. Research on modern animals including both vertebrates and invertebrates has shown that sexual selection on display and signalling traits can be a major driver of species diversification.
Sampson and Loewen concluded that centrosaurs and chasmosaurs evolved into separate ecological niches very early in ceratopsid evolution and stayed in their respective niches throughout the remaining duration of their evolution. Evidence for this comes from the lack of evolution in fitness related characters. Although their niches were stable, the environment changed rapidly isolating different populations from eachother. These isolated populations would diverge as the genetic basis for female mate preferences evolved independantly and shaped their respective populations horns and frills. Gradual accumulations of genetic differences would have limited or prevented interbreeding if these populations were ever again in contact.
The new species would have been so similar to its parent population that they would be in direct competition for resources. It's likely that one of these competing varieties would quickly dominate the other, usually causing the extinction of the other.
Multiple factors probably contributed to the short lifespans of ceratopsid species. These include cyclical envrinomental changes like transgression and regrassion of the Cretaceous Western Interior Seaway splitting larger populations into smaller ones. Also, there would have been high levels of mate competition within mixed-sex herds which might have led to rapid changes in mating signals. Also the existence of mating signals not tied to survival traits. Also the lack of changes in fitness-based traits prevented divergence into new niches for closeley related species. Succinctly, many new species were evolving in an environment that lacked the capacity to hold all of them led to the development of a pattern of continuous replacement.
This pattern contrasts with other groups of dinosaurs that lived at the same time, but for much longer periods. One example would be the ankylosaurid Eoplocephalus tutus in the Dinosaur Park Formation, which wouldn't have had the high levels of mate competition.
Sampson and Loewen admitted that this scenario was highly speculative. It predicts that there would be a rapid pattern of replacement where two taxa from the same lineage overlap only for a brief period of time. This brief period of overlap implies that there would be little evidence for the co-occurrence in the fossil record. The fossil record implied by this scenario would closely resemble one left by an anagenetic series where one taxon directly gave rise to another, as proposed by Horner and others in 1992. Sampson and Loewen's scenario, however, is cladogenetic with the formation of each new branch in the lineage followed by an extinction event. One of the predictive differences between the above scenarioes is that Sampson and Loewen's would predict relative morphological stasis within species instead of gradual drift in the direction of their descendant taxon's as would be expected of the anagenetic scenario. Instead, while there may be variation about a mean, the species should remain roughly the same until they go extinct.
Sampson and Loewen noted that morphometric analysis could turn up evidence for gradual evolution within species that would weigh against their own scenario and in favor of the anagenetic scenario. The reaffirmed that the current evidence, however, did support their scenario.
Sampson and Loewen affirmed the value understanding the ceratopsids as existing within a broader evolutionary context that includes the other ceratopsian dinosaurs. The evolution of these more primitive ceratopsians differed from the evolution of ceratopsids in that the more primitive taxa varied from one another significantly in terms of fitness related traits. These variant traits include ones like gait, which in some forms was bipedal and in others quadrupedal, or jaw function, as some taxa had broad planar cutting surfaces on their teeth while others had complex crowns that combine shearing and crushing.
This isn't just due to looking at the total variety of non-ceratopsid ceratopsians, there really does appear to be differences over time within clade and differences between them. In a 2003 paper, Streelman and Danley argued that many vertebrate radiations pass through a series of stages. They used phylogenetics and population genetics to conclude that groups tend to pass through a three stage sequence; habitat, trophic morphology, and communication. Divergence in habitat and trophic morphology proceeds according to ecological speciation models. However, divergence with regards to communication tends to follow sexual selection models.
Sampson and Loewen noted that one could interpret among ceratopsians the psittacosaurs and basal neoceratopsians representing divergence along the habitat and trophic morphology axes while ceratopsids diverged in according with the communication axis where sexual selection was a bigger driving force in their evolution than natural selection. Streelman and Danley proposed that a scarcity of traits amenable to sexual selection is one of the biggest limiting factors in vertebrate evolution. However, ceratopsian horns and frills, were highly variable and variations in their form seem to have had little impact on the fitness of the animals themselves. Sampson and Loewen "tentatively" proposed that the ceratopsids represented the third of Streelman and Danley's three stages, evolution along the communication axis, for the ceratopsians.
It's not known whether species turnover of different groups of dinosaurs living in Late Cretaceous western North America coincided or occurred with unrelated timing of speciation events. Evolution in different groups of mammals from Miocene to recent times seems to have occurred in coordinated burst across groups. Similar coordinated bursts between groups are known in marine invertebrate fossils. Thorough understanding of faunal turnover requires abundant fossils within a well-understood stratigraphic interval. The northern region of the Late Cretaceous Western Interior Basin, especially the Dinosaur Park Formation and Two Medicine Formation, best fits this description.
In 2005 Ryan and Evans described three faunal zones for the Dinosaur Park Formation based on co-occurrences of ceratopsians and hadrosaurs. The existence of these zones reduces the variety of dinosaurs alive at any given time during the late Campanian. Preliminary findings are suggestive that changes in the environment corresponded to bursts evolution occurring at the same time across different dinosaur groups. In 1992 Horner and others regarded the expansion of the Cretaceous Western Interior Seaway as an impetus for bursts of extinction and evolution across dinosaur groups. There is some evidence for this. Achelousaurus hornei appears to be a member of the Two Medicine ceratopsid lineage. This taxon may have been documented in both the highest sediments of the Two Medicine Formation and Dinosaur Park Formation farther north. These remains date just prior to the farthest extent of the Bearpaw transgression. Achelousaurus horneri is the oldest known Pachyrhinosaurus-like ceratopsid with facial bosses. All later centrosaurines, between 74 and 70 million years ago, in the area show similar traits. This may imply that the Bearpaw transgression created a habitat bottleneck that killed the Centrosaurus-Styracosaurus lineage but allowed the Achelousaurus-Pachyrhinosaurus lineage to survive.
If it turns out that pulses of speciation did occur simultaneously across groups it must then be ascertained whether or not this pattern holds across multiple geologic formations. It must also be ascertained how similar or different patterns of species turnover were between the northern and souther regions of western North America. If the expansion and retraction of the western interior seaway were responsible for the species turnover events then they wouldn't have occurred at the same time in the north and south since expansion and retraction events of the Cretaceous Western Interior Seaway didn't occur at the same time across all regions of western North America.
The two earliest known ceratopsids are Albertaceratops nesmoi and Wahweap new taxon are evidence that the ceratopsian body plan was in place between 79 and 77 million years ago. These taxa have traits like large nostrils, dental batteries, and large ornate frills. The fitness related traits in either chasmosaurs and centrosaurs didn't change significantly for a period of 14 million years despite abundant speciation. Ceratopsid evolution was dominated by sexual selection of signalling traits. Natural selection had little impact on what niches ceratopsids would have occupied. At any single point in time there were likely only a single species of chasmosaur and centrosaur in the nothern and southern regions of western North America.
The low ceratopsid species diversity at any given point in time may have important implications for understanding Late Cretaceous dinosaur diversity in general. Many scientists have argued that in North America dinosaur diversity peaked in the late Campanian and then dwindled to very low diversity near the end of the Cretaceous. Much of the support for this dwindling diversity hypothesis has been based on the large variety of dinosaur found in the Dinosaur Park Formation. However, the Dinosaur Park Formation preserves a series of dinosaur faunas, so most of its preserved diversity would not have been alive at one time. In 2002, Russell and Manabe published evidence that the Hell Creek Formation is more diverse than previously credited. Some dinosaur groups present in the late Campanian are absent in the late Maastrichtian. These include lambeosaurs, centrosaurs, and albertosaurs. The reason the Dinosaur Park Formation preserved so many different species is because they existed for brief periods before evolving and going extinct. This led to a high number of species being preserved over the period of deposition.
Dinosaur diversity during the Campanian was still high despite the above comments. At any given time during the late Campanian just among large herbivores it would be typical to find a centrosaur, chasmosaur, lambeosaur, hadrosaur, nodosaur and ankylosaur. The newly discovered Kaiparowits Formation fauna in Utah suggests the south was just as biodiverse as the north at the time. This implies that during the late Campanian a land area a fraction the size of modern western North America supported a dozen or more herbivores ranging in sizes comparable to rhinos or elephants.
This high diversity of large herbivores crowded into such a small land area suggests a greater abundance of plant material, lower metabolisms, or both, compared to those enjoyed by modern mammals. Science still has much to learn about ceratopsid dinosaurs. This can be said despite the fact that ceratopsids are probably the best documented clade of dinosaurs. North America probably still has many new kinds of ceratopsid dinosaurs waiting to be discovered. There is a complete lack of overlap in time between genera and species from the north and south of the Western Interior Basin.
Much has yet to be learned about the following subjects:
1.) The origin and early divsersification of ceratopsids from basal neoceratopsians. 2.) The diversity of ceratopsids during the Maastrichtian. 3.) The number and size of regional dinosaur faunas of western North America during the Cretaceous. 4.) Whether species evolution and extinction occurred at the same time in different dinosaur groups
- "Abstract," Sampson and Loewen (2010); page 405.
- "Introduction," Sampson and Loewen (2010); page 406.
- "Introduction," Sampson and Loewen (2010); page 407.
- "Diversity and Taxonomy," Sampson and Loewen (2010); page 407.
- "Diversity and Taxonomy," Sampson and Loewen (2010); page 408.
- "Phylogeny," Sampson and Loewen (2010); page 411.
- "Stratigraphy," Sampson and Loewen (2010); page 411.
- "Stratigraphy," Sampson and Loewen (2010); pages 411-413.
- "Stratigraphy," Sampson and Loewen (2010); page 413.
- "Stratigraphy," Sampson and Loewen (2010); pages 413-414.
- "Stratigraphy," Sampson and Loewen (2010); page 414.
- "Paleoenvironment," Sampson and Loewen (2010); page 414.
- "Paleoenvironment," Sampson and Loewen (2010); pages 414-415.
- "Paleoenvironment," Sampson and Loewen (2010); page 415.
- "Paleoenvironment," Sampson and Loewen (2010); pages 415-416.
- "Paleoenvironment," Sampson and Loewen (2010); page 416.
- "Biogeography," Sampson and Loewen (2010); page 416.
- "Biogeography," Sampson and Loewen (2010); pages 416-417.
- "Biogeography," Sampson and Loewen (2010); page 417.
- "Biogeography," Sampson and Loewen (2010); pages 417-418.
- "Biogeography," Sampson and Loewen (2010); page 418.
- "Discussion: Bridges and Gaps," Sampson and Loewen (2010); page 418.
- "Discussion: Bridges and Gaps," Sampson and Loewen (2010); page 419.
- "Discussion: Species Diversity and Turnover," Sampson and Loewen (2010); page 419.
- "Discussion: Species Diversity and Turnover," Sampson and Loewen (2010); pages 419-420.
- "Discussion: Species Diversity and Turnover," Sampson and Loewen (2010); page 420.
- "Discussion: Species Diversity and Turnover," Sampson and Loewen (2010); pages 420-421.
- "Discussion: Species Diversity and Turnover," Sampson and Loewen (2010); page 421.
- "Discussion: Species Diversity and Turnover," Sampson and Loewen (2010); pages 421-422.
- "Discussion: Species Diversity and Turnover," Sampson and Loewen (2010); page 422.
- "Conclusion," Sampson and Loewen (2010); page 422.
- "Conclusion," Sampson and Loewen (2010); pages 422-423.
- "Conclusion," Sampson and Loewen (2010); page 423.
- Sampson, S. D. and Loewen, M. A. 2010. Unraveling a radiation: a review of the diversity, stratigraphic distribution, biogeography, and evolution of horned dinosaurs. (Ornithischia:Ceratopsidae). Pp. 405-427, In: Michael J. Ryan, Brenda J. Chinnery-Allgeier, and David A. Eberth (eds), New Perspectives on Horned Dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium, Indiana University Press, 656 pp. ISBN 0253353580.
Info used in
- Scott D. Sampson
- Mark A. Loewen
- Western Inerior Basin
- geologic formation
- Wahweap Formation
- Lethbridge Coal Zone
- Dinosaur Park Formation
- TMP 2002.76.1
- Two Medicine Formation
- Bearpaw Formation
- sister taxon
- Laser-fusion Ar40/Ar39
- Kaiparowits Formation
- Judith River Formation
- New Mexican
- Fruitland Formation
- Kirtland Formation
- Aguja Formation
- Upper Shale Member
- Wahweap Formation
- radiometric dates
- Triceratops horridus
- Sevier Fault Belt
- intermontane basins
- Bearpaw Transgression
- Canonball Sea
- Dinosaur Renaissance
- clastic wedge
- Bearpaw Formation
- Eotriceratops xerinsularis
- Egg Mountain
- Laramide orogeny
- Fruitland Formations
- Frenchman Formation
- North Horn Formation
- McCrae Formation
- New Mexico
- Javelina Formation
- Andrew Farke
- Parras Basin
- Wahweap Formation
- Arrhinoceratops brachyops
- Eotriceratops xerinsularis
- Diceratops hatcheri
- Eoplocephalus tutus
- morphometric analysis
- sexual selection
- Achelousaurus hornei
- Bearpaw transgression