Monday, 30 September 2019
Common animal inhabitants include mule deer, black-tailed jackrabbits, cottontails, western bluebirds, pinyon jays, chukar partridge, whiptail lizards, western fence lizards, gophers and snakes.
Nestled at 7,000 feet on the western slope of central Nevada’s Shoshone mountain range, the park provides an array of stimulating recreational opportunities. The forested slopes provide shade, and breezes help to moderate the summer temperatures which seldom exceed 90˚F. Late spring and early fall can often be the nicest times of the year. Winter visits to the park are possible, but call for weather and road conditions before visiting.
Sunday, 29 September 2019
This classical Tethyan Mediterranean specimen is very well preserved, showing much of his delicate suturing in intricate detail. Phylloceras were primitive ammonites with involute, laterally flattened shells.
They were smooth, with very little ornamentation, which led researchers to think of them resembling plant leaves and gave rise to their name, which means "leaf-horn."
They can be found in three regions that I know of. In the Jurassic of Italy near western Sicily's Rosso Ammonitico Formation, Lower Kimmeridgian fossiliferous beds of Monte Inici East and Castello Inici (38.0° N, 12.9° E: 26.7° N, 15.4° E) and in the Armine area, southeastern Toyama Prefecture, northern central Japan, roughly (36.5° N, 137.5° E: 43.6° N, 140.6° E).
Saturday, 28 September 2019
Some 270 million-plus years ago, had one wanted to buy waterfront property in what is now British Columbia, you’d be looking somewhere between Prince George and the Alberta border. The rest of the province had yet to arrive but would be made up of over twenty major terranes from around the Pacific. The rock that would eventually become the Cariboo Mountains and form the lakes and valleys of Bowron was far out in the Pacific Ocean, down near the equator.
With tectonic shifting, these rocks drifted north-eastward, riding their continental plate, until they collided with and joined the Cordillera in what is now British Columbia. Continued pressure and volcanic activity helped create the tremendous slopes of the Cariboo Range we see today with repeated bouts of glaciation during the Pleistocene carving their final shape. Warm and dry with bellies filled full of soup and crisps, we head back out to explore more of nature's bounty.
Friday, 27 September 2019
The shells had many chambers divided by walls called septa. The chambers were connected by a tube called a siphuncle which allowed for the control of buoyancy with the hollow inner chambers of the shell acting as air tanks to help them float.
We can see the edges of this specimen's shell where it would have continued out to the last chamber, the body chamber, where the ammonite lived. Picture a squid or octopus, now add a shell and a ton of water.
Thursday, 26 September 2019
The body is black, brown, or rusty, and thickset, with thick legs. The antennae are moniliform. The front tibiae bear large strong spurs or a circlet of spines. The tarsi are five-segmented and bear tarsal claws, pulvilli, and a well-developed empodium. As it is with many species, these guys included, the teens of this species are troublesome but the adults turn out alright. As larvae, Bibionidae is an agricultural pest, devouring all those tasty young seedlings you've just planted.
Then, as they mature their tastes turn to the nectar of flowers from fruit trees and la voila, they become your best friends again. With their physical and behavioural transformation complete, Bibionidae becomes a welcome garden visitor, pulling their weight in the ecosystems they live in by being important pollinators.
Wednesday, 25 September 2019
The megaterios were large terrestrial sloths belonging to the group, Xenarthra. These herbivores inhabited large areas of land on the American continent. Their powerful skeleton enabled them to stand on their hind legs to reach leaves high in the trees, a huge advantage given the calories needed to be consumed each day to maintain their large size.
Avocados were one of the food preferences of our dear Giant ground sloths. They ate then pooped them out, spreading the pits far and wide. The next time you enjoy avocado toast, thank this large beastie. One of his ancestors may have had a hand (or butt) in your meal.
In 1788, Bru assembled the skeleton as you see it here. It is exhibited at the Museo Nacional De Ciencias Naturales in Madrid, Spain, in its original configuration for historic value. If you look closely, you'll see it is not anatomically correct. But all good paleontology is teamwork. Based upon the drawings of Juan Bautista Bru, George Cuvier used this specimen to describe the species for the very first time.
Tuesday, 24 September 2019
This was the site of the 1905 Expedition of the University of California’s Department of Geology in Berkeley funded by the beautiful and bold, Annie Alexander, the women to whom the UCMP owes both its collection and existence. J. P. Smith joined the expedition, though he was interested in the invertebrate fauna, not the mighty marine reptiles that helped get the project funded.
J.P. Smith published on the marine fauna in the early 1900s. They formed the basis for his monograph on North American Middle Triassic marine invertebrate fauna published in 1914.
N. J. Siberling from the US Geological Survey published on these outcrops in 1962. His work included nearly a dozen successive ammonite faunas, many of which were variants on previously described species. Evidently, his collections consisted mainly of weathered material and were made without stratigraphic control because he believed that most, if not all, of these species, were coexistent. The fossiliferous beds found here, as well as localities in north-western Nevada, were designated the 'Daonella dubia' zone. Dubious would be closer to the truth. Smith joined the 1905 Expedition of the University of California’s Department of Geology in Berkeley funded by the beautiful and bold, Annie Alexander, the women to whom the UCMP owes both its collection and existence.
Monday, 23 September 2019
Knowing that we share an ancient history with every living species is rather humbling. While we diverged early on from cats, dogs, frogs and birds, we share a history and proud lineage with all the vertebrates alive today.
I was up hunting for trilobites near Cranbrook and came across a graptolite. It looked like a wee pen mark on a bit of rock. It too is one of our relatives and one of the earliest. So how did vertebrates go from worm-like marine animals like Metaspriggina, to the modern diversity of forms?
Sunday, 22 September 2019
Ammonite expert Bill Cobban used this collection to describe many Texas Cretaceous ammonites species including this species from Tarrant County, Arlington, Texas.
He was a surveyor by training and kept incredibly detailed notes on the context of his fossils.
Conlin donated his collection to the USGS and we’ve learned much by studying it along with other specimens from the Lone Star State. Almost a quarter of Texas is covered by Cretaceous strata, much of it fossiliferous. If we stepped back 95 million years, the world and what we now call Texas, was a very different place.
95 million years ago, during the Late Cretaceous, a shallow seaway separated North America into separate eastern and western landmasses. We have a pretty complete picture in the fossil record of the western groups of species but relatively little in comparison for their cohorts in the east.
At the time this fellow was swimming our ancient seas, he was sharing the Earth with carnivorous dinosaurs, duck-billed dinosaurs, mammals, crocodilians, turtles, a variety of amphibians, prehistoric bony fish, oddly prolific sea cucumbers, various invertebrates and plants. Many of these sites are just being written up now and contain new species just being discovered.
During the Late Cretaceous Period, a shallow seaway separated North America into separate eastern and western landmasses. The Woodbine Formation in Texas preserves a rare fossil record of this time for the east, but many of these fossils are isolated and incomplete, making interpretations more difficult. Preliminary excavations at the AAS are providing hints at a more complete ecosystem, preserving similar patterns of change to what we see in the west.
The AAS contains an extraordinary diversity, abundance, and quality of fossil material, preserving one of the most complete terrestrial ecosystems known for this time period and area.
The AAS has a lot to tell us about Late Cretaceous life in the east. Over 2200 individual specimens have been found belonging to numerous groups including carnivorous dinosaurs, duck-billed dinosaurs, crocodilians, turtles, mammals, amphibians, sharks, bony fish, invertebrates, and plants.
Many of the fossils found here represent brand new species and studying these fossils will help to establish the geographic and environmental forces that shaped Cretaceous ecosystems in North America by providing a necessary comparison to the fossil record of the west.
Friday, 20 September 2019
who lived during the Upper Cretaceous (late Campanian) near the town of Porvenir de Jalpa (about 64 km / 40 miles west of Saltillo) in what is now southern Coahuila (formerly Coahuila de Zaragoza), northern Mexico. Collection of Jose Ventura.
Thursday, 19 September 2019
It was in contention for the Global Stratotype Section and Point (GSSP) defining the base of the Jurassic System, Lower Jurassic Series and Hettangian Stage. However, in 2008, the Kuhjoch (Karwendel Mountains, Northern Calcareous Alps, Tyrol, Austria) was chosen over Ferguson Hill mainly because the beds containing the oldest Psiloceras (P. spelae spelae) were better preserved and contained Hettangian microfossils.
Wednesday, 18 September 2019
They lead solitary lives and are excellent at avoiding humans for the most part. Cougars have a massive range that runs from the mountainous Canadian Rockies in northwestern Canada all the way down to Patagonia in South America. These cats make their dens in mountain graggs, along rocky ledges, in dense woodland areas and under uprooted trees and debris.
Monday, 16 September 2019
They are fossil relics, the sole surviving species of the order Amiiformes. Although bowfins are highly evolved, they are often referred to as primitive fishes and living fossils as they retain many of the morphologic characteristics of their ancestors.
This specimen was found in Lower Cretaceous outcrops of the Santana Formation in the Araripe Basin UNESCO Global Geopark of northeastern Brazil. Collection of David Murphy
Sunday, 15 September 2019
So which lucky ducks evolved one? Well, ducks for one. Warm-blooded birds and mammals cheerfully claim those bragging rights. They're joined by our cold-blooded, ectothermic friends, the fish, amphibians and reptiles. All these diverse lovelies share this characteristic.
And whether they now live at sea or on land, all of these lineages evolved from a marine organism somewhere down the line, then went on to develop a notochord and spinal column. Notochords are flexible rods that run down the length of chordates and vertebrates. They are handy adaptations for muscle attachment, helping with signalling and coordinating the development of the embryonic stage. The cells from the notochord play a key role in the development of the central nervous system and the formation of motor neurons and sensory cells. Alas, we often take our evolution for granted.
Let's take a moment to appreciate just how marvellous this evolutionary gift is and what it allows us to do. Your backbone gives your body structure, holds up that heavy skull of yours and connects your tasty brain to your body and organs. Eating, walking, fishing, hunting, your morning yoga class, are all made possible because of this adaptation. Pick pretty near anything you love to do and it is only possible because of your blessed spine.
And it sets us apart from our invertebrate friends.
While seventy thousand may seem like a large number, it represents less than three to five percent of all described animal species. The rest is made up of the whopping 97%'ers, our dear invertebrates who include the arthropods (insects, arachnids, crustaceans, and myriapods), mollusks (our dear chitons, snails, bivalves, squid, and octopus), annelids (the often misunderstood earthworms and leeches), and cnidarians (our beautiful hydras, jellyfish, sea anemones, and corals). You'll notice that many of our invertebrate friends occur as tasty snacks. Having a backbone provides a supreme advantage to your placement in the food chain. Not always, as you may include fish and game on your menu. But generally, having a backbone means you're more likely to be holding the menu versus being listed as an appetizer. So, enjoy your Sunday 'downward dog' and thank your backbone for the magical gift it is.
Saturday, 14 September 2019
These columns tell a story of the cooling and freezing of the lava flows that formed them. As lava at the surface cools and freezes, it also shrinks as its molecules rearrange themselves into a solid structure. This happens much more quickly at the surface where the lava comes in contact with moist, cool air. As the basalt cools and shrinks, pressure increases in intensity and cracks begin to form. A way to dissipate this huge stress is to crack at an angle of 120 degrees, the angle that gives us a hexagon.
We see this beautifully illustrated at the Giant's Causeway in Ireland. Here, highly fluid molten basalt intruded through chalk beds which later cooled, contracted and cracked into hexagonal columns, creating a surreal visual against a dark and stormy Irish Sea.
Friday, 13 September 2019
The carapace is deeply grooved with conspicuous wart-like tubercles; anterolateral margin, between the base of the exorbital tooth and cervical groove, smooth, without tubercles or denticles.
The teeth on the lower orbital margin in the cluster. Carpus of cheliped distinctly granulated on the upper surface and with a conspicuous row of granules along the anterior margin. Though missing here, the merus of second and third pereiopods are almost cylindrical. (Türkay 1995). This specimen was collected and is the collection of the deeply awesome Takashi Ito of Japan
These early fishes and many of the Pteraspidomorphi possessed large plates of dermal bone at the anterior end of their bodies. This dermal armour was very common in early vertebrates, but it was lost in their descendants. Arandaspida is represented by two well-known genera: Sacabampaspis, from South America and Arandaspis from Australia. Arandaspis have large, simple, dorsal and ventral head shields. Their bodies were fusiform, which means they were shaped sort of like a spindle, fat in the middle and tapering at both ends. Picture a sausage that is a bit wider near the centre with a crisp outer shell. Image: Tamura (http://spinops.blogspot.com) - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=19460450
Thursday, 12 September 2019
Tuesday, 10 September 2019
They were mammals and looked like large rodents. They were also quite small by whale standards, reaching about four-feet in length. They ate meat, sometimes fish and are the ancestors of whales, porpoises and dolphins.
The only real clue of their connection to our aquatic friends is the shape of their skulls. Pakicetus had a long skull and an ear bone that is unique to whales. Oddly, they also had ankle bones that share characteristics with some of our even-toed mammals. They lived along the shores of a large shallow sea known as the Tethys. Although rare, there are several examples of mammals heading back to a life at sea. Photo: Kevin Guertin from Ottawa, Ontario, Canada - DSCF1201, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=36657302
Monday, 9 September 2019
|Northumberland Fm, Upper Cretaceous Nanaimo Group|
Thee Upper Cretaceous Nanaimo Group of southwest British Columbia is a >4 km-thick succession consisting mostly of deep marine siliciclastics deposited directly on the Insular Superterrane. As such, this succession has been the focus of several paleomagnetic, isotope geochemistry, paleontology, and sedimentology studies in attempts to elucidate the tectonic history and paleolatitude of the Insular Superterrane and associated entities during the critical time of Nanaimo Group deposition, 90 to 65 million years ago. The upper two-thirds of the succession is continuously and well exposed on Denman and Hornby islands and represents the best example of this part of the succession in the northern half of what we consider the single Nanaimo Basin. A concretion found on the beach at Denman, eroding out of the grey shales of the Upper Cretaceous Nanaimo Group
Sunday, 8 September 2019
|Nootka Sound, Photo: Dan Bowen|
The area is known for its exceptional natural beauty and bounty of beautifully preserved decapod fossil specimens. The formation is named for the Hesquiaht people of the Nuu-chah-nulth, of Nootka Sound. The VIPS has led many research expeditions to remote sites on our West Coast. Their efforts have been rewarded with many new species being identified and excellent cooperation with paleontological researchers from around the globe.
Saturday, 7 September 2019
During the upper Cretaceous, between ~90 to 65 Ma, sediments derived from the Coast Belt to the east and the Cascades to the southeast poured seaward to the west and northwest into what was the large ancestral Georgia Basin. This major forearc basin was situated between Vancouver Island and the mainland of British Columbia.
The Nanaimo Group as a whole represents largely coarse-grained units deposited in deep-sea fan systems. In this environment, deeper channels continuously cut through successive shale and sandstone bodies. The channels funnelled density currents into the basin, while also building levee deposits. Turbidity currents travelled down the channels, and also overtopped the levees spilling across backslope areas. The sequential sediment formations, from significantly coarse-grained sandstones and conglomerates to fine silts and shale units of the Nanaimo Group, are considered to be partly due to eustacy, but more significantly related to relative sea-level changes induced by regional tectonics in an active forearc setting.
The Northumberland Fm consists of a massive, dark-grey mudstone which is locally interlaminated and interbedded with siltstone and fine-grained sandstone. There are abundant calcium carbonate concretions, parallel and current ripple laminations, clastic dikes and folded layers due to slumping. In the Gulf Islands to the south, this formation has been found to contain abundant and diverse foraminifera indicating marine paleodepths of 150-1200 m.
The more resistive Geoffrey Fm consists of thick-bedded sandstone and conglomerate. It is highly channelized, and some sandstone has exposed parallel and ripple laminations. The Spray Fm exposed on the east end of the island is a massive olive-grey mudstone with interlaminations of sandstone.
Furthest to the east, the youngest exposures on Hornby Island are from the Gabriola Fm, which outcrops on the eastern peninsula. This is again a thick-bedded and channelized sequence of conglomerates and massive sandstone with minor mudstone interbeds. South, in the Gulf Islands, this formation has contained ammonites, gastropods and pelecypods. Paleowater-depth from foraminiferal assemblages has been set at 200 m.
Katnick, D.C. and P.S. Mustard (2001): Geology of Denman and Hornby Islands, British Columbia (NTS 92F/7E, 10); British Columbia Geological Survey Branch, Geoscience Map 2001-3.
England, T.D.J. and R. N. Hiscott (1991): Upper Nanaimo Group and younger strata, outer Gulf Islands, southwestern British Columbia: in Current Research, Part E; Geological Survey of Canada, Paper 91-1E, p. 117-125.
Friday, 6 September 2019
Since that time, a shocking half a billion years ago, our seas have played host to an astonishing array of species. If I'd visited our Earth back in the Cambrian, I would have bet good money that our watery planet's future was in the seas not on the land. But that 's not the case. Quite surprisingly, it is our humble rock and soil who now boast more species. Five times that of those living in the oceans. I know, shocking but true. Our oceans certainly had the running start on both numbers and diversity of species. But it is our fungi, our flowering plants, mindblowing variety of insects, trees, bees and fleas that make up the bulk of Earth's species these days.
It is something I'm interested in learning more about as it does not make good sense to me. 80 percent of Earth's species live on land today. About 15 percent call our oceans home and another 5% or so live in freshwater. Why more species live on land than in the ocean has puzzled others as well. Robert May, a zoologist at the University of Oxford, mulls this very question in an article from 1994 titled, “Biological Diversity: Differences between Land and Sea.” He continued with his research and published "The future of biological diversity in a crowded world," in Current Science, Vol. 82, No. 11 (10 June 2002), pp. 1325-1331.
Here he questions how well we know the plants, animals and micro-organisms with which we share this beautiful planet. His focus in the paper was to question how many species are there and how fast are some going extinct? You'll be interested to know that his best guess in 2002 was somewhere between 1.7 to 1.8 million. That's a considerable increase from Carl Linneaus' work back in 1758, the Swedish botanist, zoologist, and physician took a stab at the same question and came up with an estimate of about 9,000 species. While his numbers were off by a long margin, he did give us the binomial nomenclature system we use for naming organisms, so he still gets a hall pass.
May is a boy about town. His work is referenced everywhere. You may enjoy an article by the Atlantic from 2017 that delves into the topic for the lay audience with an eye to popularized reading. May, R. (2002). The future of biological diversity in a crowded world. Current Science, 82(11), 1325-1331. Retrieved from http://www.jstor.org/stable/24105996 / The Atlantic article: https://www.theatlantic.com/science/archive/2017/07/why-are-there-so-many-more-species-on-land-than-in-the-sea/533247/
Thursday, 5 September 2019
"Ichthyosaurs are interesting because they have many traits in common with dolphins, but are not at all closely related to those sea-dwelling mammals," says research co-author Mary Schweitzer, professor of biological sciences at NC State with a joint appointment at the North Carolina Museum of Natural Sciences and visiting professor at Lund University. "We aren't exactly sure of their biology either. They have many features in common with living marine reptiles like sea turtles, but we know from the fossil record that they gave live birth, which is associated with warm-bloodedness. This study reveals some of those biological mysteries."
We find their fossil remains in outcrops spanning from the mid-Cretaceous to the earliest Triassic. As we look through the fossils, we see a slow evolution in body design moving towards that enjoyed by dolphins and tuna by the Upper Triassic, albeit with a more narrower, more pointed snout.
Johan Lindgren, Associate Professor at Sweden's Lund University and lead author of a paper, describing the work, worked with twenty-one other ichthyosaur researchers to analyze the 180 million-year-old specimen, Stenopterygius, from outcrops in the Holzmaden quarry in Germany.
"Both the body outline and remnants of internal organs are clearly visible," says Lindgren. "Remarkably, the fossil is so well-preserved that it is possible to observe individual cellular layers within its skin." Researchers identified cell-like microstructures containing pigment organelles on the surface of the fossil. This ancient skin revealed a feature we recognized from marine dwelling animals, the ability to change colour, providing camouflage from potential predators. They also found traces of what might have been the animal's liver. When they put some of the tissue through chemical analysis, it was consistent with what we'd look for in adipose tissue or blubber. Not surprising as dolphins today use blubber for buoyancy and to help thermally insulate for thermal regulation in cold seas. might find in a vertebrate that uses blubber as a means of maintaining body temperatures independent of ambient conditions.
Today, blubber is an important part of the anatomy of seals, whales and walruses. It covers the core of their bodies, storing energy, insulating them from cold seas and provide extra buoyancy. While they do not have blubber on their fins, flippers and flukes. Not all marine animals need blubber. Our cold-blooded marine friends: sharks, crabs, fish, are able to let their body temperatures dropdown to very chilly levels, some as low as 36 degrees Fahrenheit. They have a few tricks up their sleeves to make this happen. Sharks have evolved specialized physiology to keep their metabolic rate high and their hearts are able to contract in the icy depths because of a special protein. These adaptations allow sharks to enjoy a wide range of habitats and follow their food from warm tropical seas to the icy waters of the North Pacific.
Johan Lindgren, Peter Sjövall, Volker Thiel, Wenxia Zheng, Shosuke Ito, Kazumasa Wakamatsu, Rolf Hauff, Benjamin P. Kear, Anders Engdahl, Carl Alwmark, Mats E. Eriksson, Martin Jarenmark, Sven Sachs, Per E. Ahlberg, Federica Marone, Takeo Kuriyama, Ola Gustafsson, Per Malmberg, Aurélien Thomen, Irene Rodríguez-Meizoso, Per Uvdal, Makoto Ojika, Mary H. Schweitzer. Soft-tissue evidence for homeothermy and crypsis in a Jurassic ichthyosaur. Nature, 2018; DOI: 10.1038/s41586-018-0775-x
North Carolina State University. (2018, December 5). Soft tissue shows Jurassic ichthyosaur was warm-blooded, had blubber and camouflage. ScienceDaily. Retrieved September 7, 2019, from www.sciencedaily.com/releases/2018/12/181205134118.htm
Wednesday, 4 September 2019
We had stormy clouds, rain, sun, and afternoons of wind while walking through time and collecting marine fauna.
Tuesday, 3 September 2019
- Haikouichthys ercaicunensis
- Myllokunmingia fengjiaoa
- Zhongjianichthys rostratus
A friend of mine, Eldon Grupp from the USA, found an 18 mm specimen on a mortality slab of Haikouella from Chengjiang. Apparently, no one had noticed it before shipping. Not surprising as Zhongjianichthys are easy to overlook. I've asked him if I can get a photo of that mortality plate to share with you. It's quite stunning. Haikouella, of course, are not vertebrates, but advanced craniate chordates. The specimen in question, however, was a vertebrate. Eldon has assigned this specimen to genus Zhongjianichthys based on its eel-like characteristics and its large eyes located behind the anterior or rostral lobe instead of within it. Even so, family affiliation is uncertain.
Monday, 2 September 2019
Both of these lovelies from the order Sirenia went from terrestrial to marine, taking to the water in search of more prosperous pastures, as it were.
They are the extant and extinct forms of the oddball manatees and dugongs. They inhabit rivers and shallow coastal waters, making the best use of their fusiform bodies that lack dorsal fins and hind limbs. I've been thinking about them in the context of some of the primitive armoured fish we find in the Chengjiang biota of China, specifically those primitive species that were also fusiform.
We find dugongs today in waters near northern Australia and parts of the Indian and Pacific Oceans. They favour locations where seagrass, their food of choice, grows plentiful and they eat it roots and all. While seagrass low in fibre, high in nitrogen and easily digestible is preferred, dugongs will also dine on lower grade seagrass, algae and invertebrates should the opportunity arise. They've been known to eat jellyfish, sea squirts and shellfish over the course of their long lives. Some of the oldest dugongs have been known to live 70+ years, which is another statistic I find surprising. They are large, passive, have poor eyesight and look pretty tasty floating in the water; a defenceless floating buffet. Their population is in decline but yet they live on.
Sunday, 1 September 2019
Our dear penguins, seals, sea lions, walruses, whales, crocodiles and sea turtles were once entirely terrestrial. Some dipped a toe or two into freshwater ponds, but make no mistake, they were terrestrial. Each of these animals had ancestors that tried out the sea and decided to stay. They evolved and employed a variety of adaptations to meet their new saltwater challenges. Some adapted legs as fins, others became more streamlined, and still, others developed specialized organs to extract dissolved oxygen from the water through their skin or gills. The permutations are endless.
Returning to the sea comes with a whole host of benefits but some serious challenges as well. Life at sea is very different from life on land. Water is denser than air, impacting how an animal moves, sees and hears. More importantly, it impacts an air-breathing animal's movement on a pretty frequent basis. If you need air and haven't evolved gills, you need to surface frequently. Keeping your body temperature at a homeostatic level is also a challenge as water conducts heat much better than air. Even with all of these challenges, the lure of additional food sources and freedom of movement kept those who tried the sea in the sea and they evolved accordingly.
This is an interesting article from Alicia Ault writing for the Smithsonian who interviewed Nick Pysenson and Neil Kelley about some of their research that touches on this area. They published a paper on it in the journal Science back in 2015.
Here's the link: https://science.sciencemag.org/content/348/6232/aaa3716
And Ault's work is definitely worth a read: https://www.smithsonianmag.com/smithsonian-institution/take-deep-dive-reasons-land-animals-moved-seas-180955007/